Commit 5ebe0ee8 authored by Linus Torvalds's avatar Linus Torvalds

Merge tag 'docs-for-linus' of git://git.lwn.net/linux

Pull documentation update from Jon Corbet:
 "There is a nice new document from Neil on how pathname lookups work
  and some new CAN driver documentation.  Beyond that, we have
  kernel-doc fixes, a bit more work to support reproducible builds, and
  the usual collection of small fixes"

* tag 'docs-for-linus' of git://git.lwn.net/linux: (34 commits)
  Documentation: add new description of path-name lookup.
  Documentation/vm/slub.txt: document slabinfo-gnuplot.sh
  Doc: ABI/stable: Fix typo in ABI/stable
  doc: Clarify that nmi_watchdog param is for hardlockups
  Typo correction for description in gpio document.
  DocBook: Fix kernel-doc to be case-insensitive for private:
  kernel-docs.txt: update kernelnewbies reference
  Doc:kvm: Fix typo in Doc/virtual/kvm
  Documentation/Changes: Add bc in "Current Minimal Requirements" section
  Documentation/email-clients.txt: remove trailing whitespace
  DocBook: Use a fixed encoding for output
  MAINTAINERS: The docs tree has moved
  Docs/kernel-parameters: Add earlycon devicetree usage
  SubmittingPatches: make Subject examples match the de facto standard
  Documentation: gpio: mention that <function>-gpio has been deprecated
  Documentation: cgroups: just fix a few typos
  Documentation: Update kselftest.txt
  Documentation: DMA API: Be more explicit that nents is always the same
  Documentation: Update the default value of crashkernel low
  zram: update documentation
  ...
parents 1873499e 3ce96239
...@@ -116,7 +116,7 @@ Description: The "pubek" property will return the TPM's public endorsement ...@@ -116,7 +116,7 @@ Description: The "pubek" property will return the TPM's public endorsement
owner's authorization. Since the TPM driver doesn't store any owner's authorization. Since the TPM driver doesn't store any
secrets, it can't authorize its own request for the pubek, secrets, it can't authorize its own request for the pubek,
making it unaccessible. The public endorsement key is gener- making it unaccessible. The public endorsement key is gener-
ated at TPM menufacture time and exists for the life of the ated at TPM manufacture time and exists for the life of the
chip. chip.
Example output: Example output:
...@@ -163,7 +163,7 @@ Date: April 2006 ...@@ -163,7 +163,7 @@ Date: April 2006
KernelVersion: 2.6.17 KernelVersion: 2.6.17
Contact: tpmdd-devel@lists.sf.net Contact: tpmdd-devel@lists.sf.net
Description: The "temp_deactivated" property returns a '1' if the chip has Description: The "temp_deactivated" property returns a '1' if the chip has
been temporarily dectivated, usually until the next power been temporarily deactivated, usually until the next power
cycle. Whether a warm boot (reboot) will clear a TPM chip cycle. Whether a warm boot (reboot) will clear a TPM chip
from a temp_deactivated state is platform specific. from a temp_deactivated state is platform specific.
......
...@@ -57,4 +57,4 @@ Description: ...@@ -57,4 +57,4 @@ Description:
Shortly after acknowledging it, the log Shortly after acknowledging it, the log
entry will be removed from sysfs. entry will be removed from sysfs.
Reading this file will list the supported Reading this file will list the supported
operations (curently just acknowledge). operations (currently just acknowledge).
\ No newline at end of file
...@@ -44,6 +44,7 @@ o grub 0.93 # grub --version || grub-insta ...@@ -44,6 +44,7 @@ o grub 0.93 # grub --version || grub-insta
o mcelog 0.6 # mcelog --version o mcelog 0.6 # mcelog --version
o iptables 1.4.2 # iptables -V o iptables 1.4.2 # iptables -V
o openssl & libcrypto 1.0.0 # openssl version o openssl & libcrypto 1.0.0 # openssl version
o bc 1.06.95 # bc --version
Kernel compilation Kernel compilation
......
...@@ -681,6 +681,11 @@ or: ...@@ -681,6 +681,11 @@ or:
as appropriate. as appropriate.
PLEASE NOTE: The 'nents' argument to dma_sync_sg_for_cpu() and
dma_sync_sg_for_device() must be the same passed to
dma_map_sg(). It is _NOT_ the count returned by
dma_map_sg().
After the last DMA transfer call one of the DMA unmap routines After the last DMA transfer call one of the DMA unmap routines
dma_unmap_{single,sg}(). If you don't touch the data from the first dma_unmap_{single,sg}(). If you don't touch the data from the first
dma_map_*() call till dma_unmap_*(), then you don't have to call the dma_map_*() call till dma_unmap_*(), then you don't have to call the
......
...@@ -340,7 +340,7 @@ accessed sg->address and sg->length as shown above. ...@@ -340,7 +340,7 @@ accessed sg->address and sg->length as shown above.
void void
dma_unmap_sg(struct device *dev, struct scatterlist *sg, dma_unmap_sg(struct device *dev, struct scatterlist *sg,
int nhwentries, enum dma_data_direction direction) int nents, enum dma_data_direction direction)
Unmap the previously mapped scatter/gather list. All the parameters Unmap the previously mapped scatter/gather list. All the parameters
must be the same as those and passed in to the scatter/gather mapping must be the same as those and passed in to the scatter/gather mapping
...@@ -356,10 +356,10 @@ void ...@@ -356,10 +356,10 @@ void
dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size, dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
enum dma_data_direction direction) enum dma_data_direction direction)
void void
dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems, dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nents,
enum dma_data_direction direction) enum dma_data_direction direction)
void void
dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems, dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nents,
enum dma_data_direction direction) enum dma_data_direction direction)
Synchronise a single contiguous or scatter/gather mapping for the CPU Synchronise a single contiguous or scatter/gather mapping for the CPU
......
...@@ -11,5 +11,7 @@ ...@@ -11,5 +11,7 @@
*.png *.png
*.gif *.gif
*.svg *.svg
*.proc
*.db
media-indices.tmpl media-indices.tmpl
media-entities.tmpl media-entities.tmpl
...@@ -69,6 +69,12 @@ installmandocs: mandocs ...@@ -69,6 +69,12 @@ installmandocs: mandocs
KERNELDOCXMLREF = $(srctree)/scripts/kernel-doc-xml-ref KERNELDOCXMLREF = $(srctree)/scripts/kernel-doc-xml-ref
KERNELDOC = $(srctree)/scripts/kernel-doc KERNELDOC = $(srctree)/scripts/kernel-doc
DOCPROC = $(objtree)/scripts/docproc DOCPROC = $(objtree)/scripts/docproc
CHECK_LC_CTYPE = $(objtree)/scripts/check-lc_ctype
# Use a fixed encoding - UTF-8 if the C library has support built-in
# or ASCII if not
LC_CTYPE := $(call try-run, LC_CTYPE=C.UTF-8 $(CHECK_LC_CTYPE),C.UTF-8,C)
export LC_CTYPE
XMLTOFLAGS = -m $(srctree)/$(src)/stylesheet.xsl XMLTOFLAGS = -m $(srctree)/$(src)/stylesheet.xsl
XMLTOFLAGS += --skip-validation XMLTOFLAGS += --skip-validation
......
...@@ -659,8 +659,8 @@ succinct and descriptive, but that is what a well-written summary ...@@ -659,8 +659,8 @@ succinct and descriptive, but that is what a well-written summary
should do. should do.
The "summary phrase" may be prefixed by tags enclosed in square The "summary phrase" may be prefixed by tags enclosed in square
brackets: "Subject: [PATCH tag] <summary phrase>". The tags are not brackets: "Subject: [PATCH <tag>...] <summary phrase>". The tags are
considered part of the summary phrase, but describe how the patch not considered part of the summary phrase, but describe how the patch
should be treated. Common tags might include a version descriptor if should be treated. Common tags might include a version descriptor if
the multiple versions of the patch have been sent out in response to the multiple versions of the patch have been sent out in response to
comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for
...@@ -672,8 +672,8 @@ the patch series. ...@@ -672,8 +672,8 @@ the patch series.
A couple of example Subjects: A couple of example Subjects:
Subject: [patch 2/5] ext2: improve scalability of bitmap searching Subject: [PATCH 2/5] ext2: improve scalability of bitmap searching
Subject: [PATCHv2 001/207] x86: fix eflags tracking Subject: [PATCH v2 01/27] x86: fix eflags tracking
The "from" line must be the very first line in the message body, The "from" line must be the very first line in the message body,
and has the form: and has the form:
......
...@@ -14,8 +14,43 @@ Statistics for individual zram devices are exported through sysfs nodes at ...@@ -14,8 +14,43 @@ Statistics for individual zram devices are exported through sysfs nodes at
* Usage * Usage
There are several ways to configure and manage zram device(-s):
a) using zram and zram_control sysfs attributes
b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org).
In this document we will describe only 'manual' zram configuration steps,
IOW, zram and zram_control sysfs attributes.
In order to get a better idea about zramctl please consult util-linux
documentation, zramctl man-page or `zramctl --help'. Please be informed
that zram maintainers do not develop/maintain util-linux or zramctl, should
you have any questions please contact util-linux@vger.kernel.org
Following shows a typical sequence of steps for using zram. Following shows a typical sequence of steps for using zram.
WARNING
=======
For the sake of simplicity we skip error checking parts in most of the
examples below. However, it is your sole responsibility to handle errors.
zram sysfs attributes always return negative values in case of errors.
The list of possible return codes:
-EBUSY -- an attempt to modify an attribute that cannot be changed once
the device has been initialised. Please reset device first;
-ENOMEM -- zram was not able to allocate enough memory to fulfil your
needs;
-EINVAL -- invalid input has been provided.
If you use 'echo', the returned value that is changed by 'echo' utility,
and, in general case, something like:
echo 3 > /sys/block/zram0/max_comp_streams
if [ $? -ne 0 ];
handle_error
fi
should suffice.
1) Load Module: 1) Load Module:
modprobe zram num_devices=4 modprobe zram num_devices=4
This creates 4 devices: /dev/zram{0,1,2,3} This creates 4 devices: /dev/zram{0,1,2,3}
...@@ -47,7 +82,7 @@ max_comp_streams adjustment. ...@@ -47,7 +82,7 @@ max_comp_streams adjustment.
3) Select compression algorithm 3) Select compression algorithm
Using comp_algorithm device attribute one can see available and Using comp_algorithm device attribute one can see available and
currently selected (shown in square brackets) compression algortithms, currently selected (shown in square brackets) compression algorithms,
change selected compression algorithm (once the device is initialised change selected compression algorithm (once the device is initialised
there is no way to change compression algorithm). there is no way to change compression algorithm).
...@@ -119,7 +154,7 @@ execute ...@@ -119,7 +154,7 @@ execute
8) Stats: 8) Stats:
Per-device statistics are exported as various nodes under /sys/block/zram<id>/ Per-device statistics are exported as various nodes under /sys/block/zram<id>/
A brief description of exported device attritbutes. For more details please A brief description of exported device attributes. For more details please
read Documentation/ABI/testing/sysfs-block-zram. read Documentation/ABI/testing/sysfs-block-zram.
Name access description Name access description
...@@ -140,8 +175,9 @@ zero_pages RO the number of zero filled pages written to this disk ...@@ -140,8 +175,9 @@ zero_pages RO the number of zero filled pages written to this disk
orig_data_size RO uncompressed size of data stored in this disk orig_data_size RO uncompressed size of data stored in this disk
compr_data_size RO compressed size of data stored in this disk compr_data_size RO compressed size of data stored in this disk
mem_used_total RO the amount of memory allocated for this disk mem_used_total RO the amount of memory allocated for this disk
mem_used_max RW the maximum amount memory zram have consumed to mem_used_max RW the maximum amount of memory zram have consumed to
store compressed data store the data (to reset this counter to the actual
current value, write 1 to this attribute)
mem_limit RW the maximum amount of memory ZRAM can use to store mem_limit RW the maximum amount of memory ZRAM can use to store
the compressed data the compressed data
pages_compacted RO the number of pages freed during compaction pages_compacted RO the number of pages freed during compaction
......
...@@ -59,7 +59,7 @@ cgroups. Here is what you can do. ...@@ -59,7 +59,7 @@ cgroups. Here is what you can do.
- At macro level, first dd should finish first. To get more precise data, keep - At macro level, first dd should finish first. To get more precise data, keep
on looking at (with the help of script), at blkio.disk_time and on looking at (with the help of script), at blkio.disk_time and
blkio.disk_sectors files of both test1 and test2 groups. This will tell how blkio.disk_sectors files of both test1 and test2 groups. This will tell how
much disk time (in milli seconds), each group got and how many secotors each much disk time (in milliseconds), each group got and how many sectors each
group dispatched to the disk. We provide fairness in terms of disk time, so group dispatched to the disk. We provide fairness in terms of disk time, so
ideally io.disk_time of cgroups should be in proportion to the weight. ideally io.disk_time of cgroups should be in proportion to the weight.
......
...@@ -50,7 +50,7 @@ being frozen. This allows the bash example above and gdb to run as ...@@ -50,7 +50,7 @@ being frozen. This allows the bash example above and gdb to run as
expected. expected.
The cgroup freezer is hierarchical. Freezing a cgroup freezes all The cgroup freezer is hierarchical. Freezing a cgroup freezes all
tasks beloning to the cgroup and all its descendant cgroups. Each tasks belonging to the cgroup and all its descendant cgroups. Each
cgroup has its own state (self-state) and the state inherited from the cgroup has its own state (self-state) and the state inherited from the
parent (parent-state). Iff both states are THAWED, the cgroup is parent (parent-state). Iff both states are THAWED, the cgroup is
THAWED. THAWED.
......
...@@ -494,7 +494,7 @@ may be specified in any order and not all pairs have to be specified. ...@@ -494,7 +494,7 @@ may be specified in any order and not all pairs have to be specified.
${R|W}BPS are read/write bytes per second and ${R|W}IOPS are ${R|W}BPS are read/write bytes per second and ${R|W}IOPS are
read/write IOs per second. "max" indicates no limit. Writing read/write IOs per second. "max" indicates no limit. Writing
to the file follows the same format but the individual to the file follows the same format but the individual
settings may be ommitted or specified in any order. settings may be omitted or specified in any order.
This file is available only on non-root cgroups. This file is available only on non-root cgroups.
......
...@@ -220,7 +220,7 @@ to coerce it into behaving. ...@@ -220,7 +220,7 @@ to coerce it into behaving.
Compose dialog. Compose dialog.
Please note that "external editor" requires that your editor must not Please note that "external editor" requires that your editor must not
fork, or in other words, the editor must not return before closing. fork, or in other words, the editor must not return before closing.
You may have to pass additional flags or change the settings of your You may have to pass additional flags or change the settings of your
editor. Most notably if you are using gvim then you must pass the -f editor. Most notably if you are using gvim then you must pass the -f
option to gvim by putting "/usr/bin/gvim -f" (if the binary is in option to gvim by putting "/usr/bin/gvim -f" (if the binary is in
......
<head>
<style> p { max-width:50em} ol, ul {max-width: 40em}</style>
</head>
Pathname lookup in Linux.
=========================
This write-up is based on three articles published at lwn.net:
- <https://lwn.net/Articles/649115/> Pathname lookup in Linux
- <https://lwn.net/Articles/649729/> RCU-walk: faster pathname lookup in Linux
- <https://lwn.net/Articles/650786/> A walk among the symlinks
Written by Neil Brown with help from Al Viro and Jon Corbet.
Introduction
------------
The most obvious aspect of pathname lookup, which very little
exploration is needed to discover, is that it is complex. There are
many rules, special cases, and implementation alternatives that all
combine to confuse the unwary reader. Computer science has long been
acquainted with such complexity and has tools to help manage it. One
tool that we will make extensive use of is "divide and conquer". For
the early parts of the analysis we will divide off symlinks - leaving
them until the final part. Well before we get to symlinks we have
another major division based on the VFS's approach to locking which
will allow us to review "REF-walk" and "RCU-walk" separately. But we
are getting ahead of ourselves. There are some important low level
distinctions we need to clarify first.
There are two sorts of ...
--------------------------
[`openat()`]: http://man7.org/linux/man-pages/man2/openat.2.html
Pathnames (sometimes "file names"), used to identify objects in the
filesystem, will be familiar to most readers. They contain two sorts
of elements: "slashes" that are sequences of one or more "`/`"
characters, and "components" that are sequences of one or more
non-"`/`" characters. These form two kinds of paths. Those that
start with slashes are "absolute" and start from the filesystem root.
The others are "relative" and start from the current directory, or
from some other location specified by a file descriptor given to a
"xxx`at`" system call such as "[`openat()`]".
[`execveat()`]: http://man7.org/linux/man-pages/man2/execveat.2.html
It is tempting to describe the second kind as starting with a
component, but that isn't always accurate: a pathname can lack both
slashes and components, it can be empty, in other words. This is
generally forbidden in POSIX, but some of those "xxx`at`" system calls
in Linux permit it when the `AT_EMPTY_PATH` flag is given. For
example, if you have an open file descriptor on an executable file you
can execute it by calling [`execveat()`] passing the file descriptor,
an empty path, and the `AT_EMPTY_PATH` flag.
These paths can be divided into two sections: the final component and
everything else. The "everything else" is the easy bit. In all cases
it must identify a directory that already exists, otherwise an error
such as `ENOENT` or `ENOTDIR` will be reported.
The final component is not so simple. Not only do different system
calls interpret it quite differently (e.g. some create it, some do
not), but it might not even exist: neither the empty pathname nor the
pathname that is just slashes have a final component. If it does
exist, it could be "`.`" or "`..`" which are handled quite differently
from other components.
[POSIX]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
If a pathname ends with a slash, such as "`/tmp/foo/`" it might be
tempting to consider that to have an empty final component. In many
ways that would lead to correct results, but not always. In
particular, `mkdir()` and `rmdir()` each create or remove a directory named
by the final component, and they are required to work with pathnames
ending in "`/`". According to [POSIX]
> A pathname that contains at least one non- &lt;slash> character and
> that ends with one or more trailing &lt;slash> characters shall not
> be resolved successfully unless the last pathname component before
> the trailing <slash> characters names an existing directory or a
> directory entry that is to be created for a directory immediately
> after the pathname is resolved.
The Linux pathname walking code (mostly in `fs/namei.c`) deals with
all of these issues: breaking the path into components, handling the
"everything else" quite separately from the final component, and
checking that the trailing slash is not used where it isn't
permitted. It also addresses the important issue of concurrent
access.
While one process is looking up a pathname, another might be making
changes that affect that lookup. One fairly extreme case is that if
"a/b" were renamed to "a/c/b" while another process were looking up
"a/b/..", that process might successfully resolve on "a/c".
Most races are much more subtle, and a big part of the task of
pathname lookup is to prevent them from having damaging effects. Many
of the possible races are seen most clearly in the context of the
"dcache" and an understanding of that is central to understanding
pathname lookup.
More than just a cache.
-----------------------
The "dcache" caches information about names in each filesystem to
make them quickly available for lookup. Each entry (known as a
"dentry") contains three significant fields: a component name, a
pointer to a parent dentry, and a pointer to the "inode" which
contains further information about the object in that parent with
the given name. The inode pointer can be `NULL` indicating that the
name doesn't exist in the parent. While there can be linkage in the
dentry of a directory to the dentries of the children, that linkage is
not used for pathname lookup, and so will not be considered here.
The dcache has a number of uses apart from accelerating lookup. One
that will be particularly relevant is that it is closely integrated
with the mount table that records which filesystem is mounted where.
What the mount table actually stores is which dentry is mounted on top
of which other dentry.
When considering the dcache, we have another of our "two types"
distinctions: there are two types of filesystems.
Some filesystems ensure that the information in the dcache is always
completely accurate (though not necessarily complete). This can allow
the VFS to determine if a particular file does or doesn't exist
without checking with the filesystem, and means that the VFS can
protect the filesystem against certain races and other problems.
These are typically "local" filesystems such as ext3, XFS, and Btrfs.
Other filesystems don't provide that guarantee because they cannot.
These are typically filesystems that are shared across a network,
whether remote filesystems like NFS and 9P, or cluster filesystems
like ocfs2 or cephfs. These filesystems allow the VFS to revalidate
cached information, and must provide their own protection against
awkward races. The VFS can detect these filesystems by the
`DCACHE_OP_REVALIDATE` flag being set in the dentry.
REF-walk: simple concurrency management with refcounts and spinlocks
--------------------------------------------------------------------
With all of those divisions carefully classified, we can now start
looking at the actual process of walking along a path. In particular
we will start with the handling of the "everything else" part of a
pathname, and focus on the "REF-walk" approach to concurrency
management. This code is found in the `link_path_walk()` function, if
you ignore all the places that only run when "`LOOKUP_RCU`"
(indicating the use of RCU-walk) is set.
[Meet the Lockers]: https://lwn.net/Articles/453685/
REF-walk is fairly heavy-handed with locks and reference counts. Not
as heavy-handed as in the old "big kernel lock" days, but certainly not
afraid of taking a lock when one is needed. It uses a variety of
different concurrency controls. A background understanding of the
various primitives is assumed, or can be gleaned from elsewhere such
as in [Meet the Lockers].
The locking mechanisms used by REF-walk include:
### dentry->d_lockref ###
This uses the lockref primitive to provide both a spinlock and a
reference count. The special-sauce of this primitive is that the
conceptual sequence "lock; inc_ref; unlock;" can often be performed
with a single atomic memory operation.
Holding a reference on a dentry ensures that the dentry won't suddenly
be freed and used for something else, so the values in various fields
will behave as expected. It also protects the `->d_inode` reference
to the inode to some extent.
The association between a dentry and its inode is fairly permanent.
For example, when a file is renamed, the dentry and inode move
together to the new location. When a file is created the dentry will
initially be negative (i.e. `d_inode` is `NULL`), and will be assigned
to the new inode as part of the act of creation.
When a file is deleted, this can be reflected in the cache either by
setting `d_inode` to `NULL`, or by removing it from the hash table
(described shortly) used to look up the name in the parent directory.
If the dentry is still in use the second option is used as it is
perfectly legal to keep using an open file after it has been deleted
and having the dentry around helps. If the dentry is not otherwise in
use (i.e. if the refcount in `d_lockref` is one), only then will
`d_inode` be set to `NULL`. Doing it this way is more efficient for a
very common case.
So as long as a counted reference is held to a dentry, a non-`NULL` `->d_inode`
value will never be changed.
### dentry->d_lock ###
`d_lock` is a synonym for the spinlock that is part of `d_lockref` above.
For our purposes, holding this lock protects against the dentry being
renamed or unlinked. In particular, its parent (`d_parent`), and its
name (`d_name`) cannot be changed, and it cannot be removed from the
dentry hash table.
When looking for a name in a directory, REF-walk takes `d_lock` on
each candidate dentry that it finds in the hash table and then checks
that the parent and name are correct. So it doesn't lock the parent
while searching in the cache; it only locks children.
When looking for the parent for a given name (to handle "`..`"),
REF-walk can take `d_lock` to get a stable reference to `d_parent`,
but it first tries a more lightweight approach. As seen in
`dget_parent()`, if a reference can be claimed on the parent, and if
subsequently `d_parent` can be seen to have not changed, then there is
no need to actually take the lock on the child.
### rename_lock ###
Looking up a given name in a given directory involves computing a hash
from the two values (the name and the dentry of the directory),
accessing that slot in a hash table, and searching the linked list
that is found there.
When a dentry is renamed, the name and the parent dentry can both
change so the hash will almost certainly change too. This would move the
dentry to a different chain in the hash table. If a filename search
happened to be looking at a dentry that was moved in this way,
it might end up continuing the search down the wrong chain,
and so miss out on part of the correct chain.
The name-lookup process (`d_lookup()`) does _not_ try to prevent this
from happening, but only to detect when it happens.
`rename_lock` is a seqlock that is updated whenever any dentry is
renamed. If `d_lookup` finds that a rename happened while it
unsuccessfully scanned a chain in the hash table, it simply tries
again.
### inode->i_mutex ###
`i_mutex` is a mutex that serializes all changes to a particular
directory. This ensures that, for example, an `unlink()` and a `rename()`
cannot both happen at the same time. It also keeps the directory
stable while the filesystem is asked to look up a name that is not
currently in the dcache.
This has a complementary role to that of `d_lock`: `i_mutex` on a
directory protects all of the names in that directory, while `d_lock`
on a name protects just one name in a directory. Most changes to the
dcache hold `i_mutex` on the relevant directory inode and briefly take
`d_lock` on one or more the dentries while the change happens. One
exception is when idle dentries are removed from the dcache due to
memory pressure. This uses `d_lock`, but `i_mutex` plays no role.
The mutex affects pathname lookup in two distinct ways. Firstly it
serializes lookup of a name in a directory. `walk_component()` uses
`lookup_fast()` first which, in turn, checks to see if the name is in the cache,
using only `d_lock` locking. If the name isn't found, then `walk_component()`
falls back to `lookup_slow()` which takes `i_mutex`, checks again that
the name isn't in the cache, and then calls in to the filesystem to get a
definitive answer. A new dentry will be added to the cache regardless of
the result.
Secondly, when pathname lookup reaches the final component, it will
sometimes need to take `i_mutex` before performing the last lookup so
that the required exclusion can be achieved. How path lookup chooses
to take, or not take, `i_mutex` is one of the
issues addressed in a subsequent section.
### mnt->mnt_count ###
`mnt_count` is a per-CPU reference counter on "`mount`" structures.
Per-CPU here means that incrementing the count is cheap as it only
uses CPU-local memory, but checking if the count is zero is expensive as
it needs to check with every CPU. Taking a `mnt_count` reference
prevents the mount structure from disappearing as the result of regular
unmount operations, but does not prevent a "lazy" unmount. So holding
`mnt_count` doesn't ensure that the mount remains in the namespace and,
in particular, doesn't stabilize the link to the mounted-on dentry. It
does, however, ensure that the `mount` data structure remains coherent,
and it provides a reference to the root dentry of the mounted
filesystem. So a reference through `->mnt_count` provides a stable
reference to the mounted dentry, but not the mounted-on dentry.
### mount_lock ###
`mount_lock` is a global seqlock, a bit like `rename_lock`. It can be used to
check if any change has been made to any mount points.
While walking down the tree (away from the root) this lock is used when
crossing a mount point to check that the crossing was safe. That is,
the value in the seqlock is read, then the code finds the mount that
is mounted on the current directory, if there is one, and increments
the `mnt_count`. Finally the value in `mount_lock` is checked against
the old value. If there is no change, then the crossing was safe. If there
was a change, the `mnt_count` is decremented and the whole process is
retried.
When walking up the tree (towards the root) by following a ".." link,
a little more care is needed. In this case the seqlock (which
contains both a counter and a spinlock) is fully locked to prevent
any changes to any mount points while stepping up. This locking is
needed to stabilize the link to the mounted-on dentry, which the
refcount on the mount itself doesn't ensure.
### RCU ###
Finally the global (but extremely lightweight) RCU read lock is held
from time to time to ensure certain data structures don't get freed
unexpectedly.
In particular it is held while scanning chains in the dcache hash
table, and the mount point hash table.
Bringing it together with `struct nameidata`
--------------------------------------------
[First edition Unix]: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s
Throughout the process of walking a path, the current status is stored
in a `struct nameidata`, "namei" being the traditional name - dating
all the way back to [First Edition Unix] - of the function that
converts a "name" to an "inode". `struct nameidata` contains (among
other fields):
### `struct path path` ###
A `path` contains a `struct vfsmount` (which is
embedded in a `struct mount`) and a `struct dentry`. Together these
record the current status of the walk. They start out referring to the
starting point (the current working directory, the root directory, or some other
directory identified by a file descriptor), and are updated on each
step. A reference through `d_lockref` and `mnt_count` is always
held.
### `struct qstr last` ###
This is a string together with a length (i.e. _not_ `nul` terminated)
that is the "next" component in the pathname.
### `int last_type` ###
This is one of `LAST_NORM`, `LAST_ROOT`, `LAST_DOT`, `LAST_DOTDOT`, or
`LAST_BIND`. The `last` field is only valid if the type is
`LAST_NORM`. `LAST_BIND` is used when following a symlink and no
components of the symlink have been processed yet. Others should be
fairly self-explanatory.
### `struct path root` ###
This is used to hold a reference to the effective root of the
filesystem. Often that reference won't be needed, so this field is
only assigned the first time it is used, or when a non-standard root
is requested. Keeping a reference in the `nameidata` ensures that
only one root is in effect for the entire path walk, even if it races
with a `chroot()` system call.
The root is needed when either of two conditions holds: (1) either the
pathname or a symbolic link starts with a "'/'", or (2) a "`..`"
component is being handled, since "`..`" from the root must always stay
at the root. The value used is usually the current root directory of
the calling process. An alternate root can be provided as when
`sysctl()` calls `file_open_root()`, and when NFSv4 or Btrfs call
`mount_subtree()`. In each case a pathname is being looked up in a very
specific part of the filesystem, and the lookup must not be allowed to
escape that subtree. It works a bit like a local `chroot()`.
Ignoring the handling of symbolic links, we can now describe the
"`link_path_walk()`" function, which handles the lookup of everything
except the final component as:
> Given a path (`name`) and a nameidata structure (`nd`), check that the
> current directory has execute permission and then advance `name`
> over one component while updating `last_type` and `last`. If that
> was the final component, then return, otherwise call
> `walk_component()` and repeat from the top.
`walk_component()` is even easier. If the component is `LAST_DOTS`,
it calls `handle_dots()` which does the necessary locking as already
described. If it finds a `LAST_NORM` component it first calls
"`lookup_fast()`" which only looks in the dcache, but will ask the
filesystem to revalidate the result if it is that sort of filesystem.
If that doesn't get a good result, it calls "`lookup_slow()`" which
takes the `i_mutex`, rechecks the cache, and then asks the filesystem
to find a definitive answer. Each of these will call
`follow_managed()` (as described below) to handle any mount points.
In the absence of symbolic links, `walk_component()` creates a new
`struct path` containing a counted reference to the new dentry and a
reference to the new `vfsmount` which is only counted if it is
different from the previous `vfsmount`. It then calls
`path_to_nameidata()` to install the new `struct path` in the
`struct nameidata` and drop the unneeded references.
This "hand-over-hand" sequencing of getting a reference to the new
dentry before dropping the reference to the previous dentry may
seem obvious, but is worth pointing out so that we will recognize its
analogue in the "RCU-walk" version.
Handling the final component.
-----------------------------
`link_path_walk()` only walks as far as setting `nd->last` and
`nd->last_type` to refer to the final component of the path. It does
not call `walk_component()` that last time. Handling that final
component remains for the caller to sort out. Those callers are
`path_lookupat()`, `path_parentat()`, `path_mountpoint()` and
`path_openat()` each of which handles the differing requirements of
different system calls.
`path_parentat()` is clearly the simplest - it just wraps a little bit
of housekeeping around `link_path_walk()` and returns the parent
directory and final component to the caller. The caller will be either
aiming to create a name (via `filename_create()`) or remove or rename
a name (in which case `user_path_parent()` is used). They will use
`i_mutex` to exclude other changes while they validate and then
perform their operation.
`path_lookupat()` is nearly as simple - it is used when an existing
object is wanted such as by `stat()` or `chmod()`. It essentially just
calls `walk_component()` on the final component through a call to
`lookup_last()`. `path_lookupat()` returns just the final dentry.
`path_mountpoint()` handles the special case of unmounting which must
not try to revalidate the mounted filesystem. It effectively
contains, through a call to `mountpoint_last()`, an alternate
implementation of `lookup_slow()` which skips that step. This is
important when unmounting a filesystem that is inaccessible, such as
one provided by a dead NFS server.
Finally `path_openat()` is used for the `open()` system call; it
contains, in support functions starting with "`do_last()`", all the
complexity needed to handle the different subtleties of O_CREAT (with
or without O_EXCL), final "`/`" characters, and trailing symbolic
links. We will revisit this in the final part of this series, which
focuses on those symbolic links. "`do_last()`" will sometimes, but
not always, take `i_mutex`, depending on what it finds.
Each of these, or the functions which call them, need to be alert to
the possibility that the final component is not `LAST_NORM`. If the
goal of the lookup is to create something, then any value for
`last_type` other than `LAST_NORM` will result in an error. For
example if `path_parentat()` reports `LAST_DOTDOT`, then the caller
won't try to create that name. They also check for trailing slashes
by testing `last.name[last.len]`. If there is any character beyond
the final component, it must be a trailing slash.
Revalidation and automounts
---------------------------
Apart from symbolic links, there are only two parts of the "REF-walk"
process not yet covered. One is the handling of stale cache entries
and the other is automounts.
On filesystems that require it, the lookup routines will call the
`->d_revalidate()` dentry method to ensure that the cached information
is current. This will often confirm validity or update a few details
from a server. In some cases it may find that there has been change
further up the path and that something that was thought to be valid
previously isn't really. When this happens the lookup of the whole
path is aborted and retried with the "`LOOKUP_REVAL`" flag set. This
forces revalidation to be more thorough. We will see more details of
this retry process in the next article.
Automount points are locations in the filesystem where an attempt to
lookup a name can trigger changes to how that lookup should be
handled, in particular by mounting a filesystem there. These are
covered in greater detail in autofs4.txt in the Linux documentation
tree, but a few notes specifically related to path lookup are in order
here.
The Linux VFS has a concept of "managed" dentries which is reflected
in function names such as "`follow_managed()`". There are three
potentially interesting things about these dentries corresponding
to three different flags that might be set in `dentry->d_flags`:
### `DCACHE_MANAGE_TRANSIT` ###
If this flag has been set, then the filesystem has requested that the
`d_manage()` dentry operation be called before handling any possible
mount point. This can perform two particular services:
It can block to avoid races. If an automount point is being
unmounted, the `d_manage()` function will usually wait for that
process to complete before letting the new lookup proceed and possibly
trigger a new automount.
It can selectively allow only some processes to transit through a
mount point. When a server process is managing automounts, it may
need to access a directory without triggering normal automount
processing. That server process can identify itself to the `autofs`
filesystem, which will then give it a special pass through
`d_manage()` by returning `-EISDIR`.
### `DCACHE_MOUNTED` ###
This flag is set on every dentry that is mounted on. As Linux
supports multiple filesystem namespaces, it is possible that the
dentry may not be mounted on in *this* namespace, just in some
other. So this flag is seen as a hint, not a promise.
If this flag is set, and `d_manage()` didn't return `-EISDIR`,
`lookup_mnt()` is called to examine the mount hash table (honoring the
`mount_lock` described earlier) and possibly return a new `vfsmount`
and a new `dentry` (both with counted references).
### `DCACHE_NEED_AUTOMOUNT` ###
If `d_manage()` allowed us to get this far, and `lookup_mnt()` didn't
find a mount point, then this flag causes the `d_automount()` dentry
operation to be called.
The `d_automount()` operation can be arbitrarily complex and may
communicate with server processes etc. but it should ultimately either
report that there was an error, that there was nothing to mount, or
should provide an updated `struct path` with new `dentry` and `vfsmount`.
In the latter case, `finish_automount()` will be called to safely
install the new mount point into the mount table.
There is no new locking of import here and it is important that no
locks (only counted references) are held over this processing due to
the very real possibility of extended delays.
This will become more important next time when we examine RCU-walk
which is particularly sensitive to delays.
RCU-walk - faster pathname lookup in Linux
==========================================
RCU-walk is another algorithm for performing pathname lookup in Linux.
It is in many ways similar to REF-walk and the two share quite a bit
of code. The significant difference in RCU-walk is how it allows for
the possibility of concurrent access.
We noted that REF-walk is complex because there are numerous details
and special cases. RCU-walk reduces this complexity by simply
refusing to handle a number of cases -- it instead falls back to
REF-walk. The difficulty with RCU-walk comes from a different
direction: unfamiliarity. The locking rules when depending on RCU are
quite different from traditional locking, so we will spend a little extra
time when we come to those.
Clear demarcation of roles
--------------------------
The easiest way to manage concurrency is to forcibly stop any other
thread from changing the data structures that a given thread is
looking at. In cases where no other thread would even think of
changing the data and lots of different threads want to read at the
same time, this can be very costly. Even when using locks that permit
multiple concurrent readers, the simple act of updating the count of
the number of current readers can impose an unwanted cost. So the
goal when reading a shared data structure that no other process is
changing is to avoid writing anything to memory at all. Take no
locks, increment no counts, leave no footprints.
The REF-walk mechanism already described certainly doesn't follow this
principle, but then it is really designed to work when there may well
be other threads modifying the data. RCU-walk, in contrast, is
designed for the common situation where there are lots of frequent
readers and only occasional writers. This may not be common in all
parts of the filesystem tree, but in many parts it will be. For the
other parts it is important that RCU-walk can quickly fall back to
using REF-walk.
Pathname lookup always starts in RCU-walk mode but only remains there
as long as what it is looking for is in the cache and is stable. It
dances lightly down the cached filesystem image, leaving no footprints
and carefully watching where it is, to be sure it doesn't trip. If it
notices that something has changed or is changing, or if something
isn't in the cache, then it tries to stop gracefully and switch to
REF-walk.
This stopping requires getting a counted reference on the current
`vfsmount` and `dentry`, and ensuring that these are still valid -
that a path walk with REF-walk would have found the same entries.
This is an invariant that RCU-walk must guarantee. It can only make
decisions, such as selecting the next step, that are decisions which
REF-walk could also have made if it were walking down the tree at the
same time. If the graceful stop succeeds, the rest of the path is
processed with the reliable, if slightly sluggish, REF-walk. If
RCU-walk finds it cannot stop gracefully, it simply gives up and
restarts from the top with REF-walk.
This pattern of "try RCU-walk, if that fails try REF-walk" can be
clearly seen in functions like `filename_lookup()`,
`filename_parentat()`, `filename_mountpoint()`,
`do_filp_open()`, and `do_file_open_root()`. These five
correspond roughly to the four `path_`* functions we met earlier,
each of which calls `link_path_walk()`. The `path_*` functions are
called using different mode flags until a mode is found which works.
They are first called with `LOOKUP_RCU` set to request "RCU-walk". If
that fails with the error `ECHILD` they are called again with no
special flag to request "REF-walk". If either of those report the
error `ESTALE` a final attempt is made with `LOOKUP_REVAL` set (and no
`LOOKUP_RCU`) to ensure that entries found in the cache are forcibly
revalidated - normally entries are only revalidated if the filesystem
determines that they are too old to trust.
The `LOOKUP_RCU` attempt may drop that flag internally and switch to
REF-walk, but will never then try to switch back to RCU-walk. Places
that trip up RCU-walk are much more likely to be near the leaves and
so it is very unlikely that there will be much, if any, benefit from
switching back.
RCU and seqlocks: fast and light
--------------------------------
RCU is, unsurprisingly, critical to RCU-walk mode. The
`rcu_read_lock()` is held for the entire time that RCU-walk is walking
down a path. The particular guarantee it provides is that the key
data structures - dentries, inodes, super_blocks, and mounts - will
not be freed while the lock is held. They might be unlinked or
invalidated in one way or another, but the memory will not be
repurposed so values in various fields will still be meaningful. This
is the only guarantee that RCU provides; everything else is done using
seqlocks.
As we saw above, REF-walk holds a counted reference to the current
dentry and the current vfsmount, and does not release those references
before taking references to the "next" dentry or vfsmount. It also
sometimes takes the `d_lock` spinlock. These references and locks are
taken to prevent certain changes from happening. RCU-walk must not
take those references or locks and so cannot prevent such changes.
Instead, it checks to see if a change has been made, and aborts or
retries if it has.
To preserve the invariant mentioned above (that RCU-walk may only make
decisions that REF-walk could have made), it must make the checks at
or near the same places that REF-walk holds the references. So, when
REF-walk increments a reference count or takes a spinlock, RCU-walk
samples the status of a seqlock using `read_seqcount_begin()` or a
similar function. When REF-walk decrements the count or drops the
lock, RCU-walk checks if the sampled status is still valid using
`read_seqcount_retry()` or similar.
However, there is a little bit more to seqlocks than that. If
RCU-walk accesses two different fields in a seqlock-protected
structure, or accesses the same field twice, there is no a priori
guarantee of any consistency between those accesses. When consistency
is needed - which it usually is - RCU-walk must take a copy and then
use `read_seqcount_retry()` to validate that copy.
`read_seqcount_retry()` not only checks the sequence number, but also
imposes a memory barrier so that no memory-read instruction from
*before* the call can be delayed until *after* the call, either by the
CPU or by the compiler. A simple example of this can be seen in
`slow_dentry_cmp()` which, for filesystems which do not use simple
byte-wise name equality, calls into the filesystem to compare a name
against a dentry. The length and name pointer are copied into local
variables, then `read_seqcount_retry()` is called to confirm the two
are consistent, and only then is `->d_compare()` called. When
standard filename comparison is used, `dentry_cmp()` is called
instead. Notably it does _not_ use `read_seqcount_retry()`, but
instead has a large comment explaining why the consistency guarantee
isn't necessary. A subsequent `read_seqcount_retry()` will be
sufficient to catch any problem that could occur at this point.
With that little refresher on seqlocks out of the way we can look at
the bigger picture of how RCU-walk uses seqlocks.
### `mount_lock` and `nd->m_seq` ###
We already met the `mount_lock` seqlock when REF-walk used it to
ensure that crossing a mount point is performed safely. RCU-walk uses
it for that too, but for quite a bit more.
Instead of taking a counted reference to each `vfsmount` as it
descends the tree, RCU-walk samples the state of `mount_lock` at the
start of the walk and stores this initial sequence number in the
`struct nameidata` in the `m_seq` field. This one lock and one
sequence number are used to validate all accesses to all `vfsmounts`,
and all mount point crossings. As changes to the mount table are
relatively rare, it is reasonable to fall back on REF-walk any time
that any "mount" or "unmount" happens.
`m_seq` is checked (using `read_seqretry()`) at the end of an RCU-walk
sequence, whether switching to REF-walk for the rest of the path or
when the end of the path is reached. It is also checked when stepping
down over a mount point (in `__follow_mount_rcu()`) or up (in
`follow_dotdot_rcu()`). If it is ever found to have changed, the
whole RCU-walk sequence is aborted and the path is processed again by
REF-walk.
If RCU-walk finds that `mount_lock` hasn't changed then it can be sure
that, had REF-walk taken counted references on each vfsmount, the
results would have been the same. This ensures the invariant holds,
at least for vfsmount structures.
### `dentry->d_seq` and `nd->seq`. ###
In place of taking a count or lock on `d_reflock`, RCU-walk samples
the per-dentry `d_seq` seqlock, and stores the sequence number in the
`seq` field of the nameidata structure, so `nd->seq` should always be
the current sequence number of `nd->dentry`. This number needs to be
revalidated after copying, and before using, the name, parent, or
inode of the dentry.
The handling of the name we have already looked at, and the parent is
only accessed in `follow_dotdot_rcu()` which fairly trivially follows
the required pattern, though it does so for three different cases.
When not at a mount point, `d_parent` is followed and its `d_seq` is
collected. When we are at a mount point, we instead follow the
`mnt->mnt_mountpoint` link to get a new dentry and collect its
`d_seq`. Then, after finally finding a `d_parent` to follow, we must
check if we have landed on a mount point and, if so, must find that
mount point and follow the `mnt->mnt_root` link. This would imply a
somewhat unusual, but certainly possible, circumstance where the
starting point of the path lookup was in part of the filesystem that
was mounted on, and so not visible from the root.
The inode pointer, stored in `->d_inode`, is a little more
interesting. The inode will always need to be accessed at least
twice, once to determine if it is NULL and once to verify access
permissions. Symlink handling requires a validated inode pointer too.
Rather than revalidating on each access, a copy is made on the first
access and it is stored in the `inode` field of `nameidata` from where
it can be safely accessed without further validation.
`lookup_fast()` is the only lookup routine that is used in RCU-mode,
`lookup_slow()` being too slow and requiring locks. It is in
`lookup_fast()` that we find the important "hand over hand" tracking
of the current dentry.
The current `dentry` and current `seq` number are passed to
`__d_lookup_rcu()` which, on success, returns a new `dentry` and a
new `seq` number. `lookup_fast()` then copies the inode pointer and
revalidates the new `seq` number. It then validates the old `dentry`
with the old `seq` number one last time and only then continues. This
process of getting the `seq` number of the new dentry and then
checking the `seq` number of the old exactly mirrors the process of
getting a counted reference to the new dentry before dropping that for
the old dentry which we saw in REF-walk.
### No `inode->i_mutex` or even `rename_lock` ###
A mutex is a fairly heavyweight lock that can only be taken when it is
permissible to sleep. As `rcu_read_lock()` forbids sleeping,
`inode->i_mutex` plays no role in RCU-walk. If some other thread does
take `i_mutex` and modifies the directory in a way that RCU-walk needs
to notice, the result will be either that RCU-walk fails to find the
dentry that it is looking for, or it will find a dentry which
`read_seqretry()` won't validate. In either case it will drop down to
REF-walk mode which can take whatever locks are needed.
Though `rename_lock` could be used by RCU-walk as it doesn't require
any sleeping, RCU-walk doesn't bother. REF-walk uses `rename_lock` to
protect against the possibility of hash chains in the dcache changing
while they are being searched. This can result in failing to find
something that actually is there. When RCU-walk fails to find
something in the dentry cache, whether it is really there or not, it
already drops down to REF-walk and tries again with appropriate
locking. This neatly handles all cases, so adding extra checks on
rename_lock would bring no significant value.
`unlazy walk()` and `complete_walk()`
-------------------------------------
That "dropping down to REF-walk" typically involves a call to
`unlazy_walk()`, so named because "RCU-walk" is also sometimes
referred to as "lazy walk". `unlazy_walk()` is called when
following the path down to the current vfsmount/dentry pair seems to
have proceeded successfully, but the next step is problematic. This
can happen if the next name cannot be found in the dcache, if
permission checking or name revalidation couldn't be achieved while
the `rcu_read_lock()` is held (which forbids sleeping), if an
automount point is found, or in a couple of cases involving symlinks.
It is also called from `complete_walk()` when the lookup has reached
the final component, or the very end of the path, depending on which
particular flavor of lookup is used.
Other reasons for dropping out of RCU-walk that do not trigger a call
to `unlazy_walk()` are when some inconsistency is found that cannot be
handled immediately, such as `mount_lock` or one of the `d_seq`
seqlocks reporting a change. In these cases the relevant function
will return `-ECHILD` which will percolate up until it triggers a new
attempt from the top using REF-walk.
For those cases where `unlazy_walk()` is an option, it essentially
takes a reference on each of the pointers that it holds (vfsmount,
dentry, and possibly some symbolic links) and then verifies that the
relevant seqlocks have not been changed. If there have been changes,
it, too, aborts with `-ECHILD`, otherwise the transition to REF-walk
has been a success and the lookup process continues.
Taking a reference on those pointers is not quite as simple as just
incrementing a counter. That works to take a second reference if you
already have one (often indirectly through another object), but it
isn't sufficient if you don't actually have a counted reference at
all. For `dentry->d_lockref`, it is safe to increment the reference
counter to get a reference unless it has been explicitly marked as
"dead" which involves setting the counter to `-128`.
`lockref_get_not_dead()` achieves this.
For `mnt->mnt_count` it is safe to take a reference as long as
`mount_lock` is then used to validate the reference. If that
validation fails, it may *not* be safe to just drop that reference in
the standard way of calling `mnt_put()` - an unmount may have
progressed too far. So the code in `legitimize_mnt()`, when it
finds that the reference it got might not be safe, checks the
`MNT_SYNC_UMOUNT` flag to determine if a simple `mnt_put()` is
correct, or if it should just decrement the count and pretend none of
this ever happened.
Taking care in filesystems
---------------------------
RCU-walk depends almost entirely on cached information and often will
not call into the filesystem at all. However there are two places,
besides the already-mentioned component-name comparison, where the
file system might be included in RCU-walk, and it must know to be
careful.
If the filesystem has non-standard permission-checking requirements -
such as a networked filesystem which may need to check with the server
- the `i_op->permission` interface might be called during RCU-walk.
In this case an extra "`MAY_NOT_BLOCK`" flag is passed so that it
knows not to sleep, but to return `-ECHILD` if it cannot complete
promptly. `i_op->permission` is given the inode pointer, not the
dentry, so it doesn't need to worry about further consistency checks.
However if it accesses any other filesystem data structures, it must
ensure they are safe to be accessed with only the `rcu_read_lock()`
held. This typically means they must be freed using `kfree_rcu()` or
similar.
[`READ_ONCE()`]: https://lwn.net/Articles/624126/
If the filesystem may need to revalidate dcache entries, then
`d_op->d_revalidate` may be called in RCU-walk too. This interface
*is* passed the dentry but does not have access to the `inode` or the
`seq` number from the `nameidata`, so it needs to be extra careful
when accessing fields in the dentry. This "extra care" typically
involves using `ACCESS_ONCE()` or the newer [`READ_ONCE()`] to access
fields, and verifying the result is not NULL before using it. This
pattern can be see in `nfs_lookup_revalidate()`.
A pair of patterns
------------------
In various places in the details of REF-walk and RCU-walk, and also in
the big picture, there are a couple of related patterns that are worth
being aware of.
The first is "try quickly and check, if that fails try slowly". We
can see that in the high-level approach of first trying RCU-walk and
then trying REF-walk, and in places where `unlazy_walk()` is used to
switch to REF-walk for the rest of the path. We also saw it earlier
in `dget_parent()` when following a "`..`" link. It tries a quick way
to get a reference, then falls back to taking locks if needed.
The second pattern is "try quickly and check, if that fails try
again - repeatedly". This is seen with the use of `rename_lock` and
`mount_lock` in REF-walk. RCU-walk doesn't make use of this pattern -
if anything goes wrong it is much safer to just abort and try a more
sedate approach.
The emphasis here is "try quickly and check". It should probably be
"try quickly _and carefully,_ then check". The fact that checking is
needed is a reminder that the system is dynamic and only a limited
number of things are safe at all. The most likely cause of errors in
this whole process is assuming something is safe when in reality it
isn't. Careful consideration of what exactly guarantees the safety of
each access is sometimes necessary.
A walk among the symlinks
=========================
There are several basic issues that we will examine to understand the
handling of symbolic links: the symlink stack, together with cache
lifetimes, will help us understand the overall recursive handling of
symlinks and lead to the special care needed for the final component.
Then a consideration of access-time updates and summary of the various
flags controlling lookup will finish the story.
The symlink stack
-----------------
There are only two sorts of filesystem objects that can usefully
appear in a path prior to the final component: directories and symlinks.
Handling directories is quite straightforward: the new directory
simply becomes the starting point at which to interpret the next
component on the path. Handling symbolic links requires a bit more
work.
Conceptually, symbolic links could be handled by editing the path. If
a component name refers to a symbolic link, then that component is
replaced by the body of the link and, if that body starts with a '/',
then all preceding parts of the path are discarded. This is what the
"`readlink -f`" command does, though it also edits out "`.`" and
"`..`" components.
Directly editing the path string is not really necessary when looking
up a path, and discarding early components is pointless as they aren't
looked at anyway. Keeping track of all remaining components is
important, but they can of course be kept separately; there is no need
to concatenate them. As one symlink may easily refer to another,
which in turn can refer to a third, we may need to keep the remaining
components of several paths, each to be processed when the preceding
ones are completed. These path remnants are kept on a stack of
limited size.
There are two reasons for placing limits on how many symlinks can
occur in a single path lookup. The most obvious is to avoid loops.
If a symlink referred to itself either directly or through
intermediaries, then following the symlink can never complete
successfully - the error `ELOOP` must be returned. Loops can be
detected without imposing limits, but limits are the simplest solution
and, given the second reason for restriction, quite sufficient.
[outlined recently]: http://thread.gmane.org/gmane.linux.kernel/1934390/focus=1934550
The second reason was [outlined recently] by Linus:
> Because it's a latency and DoS issue too. We need to react well to
> true loops, but also to "very deep" non-loops. It's not about memory
> use, it's about users triggering unreasonable CPU resources.
Linux imposes a limit on the length of any pathname: `PATH_MAX`, which
is 4096. There are a number of reasons for this limit; not letting the
kernel spend too much time on just one path is one of them. With
symbolic links you can effectively generate much longer paths so some
sort of limit is needed for the same reason. Linux imposes a limit of
at most 40 symlinks in any one path lookup. It previously imposed a
further limit of eight on the maximum depth of recursion, but that was
raised to 40 when a separate stack was implemented, so there is now
just the one limit.
The `nameidata` structure that we met in an earlier article contains a
small stack that can be used to store the remaining part of up to two
symlinks. In many cases this will be sufficient. If it isn't, a
separate stack is allocated with room for 40 symlinks. Pathname
lookup will never exceed that stack as, once the 40th symlink is
detected, an error is returned.
It might seem that the name remnants are all that needs to be stored on
this stack, but we need a bit more. To see that, we need to move on to
cache lifetimes.
Storage and lifetime of cached symlinks
---------------------------------------
Like other filesystem resources, such as inodes and directory
entries, symlinks are cached by Linux to avoid repeated costly access
to external storage. It is particularly important for RCU-walk to be
able to find and temporarily hold onto these cached entries, so that
it doesn't need to drop down into REF-walk.
[object-oriented design pattern]: https://lwn.net/Articles/446317/
While each filesystem is free to make its own choice, symlinks are
typically stored in one of two places. Short symlinks are often
stored directly in the inode. When a filesystem allocates a `struct
inode` it typically allocates extra space to store private data (a
common [object-oriented design pattern] in the kernel). This will
sometimes include space for a symlink. The other common location is
in the page cache, which normally stores the content of files. The
pathname in a symlink can be seen as the content of that symlink and
can easily be stored in the page cache just like file content.
When neither of these is suitable, the next most likely scenario is
that the filesystem will allocate some temporary memory and copy or
construct the symlink content into that memory whenever it is needed.
When the symlink is stored in the inode, it has the same lifetime as
the inode which, itself, is protected by RCU or by a counted reference
on the dentry. This means that the mechanisms that pathname lookup
uses to access the dcache and icache (inode cache) safely are quite
sufficient for accessing some cached symlinks safely. In these cases,
the `i_link` pointer in the inode is set to point to wherever the
symlink is stored and it can be accessed directly whenever needed.
When the symlink is stored in the page cache or elsewhere, the
situation is not so straightforward. A reference on a dentry or even
on an inode does not imply any reference on cached pages of that
inode, and even an `rcu_read_lock()` is not sufficient to ensure that
a page will not disappear. So for these symlinks the pathname lookup
code needs to ask the filesystem to provide a stable reference and,
significantly, needs to release that reference when it is finished
with it.
Taking a reference to a cache page is often possible even in RCU-walk
mode. It does require making changes to memory, which is best avoided,
but that isn't necessarily a big cost and it is better than dropping
out of RCU-walk mode completely. Even filesystems that allocate
space to copy the symlink into can use `GFP_ATOMIC` to often successfully
allocate memory without the need to drop out of RCU-walk. If a
filesystem cannot successfully get a reference in RCU-walk mode, it
must return `-ECHILD` and `unlazy_walk()` will be called to return to
REF-walk mode in which the filesystem is allowed to sleep.
The place for all this to happen is the `i_op->follow_link()` inode
method. In the present mainline code this is never actually called in
RCU-walk mode as the rewrite is not quite complete. It is likely that
in a future release this method will be passed an `inode` pointer when
called in RCU-walk mode so it both (1) knows to be careful, and (2) has the
validated pointer. Much like the `i_op->permission()` method we
looked at previously, `->follow_link()` would need to be careful that
all the data structures it references are safe to be accessed while
holding no counted reference, only the RCU lock. Though getting a
reference with `->follow_link()` is not yet done in RCU-walk mode, the
code is ready to release the reference when that does happen.
This need to drop the reference to a symlink adds significant
complexity. It requires a reference to the inode so that the
`i_op->put_link()` inode operation can be called. In REF-walk, that
reference is kept implicitly through a reference to the dentry, so
keeping the `struct path` of the symlink is easiest. For RCU-walk,
the pointer to the inode is kept separately. To allow switching from
RCU-walk back to REF-walk in the middle of processing nested symlinks
we also need the seq number for the dentry so we can confirm that
switching back was safe.
Finally, when providing a reference to a symlink, the filesystem also
provides an opaque "cookie" that must be passed to `->put_link()` so that it
knows what to free. This might be the allocated memory area, or a
pointer to the `struct page` in the page cache, or something else
completely. Only the filesystem knows what it is.
In order for the reference to each symlink to be dropped when the walk completes,
whether in RCU-walk or REF-walk, the symlink stack needs to contain,
along with the path remnants:
- the `struct path` to provide a reference to the inode in REF-walk
- the `struct inode *` to provide a reference to the inode in RCU-walk
- the `seq` to allow the path to be safely switched from RCU-walk to REF-walk
- the `cookie` that tells `->put_path()` what to put.
This means that each entry in the symlink stack needs to hold five
pointers and an integer instead of just one pointer (the path
remnant). On a 64-bit system, this is about 40 bytes per entry;
with 40 entries it adds up to 1600 bytes total, which is less than
half a page. So it might seem like a lot, but is by no means
excessive.
Note that, in a given stack frame, the path remnant (`name`) is not
part of the symlink that the other fields refer to. It is the remnant
to be followed once that symlink has been fully parsed.
Following the symlink
---------------------
The main loop in `link_path_walk()` iterates seamlessly over all
components in the path and all of the non-final symlinks. As symlinks
are processed, the `name` pointer is adjusted to point to a new
symlink, or is restored from the stack, so that much of the loop
doesn't need to notice. Getting this `name` variable on and off the
stack is very straightforward; pushing and popping the references is
a little more complex.
When a symlink is found, `walk_component()` returns the value `1`
(`0` is returned for any other sort of success, and a negative number
is, as usual, an error indicator). This causes `get_link()` to be
called; it then gets the link from the filesystem. Providing that
operation is successful, the old path `name` is placed on the stack,
and the new value is used as the `name` for a while. When the end of
the path is found (i.e. `*name` is `'\0'`) the old `name` is restored
off the stack and path walking continues.
Pushing and popping the reference pointers (inode, cookie, etc.) is more
complex in part because of the desire to handle tail recursion. When
the last component of a symlink itself points to a symlink, we
want to pop the symlink-just-completed off the stack before pushing
the symlink-just-found to avoid leaving empty path remnants that would
just get in the way.
It is most convenient to push the new symlink references onto the
stack in `walk_component()` immediately when the symlink is found;
`walk_component()` is also the last piece of code that needs to look at the
old symlink as it walks that last component. So it is quite
convenient for `walk_component()` to release the old symlink and pop
the references just before pushing the reference information for the
new symlink. It is guided in this by two flags; `WALK_GET`, which
gives it permission to follow a symlink if it finds one, and
`WALK_PUT`, which tells it to release the current symlink after it has been
followed. `WALK_PUT` is tested first, leading to a call to
`put_link()`. `WALK_GET` is tested subsequently (by
`should_follow_link()`) leading to a call to `pick_link()` which sets
up the stack frame.
### Symlinks with no final component ###
A pair of special-case symlinks deserve a little further explanation.
Both result in a new `struct path` (with mount and dentry) being set
up in the `nameidata`, and result in `get_link()` returning `NULL`.
The more obvious case is a symlink to "`/`". All symlinks starting
with "`/`" are detected in `get_link()` which resets the `nameidata`
to point to the effective filesystem root. If the symlink only
contains "`/`" then there is nothing more to do, no components at all,
so `NULL` is returned to indicate that the symlink can be released and
the stack frame discarded.
The other case involves things in `/proc` that look like symlinks but
aren't really.
> $ ls -l /proc/self/fd/1
> lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4
Every open file descriptor in any process is represented in `/proc` by
something that looks like a symlink. It is really a reference to the
target file, not just the name of it. When you `readlink` these
objects you get a name that might refer to the same file - unless it
has been unlinked or mounted over. When `walk_component()` follows
one of these, the `->follow_link()` method in "procfs" doesn't return
a string name, but instead calls `nd_jump_link()` which updates the
`nameidata` in place to point to that target. `->follow_link()` then
returns `NULL`. Again there is no final component and `get_link()`
reports this by leaving the `last_type` field of `nameidata` as
`LAST_BIND`.
Following the symlink in the final component
--------------------------------------------
All this leads to `link_path_walk()` walking down every component, and
following all symbolic links it finds, until it reaches the final
component. This is just returned in the `last` field of `nameidata`.
For some callers, this is all they need; they want to create that
`last` name if it doesn't exist or give an error if it does. Other
callers will want to follow a symlink if one is found, and possibly
apply special handling to the last component of that symlink, rather
than just the last component of the original file name. These callers
potentially need to call `link_path_walk()` again and again on
successive symlinks until one is found that doesn't point to another
symlink.
This case is handled by the relevant caller of `link_path_walk()`, such as
`path_lookupat()` using a loop that calls `link_path_walk()`, and then
handles the final component. If the final component is a symlink
that needs to be followed, then `trailing_symlink()` is called to set
things up properly and the loop repeats, calling `link_path_walk()`
again. This could loop as many as 40 times if the last component of
each symlink is another symlink.
The various functions that examine the final component and possibly
report that it is a symlink are `lookup_last()`, `mountpoint_last()`
and `do_last()`, each of which use the same convention as
`walk_component()` of returning `1` if a symlink was found that needs
to be followed.
Of these, `do_last()` is the most interesting as it is used for
opening a file. Part of `do_last()` runs with `i_mutex` held and this
part is in a separate function: `lookup_open()`.
Explaining `do_last()` completely is beyond the scope of this article,
but a few highlights should help those interested in exploring the
code.
1. Rather than just finding the target file, `do_last()` needs to open
it. If the file was found in the dcache, then `vfs_open()` is used for
this. If not, then `lookup_open()` will either call `atomic_open()` (if
the filesystem provides it) to combine the final lookup with the open, or
will perform the separate `lookup_real()` and `vfs_create()` steps
directly. In the later case the actual "open" of this newly found or
created file will be performed by `vfs_open()`, just as if the name
were found in the dcache.
2. `vfs_open()` can fail with `-EOPENSTALE` if the cached information
wasn't quite current enough. Rather than restarting the lookup from
the top with `LOOKUP_REVAL` set, `lookup_open()` is called instead,
giving the filesystem a chance to resolve small inconsistencies.
If that doesn't work, only then is the lookup restarted from the top.
3. An open with O_CREAT **does** follow a symlink in the final component,
unlike other creation system calls (like `mkdir`). So the sequence:
> ln -s bar /tmp/foo
> echo hello > /tmp/foo
will create a file called `/tmp/bar`. This is not permitted if
`O_EXCL` is set but otherwise is handled for an O_CREAT open much
like for a non-creating open: `should_follow_link()` returns `1`, and
so does `do_last()` so that `trailing_symlink()` gets called and the
open process continues on the symlink that was found.
Updating the access time
------------------------
We previously said of RCU-walk that it would "take no locks, increment
no counts, leave no footprints." We have since seen that some
"footprints" can be needed when handling symlinks as a counted
reference (or even a memory allocation) may be needed. But these
footprints are best kept to a minimum.
One other place where walking down a symlink can involve leaving
footprints in a way that doesn't affect directories is in updating access times.
In Unix (and Linux) every filesystem object has a "last accessed
time", or "`atime`". Passing through a directory to access a file
within is not considered to be an access for the purposes of
`atime`; only listing the contents of a directory can update its `atime`.
Symlinks are different it seems. Both reading a symlink (with `readlink()`)
and looking up a symlink on the way to some other destination can
update the atime on that symlink.
[clearest statement]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08
It is not clear why this is the case; POSIX has little to say on the
subject. The [clearest statement] is that, if a particular implementation
updates a timestamp in a place not specified by POSIX, this must be
documented "except that any changes caused by pathname resolution need
not be documented". This seems to imply that POSIX doesn't really
care about access-time updates during pathname lookup.
[Linux 1.3.87]: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8
An examination of history shows that prior to [Linux 1.3.87], the ext2
filesystem, at least, didn't update atime when following a link.
Unfortunately we have no record of why that behavior was changed.
In any case, access time must now be updated and that operation can be
quite complex. Trying to stay in RCU-walk while doing it is best
avoided. Fortunately it is often permitted to skip the `atime`
update. Because `atime` updates cause performance problems in various
areas, Linux supports the `relatime` mount option, which generally
limits the updates of `atime` to once per day on files that aren't
being changed (and symlinks never change once created). Even without
`relatime`, many filesystems record `atime` with a one-second
granularity, so only one update per second is required.
It is easy to test if an `atime` update is needed while in RCU-walk
mode and, if it isn't, the update can be skipped and RCU-walk mode
continues. Only when an `atime` update is actually required does the
path walk drop down to REF-walk. All of this is handled in the
`get_link()` function.
A few flags
-----------
A suitable way to wrap up this tour of pathname walking is to list
the various flags that can be stored in the `nameidata` to guide the
lookup process. Many of these are only meaningful on the final
component, others reflect the current state of the pathname lookup.
And then there is `LOOKUP_EMPTY`, which doesn't fit conceptually with
the others. If this is not set, an empty pathname causes an error
very early on. If it is set, empty pathnames are not considered to be
an error.
### Global state flags ###
We have already met two global state flags: `LOOKUP_RCU` and
`LOOKUP_REVAL`. These select between one of three overall approaches
to lookup: RCU-walk, REF-walk, and REF-walk with forced revalidation.
`LOOKUP_PARENT` indicates that the final component hasn't been reached
yet. This is primarily used to tell the audit subsystem the full
context of a particular access being audited.
`LOOKUP_ROOT` indicates that the `root` field in the `nameidata` was
provided by the caller, so it shouldn't be released when it is no
longer needed.
`LOOKUP_JUMPED` means that the current dentry was chosen not because
it had the right name but for some other reason. This happens when
following "`..`", following a symlink to `/`, crossing a mount point
or accessing a "`/proc/$PID/fd/$FD`" symlink. In this case the
filesystem has not been asked to revalidate the name (with
`d_revalidate()`). In such cases the inode may still need to be
revalidated, so `d_op->d_weak_revalidate()` is called if
`LOOKUP_JUMPED` is set when the look completes - which may be at the
final component or, when creating, unlinking, or renaming, at the penultimate component.
### Final-component flags ###
Some of these flags are only set when the final component is being
considered. Others are only checked for when considering that final
component.
`LOOKUP_AUTOMOUNT` ensures that, if the final component is an automount
point, then the mount is triggered. Some operations would trigger it
anyway, but operations like `stat()` deliberately don't. `statfs()`
needs to trigger the mount but otherwise behaves a lot like `stat()`, so
it sets `LOOKUP_AUTOMOUNT`, as does "`quotactl()`" and the handling of
"`mount --bind`".
`LOOKUP_FOLLOW` has a similar function to `LOOKUP_AUTOMOUNT` but for
symlinks. Some system calls set or clear it implicitly, while
others have API flags such as `AT_SYMLINK_FOLLOW` and
`UMOUNT_NOFOLLOW` to control it. Its effect is similar to
`WALK_GET` that we already met, but it is used in a different way.
`LOOKUP_DIRECTORY` insists that the final component is a directory.
Various callers set this and it is also set when the final component
is found to be followed by a slash.
Finally `LOOKUP_OPEN`, `LOOKUP_CREATE`, `LOOKUP_EXCL`, and
`LOOKUP_RENAME_TARGET` are not used directly by the VFS but are made
available to the filesystem and particularly the `->d_revalidate()`
method. A filesystem can choose not to bother revalidating too hard
if it knows that it will be asked to open or create the file soon.
These flags were previously useful for `->lookup()` too but with the
introduction of `->atomic_open()` they are less relevant there.
End of the road
---------------
Despite its complexity, all this pathname lookup code appears to be
in good shape - various parts are certainly easier to understand now
than even a couple of releases ago. But that doesn't mean it is
"finished". As already mentioned, RCU-walk currently only follows
symlinks that are stored in the inode so, while it handles many ext4
symlinks, it doesn't help with NFS, XFS, or Btrfs. That support
is not likely to be long delayed.
...@@ -379,4 +379,4 @@ Papers and other documentation on dcache locking ...@@ -379,4 +379,4 @@ Papers and other documentation on dcache locking
2. http://lse.sourceforge.net/locking/dcache/dcache.html 2. http://lse.sourceforge.net/locking/dcache/dcache.html
3. path-lookup.md in this directory.
...@@ -17,13 +17,13 @@ the sysfs directory entries we ensure that we don't have conflicts ...@@ -17,13 +17,13 @@ the sysfs directory entries we ensure that we don't have conflicts
in the directories and applications only see a limited set of in the directories and applications only see a limited set of
the network devices. the network devices.
Each sysfs directory entry may be tagged with zero or one Each sysfs directory entry may be tagged with a namespace via the
namespaces. A sysfs_dirent is augmented with a void *s_ns. If a void *ns member of its kernfs_node. If a directory entry is tagged,
directory entry is tagged, then sysfs_dirent->s_flags will have a then kernfs_node->flags will have a flag between KOBJ_NS_TYPE_NONE
flag between KOBJ_NS_TYPE_NONE and KOBJ_NS_TYPES, and s_ns will and KOBJ_NS_TYPES, and ns will point to the namespace to which it
point to the namespace to which it belongs. belongs.
Each sysfs superblock's sysfs_super_info contains an array void Each sysfs superblock's kernfs_super_info contains an array void
*ns[KOBJ_NS_TYPES]. When a task in a tagging namespace *ns[KOBJ_NS_TYPES]. When a task in a tagging namespace
kobj_nstype first mounts sysfs, a new superblock is created. It kobj_nstype first mounts sysfs, a new superblock is created. It
will be differentiated from other sysfs mounts by having its will be differentiated from other sysfs mounts by having its
...@@ -31,7 +31,7 @@ s_fs_info->ns[kobj_nstype] set to the new namespace. Note that ...@@ -31,7 +31,7 @@ s_fs_info->ns[kobj_nstype] set to the new namespace. Note that
through bind mounting and mounts propagation, a task can easily view through bind mounting and mounts propagation, a task can easily view
the contents of other namespaces' sysfs mounts. Therefore, when a the contents of other namespaces' sysfs mounts. Therefore, when a
namespace exits, it will call kobj_ns_exit() to invalidate any namespace exits, it will call kobj_ns_exit() to invalidate any
sysfs_dirent->s_ns pointers pointing to it. kernfs_node->ns pointers pointing to it.
Users of this interface: Users of this interface:
- define a type in the kobj_ns_type enumeration. - define a type in the kobj_ns_type enumeration.
......
...@@ -40,7 +40,7 @@ ancestors of object hierarchies; i.e. the subsystems the objects ...@@ -40,7 +40,7 @@ ancestors of object hierarchies; i.e. the subsystems the objects
belong to. belong to.
Sysfs internally stores a pointer to the kobject that implements a Sysfs internally stores a pointer to the kobject that implements a
directory in the sysfs_dirent object associated with the directory. In directory in the kernfs_node object associated with the directory. In
the past this kobject pointer has been used by sysfs to do reference the past this kobject pointer has been used by sysfs to do reference
counting directly on the kobject whenever the file is opened or closed. counting directly on the kobject whenever the file is opened or closed.
With the current sysfs implementation the kobject reference count is With the current sysfs implementation the kobject reference count is
...@@ -191,9 +191,10 @@ implementations: ...@@ -191,9 +191,10 @@ implementations:
be called again, rearmed, to fill the buffer. be called again, rearmed, to fill the buffer.
- On write(2), sysfs expects the entire buffer to be passed during the - On write(2), sysfs expects the entire buffer to be passed during the
first write. Sysfs then passes the entire buffer to the store() first write. Sysfs then passes the entire buffer to the store() method.
method. A terminating null is added after the data on stores. This makes
functions like sysfs_streq() safe to use.
When writing sysfs files, userspace processes should first read the When writing sysfs files, userspace processes should first read the
entire file, modify the values it wishes to change, then write the entire file, modify the values it wishes to change, then write the
entire buffer back. entire buffer back.
......
...@@ -21,8 +21,8 @@ exact way to do it depends on the GPIO controller providing the GPIOs, see the ...@@ -21,8 +21,8 @@ exact way to do it depends on the GPIO controller providing the GPIOs, see the
device tree bindings for your controller. device tree bindings for your controller.
GPIOs mappings are defined in the consumer device's node, in a property named GPIOs mappings are defined in the consumer device's node, in a property named
either <function>-gpios or <function>-gpio, where <function> is the function <function>-gpios, where <function> is the function the driver will request
the driver will request through gpiod_get(). For example: through gpiod_get(). For example:
foo_device { foo_device {
compatible = "acme,foo"; compatible = "acme,foo";
...@@ -31,9 +31,13 @@ the driver will request through gpiod_get(). For example: ...@@ -31,9 +31,13 @@ the driver will request through gpiod_get(). For example:
<&gpio 16 GPIO_ACTIVE_HIGH>, /* green */ <&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
<&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */ <&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */
power-gpio = <&gpio 1 GPIO_ACTIVE_LOW>; power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
}; };
Properties named <function>-gpio are also considered valid and old bindings use
it but are only supported for compatibility reasons and should not be used for
newer bindings since it has been deprecated.
This property will make GPIOs 15, 16 and 17 available to the driver under the This property will make GPIOs 15, 16 and 17 available to the driver under the
"led" function, and GPIO 1 as the "power" GPIO: "led" function, and GPIO 1 as the "power" GPIO:
......
...@@ -20,14 +20,14 @@ userspace GPIO can be used to determine system configuration data that ...@@ -20,14 +20,14 @@ userspace GPIO can be used to determine system configuration data that
standard kernels won't know about. And for some tasks, simple userspace standard kernels won't know about. And for some tasks, simple userspace
GPIO drivers could be all that the system really needs. GPIO drivers could be all that the system really needs.
DO NOT ABUSE SYFS TO CONTROL HARDWARE THAT HAS PROPER KERNEL DRIVERS. DO NOT ABUSE SYSFS TO CONTROL HARDWARE THAT HAS PROPER KERNEL DRIVERS.
PLEASE READ THE DOCUMENT NAMED "drivers-on-gpio.txt" IN THIS DOCUMENTATION PLEASE READ THE DOCUMENT NAMED "drivers-on-gpio.txt" IN THIS DOCUMENTATION
DIRECTORY TO AVOID REINVENTING KERNEL WHEELS IN USERSPACE. I MEAN IT. DIRECTORY TO AVOID REINVENTING KERNEL WHEELS IN USERSPACE. I MEAN IT.
REALLY. REALLY.
Paths in Sysfs Paths in Sysfs
-------------- --------------
There are three kinds of entry in /sys/class/gpio: There are three kinds of entries in /sys/class/gpio:
- Control interfaces used to get userspace control over GPIOs; - Control interfaces used to get userspace control over GPIOs;
...@@ -106,7 +106,7 @@ read-only attributes: ...@@ -106,7 +106,7 @@ read-only attributes:
"label" ... provided for diagnostics (not always unique) "label" ... provided for diagnostics (not always unique)
"ngpio" ... how many GPIOs this manges (N to N + ngpio - 1) "ngpio" ... how many GPIOs this manages (N to N + ngpio - 1)
Board documentation should in most cases cover what GPIOs are used for Board documentation should in most cases cover what GPIOs are used for
what purposes. However, those numbers are not always stable; GPIOs on what purposes. However, those numbers are not always stable; GPIOs on
......
...@@ -696,18 +696,18 @@ ...@@ -696,18 +696,18 @@
Memory related patches, HOWTOs, links, mm developers... Don't miss Memory related patches, HOWTOs, links, mm developers... Don't miss
it if you are interested in memory management development! it if you are interested in memory management development!
* Name: "Kernel Newbies IRC Channel" * Name: "Kernel Newbies IRC Channel and Website"
URL: http://www.kernelnewbies.org URL: http://www.kernelnewbies.org
Keywords: IRC, newbies, channel, asking doubts. Keywords: IRC, newbies, channel, asking doubts.
Description: #kernelnewbies on irc.openprojects.net. From the web Description: #kernelnewbies on irc.oftc.net.
page: "#kernelnewbies is an IRC network dedicated to the 'newbie' #kernelnewbies is an IRC network dedicated to the 'newbie'
kernel hacker. The audience mostly consists of people who are kernel hacker. The audience mostly consists of people who are
learning about the kernel, working on kernel projects or learning about the kernel, working on kernel projects or
professional kernel hackers that want to help less seasoned kernel professional kernel hackers that want to help less seasoned kernel
people. [...] #kernelnewbies is on the Open Projects IRC Network, people.
try irc.openprojects.net or irc.<country>.openprojects.net as your #kernelnewbies is on the OFTC IRC Network.
server and then /join #kernelnewbies". It also hosts articles, Try irc.oftc.net as your server and then /join #kernelnewbies.
documents, FAQs... The kernelnewbies website also hosts articles, documents, FAQs...
* Name: "linux-kernel mailing list archives and search engines" * Name: "linux-kernel mailing list archives and search engines"
URL: http://vger.kernel.org/vger-lists.html URL: http://vger.kernel.org/vger-lists.html
......
...@@ -790,8 +790,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted. ...@@ -790,8 +790,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
is passed, kernel could allocate physical memory region is passed, kernel could allocate physical memory region
above 4G, that cause second kernel crash on system above 4G, that cause second kernel crash on system
that require some amount of low memory, e.g. swiotlb that require some amount of low memory, e.g. swiotlb
requires at least 64M+32K low memory. Kernel would requires at least 64M+32K low memory, also enough extra
try to allocate 72M below 4G automatically. low memory is needed to make sure DMA buffers for 32-bit
devices won't run out. Kernel would try to allocate at
at least 256M below 4G automatically.
This one let user to specify own low range under 4G This one let user to specify own low range under 4G
for second kernel instead. for second kernel instead.
0: to disable low allocation. 0: to disable low allocation.
...@@ -972,6 +974,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted. ...@@ -972,6 +974,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
earlycon= [KNL] Output early console device and options. earlycon= [KNL] Output early console device and options.
When used with no options, the early console is
determined by the stdout-path property in device
tree's chosen node.
cdns,<addr> cdns,<addr>
Start an early, polled-mode console on a cadence serial Start an early, polled-mode console on a cadence serial
port at the specified address. The cadence serial port port at the specified address. The cadence serial port
...@@ -2340,11 +2346,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted. ...@@ -2340,11 +2346,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
nmi_watchdog= [KNL,BUGS=X86] Debugging features for SMP kernels nmi_watchdog= [KNL,BUGS=X86] Debugging features for SMP kernels
Format: [panic,][nopanic,][num] Format: [panic,][nopanic,][num]
Valid num: 0 or 1 Valid num: 0 or 1
0 - turn nmi_watchdog off 0 - turn hardlockup detector in nmi_watchdog off
1 - turn nmi_watchdog on 1 - turn hardlockup detector in nmi_watchdog on
When panic is specified, panic when an NMI watchdog When panic is specified, panic when an NMI watchdog
timeout occurs (or 'nopanic' to override the opposite timeout occurs (or 'nopanic' to override the opposite
default). default). To disable both hard and soft lockup detectors,
please see 'nowatchdog'.
This is useful when you use a panic=... timeout and This is useful when you use a panic=... timeout and
need the box quickly up again. need the box quickly up again.
......
...@@ -54,6 +54,22 @@ To run the hotplug tests: ...@@ -54,6 +54,22 @@ To run the hotplug tests:
- note that some tests will require root privileges. - note that some tests will require root privileges.
Install selftests
=================
You can use kselftest_install.sh tool installs selftests in default
location which is tools/testing/selftests/kselftest or an user specified
location.
To install selftests in default location:
$ cd tools/testing/selftests
$ ./kselftest_install.sh
To install selftests in an user specified location:
$ cd tools/testing/selftests
$ ./kselftest_install.sh install_dir
Contributing new tests Contributing new tests
====================== ======================
......
...@@ -30,7 +30,7 @@ lead to false interrupt, but that doesn't harm. ...@@ -30,7 +30,7 @@ lead to false interrupt, but that doesn't harm.
ALS contains 4 different gain steps. Driver automatically ALS contains 4 different gain steps. Driver automatically
selects suitable gain step. After each measurement, reliability of the results selects suitable gain step. After each measurement, reliability of the results
is estimated and new measurement is trigged if necessary. is estimated and new measurement is triggered if necessary.
Platform data can provide tuned values to the conversion formulas if Platform data can provide tuned values to the conversion formulas if
values are known. Otherwise plain sensor default values are used. values are known. Otherwise plain sensor default values are used.
......
...@@ -29,7 +29,7 @@ Detection ...@@ -29,7 +29,7 @@ Detection
The ISL29003 does not have an ID register which could be used to identify The ISL29003 does not have an ID register which could be used to identify
it, so the detection routine will just try to read from the configured I2C it, so the detection routine will just try to read from the configured I2C
addess and consider the device to be present as soon as it ACKs the address and consider the device to be present as soon as it ACKs the
transfer. transfer.
......
...@@ -22,7 +22,7 @@ At reset, the MAX6875 reads the configuration EEPROM into its configuration ...@@ -22,7 +22,7 @@ At reset, the MAX6875 reads the configuration EEPROM into its configuration
registers. The chip then begins to operate according to the values in the registers. The chip then begins to operate according to the values in the
registers. registers.
The Maxim MAX6874 is a similar, mostly compatible device, with more intputs The Maxim MAX6874 is a similar, mostly compatible device, with more inputs
and outputs: and outputs:
vin gpi vout vin gpi vout
MAX6874 6 4 8 MAX6874 6 4 8
......
...@@ -1018,25 +1018,34 @@ solution for a couple of reasons: ...@@ -1018,25 +1018,34 @@ solution for a couple of reasons:
$ ip link set can0 type can help $ ip link set can0 type can help
Usage: ip link set DEVICE type can Usage: ip link set DEVICE type can
[ bitrate BITRATE [ sample-point SAMPLE-POINT] ] | [ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
[ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1 [ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
phase-seg2 PHASE-SEG2 [ sjw SJW ] ] phase-seg2 PHASE-SEG2 [ sjw SJW ] ]
[ loopback { on | off } ] [ dbitrate BITRATE [ dsample-point SAMPLE-POINT] ] |
[ listen-only { on | off } ] [ dtq TQ dprop-seg PROP_SEG dphase-seg1 PHASE-SEG1
[ triple-sampling { on | off } ] dphase-seg2 PHASE-SEG2 [ dsjw SJW ] ]
[ restart-ms TIME-MS ] [ loopback { on | off } ]
[ restart ] [ listen-only { on | off } ]
[ triple-sampling { on | off } ]
Where: BITRATE := { 1..1000000 } [ one-shot { on | off } ]
SAMPLE-POINT := { 0.000..0.999 } [ berr-reporting { on | off } ]
TQ := { NUMBER } [ fd { on | off } ]
PROP-SEG := { 1..8 } [ fd-non-iso { on | off } ]
PHASE-SEG1 := { 1..8 } [ presume-ack { on | off } ]
PHASE-SEG2 := { 1..8 }
SJW := { 1..4 } [ restart-ms TIME-MS ]
RESTART-MS := { 0 | NUMBER } [ restart ]
Where: BITRATE := { 1..1000000 }
SAMPLE-POINT := { 0.000..0.999 }
TQ := { NUMBER }
PROP-SEG := { 1..8 }
PHASE-SEG1 := { 1..8 }
PHASE-SEG2 := { 1..8 }
SJW := { 1..4 }
RESTART-MS := { 0 | NUMBER }
- Display CAN device details and statistics: - Display CAN device details and statistics:
...@@ -1178,7 +1187,55 @@ solution for a couple of reasons: ...@@ -1178,7 +1187,55 @@ solution for a couple of reasons:
The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall. The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.
N.B. CAN FD capable devices can also handle and send legacy CAN frames. N.B. CAN FD capable devices can also handle and send legacy CAN frames.
FIXME: Add details about the CAN FD controller configuration when available. When configuring CAN FD capable CAN controllers an additional 'data' bitrate
has to be set. This bitrate for the data phase of the CAN FD frame has to be
at least the bitrate which was configured for the arbitration phase. This
second bitrate is specified analogue to the first bitrate but the bitrate
setting keywords for the 'data' bitrate start with 'd' e.g. dbitrate,
dsample-point, dsjw or dtq and similar settings. When a data bitrate is set
within the configuration process the controller option "fd on" can be
specified to enable the CAN FD mode in the CAN controller. This controller
option also switches the device MTU to 72 (CANFD_MTU).
The first CAN FD specification presented as whitepaper at the International
CAN Conference 2012 needed to be improved for data integrity reasons.
Therefore two CAN FD implementations have to be distinguished today:
- ISO compliant: The ISO 11898-1:2015 CAN FD implementation (default)
- non-ISO compliant: The CAN FD implementation following the 2012 whitepaper
Finally there are three types of CAN FD controllers:
1. ISO compliant (fixed)
2. non-ISO compliant (fixed, like the M_CAN IP core v3.0.1 in m_can.c)
3. ISO/non-ISO CAN FD controllers (switchable, like the PEAK PCAN-USB FD)
The current ISO/non-ISO mode is announced by the CAN controller driver via
netlink and displayed by the 'ip' tool (controller option FD-NON-ISO).
The ISO/non-ISO-mode can be altered by setting 'fd-non-iso {on|off}' for
switchable CAN FD controllers only.
Example configuring 500 kbit/s arbitration bitrate and 4 Mbit/s data bitrate:
$ ip link set can0 up type can bitrate 500000 sample-point 0.75 \
dbitrate 4000000 dsample-point 0.8 fd on
$ ip -details link show can0
5: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 72 qdisc pfifo_fast state UNKNOWN \
mode DEFAULT group default qlen 10
link/can promiscuity 0
can <FD> state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0
bitrate 500000 sample-point 0.750
tq 50 prop-seg 14 phase-seg1 15 phase-seg2 10 sjw 1
pcan_usb_pro_fd: tseg1 1..64 tseg2 1..16 sjw 1..16 brp 1..1024 \
brp-inc 1
dbitrate 4000000 dsample-point 0.800
dtq 12 dprop-seg 7 dphase-seg1 8 dphase-seg2 4 dsjw 1
pcan_usb_pro_fd: dtseg1 1..16 dtseg2 1..8 dsjw 1..4 dbrp 1..1024 \
dbrp-inc 1
clock 80000000
Example when 'fd-non-iso on' is added on this switchable CAN FD adapter:
can <FD,FD-NON-ISO> state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0
6.7 Supported CAN hardware 6.7 Supported CAN hardware
......
...@@ -203,7 +203,7 @@ functions with the user provided augmentation callback when inserting ...@@ -203,7 +203,7 @@ functions with the user provided augmentation callback when inserting
and erasing nodes. and erasing nodes.
C files implementing augmented rbtree manipulation must include C files implementing augmented rbtree manipulation must include
<linux/rbtree_augmented.h> instead of <linus/rbtree.h>. Note that <linux/rbtree_augmented.h> instead of <linux/rbtree.h>. Note that
linux/rbtree_augmented.h exposes some rbtree implementations details linux/rbtree_augmented.h exposes some rbtree implementations details
you are not expected to rely on; please stick to the documented APIs you are not expected to rely on; please stick to the documented APIs
there and do not include <linux/rbtree_augmented.h> from header files there and do not include <linux/rbtree_augmented.h> from header files
......
...@@ -1774,7 +1774,7 @@ has been called, this interface is completely emulated within the kernel. ...@@ -1774,7 +1774,7 @@ has been called, this interface is completely emulated within the kernel.
To use this to emulate the LINT1 input with KVM_CREATE_IRQCHIP, use the To use this to emulate the LINT1 input with KVM_CREATE_IRQCHIP, use the
following algorithm: following algorithm:
- pause the vpcu - pause the vcpu
- read the local APIC's state (KVM_GET_LAPIC) - read the local APIC's state (KVM_GET_LAPIC)
- check whether changing LINT1 will queue an NMI (see the LVT entry for LINT1) - check whether changing LINT1 will queue an NMI (see the LVT entry for LINT1)
- if so, issue KVM_NMI - if so, issue KVM_NMI
...@@ -2798,7 +2798,7 @@ Returns: = 0 on success, ...@@ -2798,7 +2798,7 @@ Returns: = 0 on success,
< 0 on generic error (e.g. -EFAULT or -ENOMEM), < 0 on generic error (e.g. -EFAULT or -ENOMEM),
> 0 if an exception occurred while walking the page tables > 0 if an exception occurred while walking the page tables
Read or write data from/to the logical (virtual) memory of a VPCU. Read or write data from/to the logical (virtual) memory of a VCPU.
Parameters are specified via the following structure: Parameters are specified via the following structure:
......
...@@ -74,7 +74,7 @@ struct kvm_s390_vm_cpu_processor { ...@@ -74,7 +74,7 @@ struct kvm_s390_vm_cpu_processor {
KVM does not enforce or limit the cpu model data in any form. Take the information KVM does not enforce or limit the cpu model data in any form. Take the information
retrieved by means of KVM_S390_VM_CPU_MACHINE as hint for reasonable configuration retrieved by means of KVM_S390_VM_CPU_MACHINE as hint for reasonable configuration
setups. Instruction interceptions triggered by additionally set facilitiy bits that setups. Instruction interceptions triggered by additionally set facility bits that
are not handled by KVM need to by imlemented in the VM driver code. are not handled by KVM need to by imlemented in the VM driver code.
Parameters: address of buffer to store/set the processor related cpu Parameters: address of buffer to store/set the processor related cpu
......
...@@ -110,7 +110,7 @@ Flags are passed to the host in the low 12 bits of the Effective Address. ...@@ -110,7 +110,7 @@ Flags are passed to the host in the low 12 bits of the Effective Address.
The following flags are currently available for a guest to expose: The following flags are currently available for a guest to expose:
MAGIC_PAGE_FLAG_NOT_MAPPED_NX Guest handles NX bits correclty wrt magic page MAGIC_PAGE_FLAG_NOT_MAPPED_NX Guest handles NX bits correctly wrt magic page
MSR bits MSR bits
======== ========
......
...@@ -280,4 +280,63 @@ of other objects. ...@@ -280,4 +280,63 @@ of other objects.
slub_debug=FZ,dentry slub_debug=FZ,dentry
Extended slabinfo mode and plotting
-----------------------------------
The slabinfo tool has a special 'extended' ('-X') mode that includes:
- Slabcache Totals
- Slabs sorted by size (up to -N <num> slabs, default 1)
- Slabs sorted by loss (up to -N <num> slabs, default 1)
Additionally, in this mode slabinfo does not dynamically scale sizes (G/M/K)
and reports everything in bytes (this functionality is also available to
other slabinfo modes via '-B' option) which makes reporting more precise and
accurate. Moreover, in some sense the `-X' mode also simplifies the analysis
of slabs' behaviour, because its output can be plotted using the
slabinfo-gnuplot.sh script. So it pushes the analysis from looking through
the numbers (tons of numbers) to something easier -- visual analysis.
To generate plots:
a) collect slabinfo extended records, for example:
while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done
b) pass stats file(-s) to slabinfo-gnuplot.sh script:
slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN]
The slabinfo-gnuplot.sh script will pre-processes the collected records
and generates 3 png files (and 3 pre-processing cache files) per STATS
file:
- Slabcache Totals: FOO_STATS-totals.png
- Slabs sorted by size: FOO_STATS-slabs-by-size.png
- Slabs sorted by loss: FOO_STATS-slabs-by-loss.png
Another use case, when slabinfo-gnuplot can be useful, is when you need
to compare slabs' behaviour "prior to" and "after" some code modification.
To help you out there, slabinfo-gnuplot.sh script can 'merge' the
`Slabcache Totals` sections from different measurements. To visually
compare N plots:
a) Collect as many STATS1, STATS2, .. STATSN files as you need
while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done
b) Pre-process those STATS files
slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN
c) Execute slabinfo-gnuplot.sh in '-t' mode, passing all of the
generated pre-processed *-totals
slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals
This will produce a single plot (png file).
Plots, expectedly, can be large so some fluctuations or small spikes
can go unnoticed. To deal with that, `slabinfo-gnuplot.sh' has two
options to 'zoom-in'/'zoom-out':
a) -s %d,%d overwrites the default image width and heigh
b) -r %d,%d specifies a range of samples to use (for example,
in `slabinfo -X >> FOO_STATS; sleep 1;' case, using
a "-r 40,60" range will plot only samples collected
between 40th and 60th seconds).
Christoph Lameter, May 30, 2007 Christoph Lameter, May 30, 2007
Sergey Senozhatsky, October 23, 2015
...@@ -61,7 +61,7 @@ Documentation/kobject.txt 文档以获得更多关于 kobject 接口的 ...@@ -61,7 +61,7 @@ Documentation/kobject.txt 文档以获得更多关于 kobject 接口的
内核的对象层次到用户空间。sysfs 中的顶层目录代表着内核对象层次的 内核的对象层次到用户空间。sysfs 中的顶层目录代表着内核对象层次的
共同祖先;例如:某些对象属于某个子系统。 共同祖先;例如:某些对象属于某个子系统。
Sysfs 在与其目录关联的 sysfs_dirent 对象中内部保存一个指向实现 Sysfs 在与其目录关联的 kernfs_node 对象中内部保存一个指向实现
目录的 kobject 的指针。以前,这个 kobject 指针被 sysfs 直接用于 目录的 kobject 的指针。以前,这个 kobject 指针被 sysfs 直接用于
kobject 文件打开和关闭的引用计数。而现在的 sysfs 实现中,kobject kobject 文件打开和关闭的引用计数。而现在的 sysfs 实现中,kobject
引用计数只能通过 sysfs_schedule_callback() 函数直接修改。 引用计数只能通过 sysfs_schedule_callback() 函数直接修改。
......
...@@ -3528,13 +3528,15 @@ M: Jonathan Corbet <corbet@lwn.net> ...@@ -3528,13 +3528,15 @@ M: Jonathan Corbet <corbet@lwn.net>
L: linux-doc@vger.kernel.org L: linux-doc@vger.kernel.org
S: Maintained S: Maintained
F: Documentation/ F: Documentation/
F: scripts/docproc.c
F: scripts/kernel-doc*
X: Documentation/ABI/ X: Documentation/ABI/
X: Documentation/devicetree/ X: Documentation/devicetree/
X: Documentation/acpi X: Documentation/acpi
X: Documentation/power X: Documentation/power
X: Documentation/spi X: Documentation/spi
X: Documentation/DocBook/media X: Documentation/DocBook/media
T: git git://git.lwn.net/linux-2.6.git docs-next T: git git://git.lwn.net/linux.git docs-next
DOUBLETALK DRIVER DOUBLETALK DRIVER
M: "James R. Van Zandt" <jrv@vanzandt.mv.com> M: "James R. Van Zandt" <jrv@vanzandt.mv.com>
......
...@@ -1336,7 +1336,7 @@ $(help-board-dirs): help-%: ...@@ -1336,7 +1336,7 @@ $(help-board-dirs): help-%:
# Documentation targets # Documentation targets
# --------------------------------------------------------------------------- # ---------------------------------------------------------------------------
%docs: scripts_basic FORCE %docs: scripts_basic FORCE
$(Q)$(MAKE) $(build)=scripts build_docproc $(Q)$(MAKE) $(build)=scripts build_docproc build_check-lc_ctype
$(Q)$(MAKE) $(build)=Documentation/DocBook $@ $(Q)$(MAKE) $(build)=Documentation/DocBook $@
else # KBUILD_EXTMOD else # KBUILD_EXTMOD
......
...@@ -24,7 +24,7 @@ ON WHAT HARDWARE DOES IT RUN? ...@@ -24,7 +24,7 @@ ON WHAT HARDWARE DOES IT RUN?
today Linux also runs on (at least) the Compaq Alpha AXP, Sun SPARC and today Linux also runs on (at least) the Compaq Alpha AXP, Sun SPARC and
UltraSPARC, Motorola 68000, PowerPC, PowerPC64, ARM, Hitachi SuperH, Cell, UltraSPARC, Motorola 68000, PowerPC, PowerPC64, ARM, Hitachi SuperH, Cell,
IBM S/390, MIPS, HP PA-RISC, Intel IA-64, DEC VAX, AMD x86-64, AXIS CRIS, IBM S/390, MIPS, HP PA-RISC, Intel IA-64, DEC VAX, AMD x86-64, AXIS CRIS,
Xtensa, Tilera TILE, AVR32 and Renesas M32R architectures. Xtensa, Tilera TILE, AVR32, ARC and Renesas M32R architectures.
Linux is easily portable to most general-purpose 32- or 64-bit architectures Linux is easily portable to most general-purpose 32- or 64-bit architectures
as long as they have a paged memory management unit (PMMU) and a port of the as long as they have a paged memory management unit (PMMU) and a port of the
......
...@@ -8,6 +8,7 @@ unifdef ...@@ -8,6 +8,7 @@ unifdef
ihex2fw ihex2fw
recordmcount recordmcount
docproc docproc
check-lc_ctype
sortextable sortextable
asn1_compiler asn1_compiler
extract-cert extract-cert
......
...@@ -7,6 +7,7 @@ ...@@ -7,6 +7,7 @@
# conmakehash: Create chartable # conmakehash: Create chartable
# conmakehash: Create arrays for initializing the kernel console tables # conmakehash: Create arrays for initializing the kernel console tables
# docproc: Used in Documentation/DocBook # docproc: Used in Documentation/DocBook
# check-lc_ctype: Used in Documentation/DocBook
HOST_EXTRACFLAGS += -I$(srctree)/tools/include HOST_EXTRACFLAGS += -I$(srctree)/tools/include
...@@ -27,14 +28,16 @@ HOSTLOADLIBES_extract-cert = -lcrypto ...@@ -27,14 +28,16 @@ HOSTLOADLIBES_extract-cert = -lcrypto
always := $(hostprogs-y) $(hostprogs-m) always := $(hostprogs-y) $(hostprogs-m)
# The following hostprogs-y programs are only build on demand # The following hostprogs-y programs are only build on demand
hostprogs-y += unifdef docproc hostprogs-y += unifdef docproc check-lc_ctype
# These targets are used internally to avoid "is up to date" messages # These targets are used internally to avoid "is up to date" messages
PHONY += build_unifdef build_docproc PHONY += build_unifdef build_docproc build_check-lc_ctype
build_unifdef: $(obj)/unifdef build_unifdef: $(obj)/unifdef
@: @:
build_docproc: $(obj)/docproc build_docproc: $(obj)/docproc
@: @:
build_check-lc_ctype: $(obj)/check-lc_ctype
@:
subdir-$(CONFIG_MODVERSIONS) += genksyms subdir-$(CONFIG_MODVERSIONS) += genksyms
subdir-y += mod subdir-y += mod
......
/*
* Check that a specified locale works as LC_CTYPE. Used by the
* DocBook build system to probe for C.UTF-8 support.
*/
#include <locale.h>
int main(void)
{
return !setlocale(LC_CTYPE, "");
}
...@@ -206,59 +206,73 @@ my $type_env = '(\$\w+)'; ...@@ -206,59 +206,73 @@ my $type_env = '(\$\w+)';
# One for each output format # One for each output format
# these work fairly well # these work fairly well
my %highlights_html = ( $type_constant, "<i>\$1</i>", my @highlights_html = (
$type_func, "<b>\$1</b>", [$type_constant, "<i>\$1</i>"],
$type_struct_xml, "<i>\$1</i>", [$type_func, "<b>\$1</b>"],
$type_env, "<b><i>\$1</i></b>", [$type_struct_xml, "<i>\$1</i>"],
$type_param, "<tt><b>\$1</b></tt>" ); [$type_env, "<b><i>\$1</i></b>"],
[$type_param, "<tt><b>\$1</b></tt>"]
);
my $local_lt = "\\\\\\\\lt:"; my $local_lt = "\\\\\\\\lt:";
my $local_gt = "\\\\\\\\gt:"; my $local_gt = "\\\\\\\\gt:";
my $blankline_html = $local_lt . "p" . $local_gt; # was "<p>" my $blankline_html = $local_lt . "p" . $local_gt; # was "<p>"
# html version 5 # html version 5
my %highlights_html5 = ( $type_constant, "<span class=\"const\">\$1</span>", my @highlights_html5 = (
$type_func, "<span class=\"func\">\$1</span>", [$type_constant, "<span class=\"const\">\$1</span>"],
$type_struct_xml, "<span class=\"struct\">\$1</span>", [$type_func, "<span class=\"func\">\$1</span>"],
$type_env, "<span class=\"env\">\$1</span>", [$type_struct_xml, "<span class=\"struct\">\$1</span>"],
$type_param, "<span class=\"param\">\$1</span>" ); [$type_env, "<span class=\"env\">\$1</span>"],
[$type_param, "<span class=\"param\">\$1</span>]"]
);
my $blankline_html5 = $local_lt . "br /" . $local_gt; my $blankline_html5 = $local_lt . "br /" . $local_gt;
# XML, docbook format # XML, docbook format
my %highlights_xml = ( "([^=])\\\"([^\\\"<]+)\\\"", "\$1<quote>\$2</quote>", my @highlights_xml = (
$type_constant, "<constant>\$1</constant>", ["([^=])\\\"([^\\\"<]+)\\\"", "\$1<quote>\$2</quote>"],
$type_func, "<function>\$1</function>", [$type_constant, "<constant>\$1</constant>"],
$type_struct_xml, "<structname>\$1</structname>", [$type_struct_xml, "<structname>\$1</structname>"],
$type_env, "<envar>\$1</envar>", [$type_param, "<parameter>\$1</parameter>"],
$type_param, "<parameter>\$1</parameter>" ); [$type_func, "<function>\$1</function>"],
[$type_env, "<envar>\$1</envar>"]
);
my $blankline_xml = $local_lt . "/para" . $local_gt . $local_lt . "para" . $local_gt . "\n"; my $blankline_xml = $local_lt . "/para" . $local_gt . $local_lt . "para" . $local_gt . "\n";
# gnome, docbook format # gnome, docbook format
my %highlights_gnome = ( $type_constant, "<replaceable class=\"option\">\$1</replaceable>", my @highlights_gnome = (
$type_func, "<function>\$1</function>", [$type_constant, "<replaceable class=\"option\">\$1</replaceable>"],
$type_struct, "<structname>\$1</structname>", [$type_func, "<function>\$1</function>"],
$type_env, "<envar>\$1</envar>", [$type_struct, "<structname>\$1</structname>"],
$type_param, "<parameter>\$1</parameter>" ); [$type_env, "<envar>\$1</envar>"],
[$type_param, "<parameter>\$1</parameter>" ]
);
my $blankline_gnome = "</para><para>\n"; my $blankline_gnome = "</para><para>\n";
# these are pretty rough # these are pretty rough
my %highlights_man = ( $type_constant, "\$1", my @highlights_man = (
$type_func, "\\\\fB\$1\\\\fP", [$type_constant, "\$1"],
$type_struct, "\\\\fI\$1\\\\fP", [$type_func, "\\\\fB\$1\\\\fP"],
$type_param, "\\\\fI\$1\\\\fP" ); [$type_struct, "\\\\fI\$1\\\\fP"],
[$type_param, "\\\\fI\$1\\\\fP"]
);
my $blankline_man = ""; my $blankline_man = "";
# text-mode # text-mode
my %highlights_text = ( $type_constant, "\$1", my @highlights_text = (
$type_func, "\$1", [$type_constant, "\$1"],
$type_struct, "\$1", [$type_func, "\$1"],
$type_param, "\$1" ); [$type_struct, "\$1"],
[$type_param, "\$1"]
);
my $blankline_text = ""; my $blankline_text = "";
# list mode # list mode
my %highlights_list = ( $type_constant, "\$1", my @highlights_list = (
$type_func, "\$1", [$type_constant, "\$1"],
$type_struct, "\$1", [$type_func, "\$1"],
$type_param, "\$1" ); [$type_struct, "\$1"],
[$type_param, "\$1"]
);
my $blankline_list = ""; my $blankline_list = "";
# read arguments # read arguments
...@@ -273,7 +287,7 @@ my $verbose = 0; ...@@ -273,7 +287,7 @@ my $verbose = 0;
my $output_mode = "man"; my $output_mode = "man";
my $output_preformatted = 0; my $output_preformatted = 0;
my $no_doc_sections = 0; my $no_doc_sections = 0;
my %highlights = %highlights_man; my @highlights = @highlights_man;
my $blankline = $blankline_man; my $blankline = $blankline_man;
my $modulename = "Kernel API"; my $modulename = "Kernel API";
my $function_only = 0; my $function_only = 0;
...@@ -374,31 +388,31 @@ while ($ARGV[0] =~ m/^-(.*)/) { ...@@ -374,31 +388,31 @@ while ($ARGV[0] =~ m/^-(.*)/) {
my $cmd = shift @ARGV; my $cmd = shift @ARGV;
if ($cmd eq "-html") { if ($cmd eq "-html") {
$output_mode = "html"; $output_mode = "html";
%highlights = %highlights_html; @highlights = @highlights_html;
$blankline = $blankline_html; $blankline = $blankline_html;
} elsif ($cmd eq "-html5") { } elsif ($cmd eq "-html5") {
$output_mode = "html5"; $output_mode = "html5";
%highlights = %highlights_html5; @highlights = @highlights_html5;
$blankline = $blankline_html5; $blankline = $blankline_html5;
} elsif ($cmd eq "-man") { } elsif ($cmd eq "-man") {
$output_mode = "man"; $output_mode = "man";
%highlights = %highlights_man; @highlights = @highlights_man;
$blankline = $blankline_man; $blankline = $blankline_man;
} elsif ($cmd eq "-text") { } elsif ($cmd eq "-text") {
$output_mode = "text"; $output_mode = "text";
%highlights = %highlights_text; @highlights = @highlights_text;
$blankline = $blankline_text; $blankline = $blankline_text;
} elsif ($cmd eq "-docbook") { } elsif ($cmd eq "-docbook") {
$output_mode = "xml"; $output_mode = "xml";
%highlights = %highlights_xml; @highlights = @highlights_xml;
$blankline = $blankline_xml; $blankline = $blankline_xml;
} elsif ($cmd eq "-list") { } elsif ($cmd eq "-list") {
$output_mode = "list"; $output_mode = "list";
%highlights = %highlights_list; @highlights = @highlights_list;
$blankline = $blankline_list; $blankline = $blankline_list;
} elsif ($cmd eq "-gnome") { } elsif ($cmd eq "-gnome") {
$output_mode = "gnome"; $output_mode = "gnome";
%highlights = %highlights_gnome; @highlights = @highlights_gnome;
$blankline = $blankline_gnome; $blankline = $blankline_gnome;
} elsif ($cmd eq "-module") { # not needed for XML, inherits from calling document } elsif ($cmd eq "-module") { # not needed for XML, inherits from calling document
$modulename = shift @ARGV; $modulename = shift @ARGV;
...@@ -1746,7 +1760,7 @@ sub output_declaration { ...@@ -1746,7 +1760,7 @@ sub output_declaration {
my $func = "output_${functype}_$output_mode"; my $func = "output_${functype}_$output_mode";
if (($function_only==0) || if (($function_only==0) ||
( $function_only == 1 && defined($function_table{$name})) || ( $function_only == 1 && defined($function_table{$name})) ||
( $function_only == 2 && !defined($function_table{$name}))) ( $function_only == 2 && !($functype eq "function" && defined($function_table{$name}))))
{ {
&$func(@_); &$func(@_);
$section_counter++; $section_counter++;
...@@ -1791,8 +1805,8 @@ sub dump_struct($$) { ...@@ -1791,8 +1805,8 @@ sub dump_struct($$) {
$nested = $1; $nested = $1;
# ignore members marked private: # ignore members marked private:
$members =~ s/\/\*\s*private:.*?\/\*\s*public:.*?\*\///gos; $members =~ s/\/\*\s*private:.*?\/\*\s*public:.*?\*\///gosi;
$members =~ s/\/\*\s*private:.*//gos; $members =~ s/\/\*\s*private:.*//gosi;
# strip comments: # strip comments:
$members =~ s/\/\*.*?\*\///gos; $members =~ s/\/\*.*?\*\///gos;
$nested =~ s/\/\*.*?\*\///gos; $nested =~ s/\/\*.*?\*\///gos;
...@@ -2416,12 +2430,13 @@ sub process_file($) { ...@@ -2416,12 +2430,13 @@ sub process_file($) {
my $descr; my $descr;
my $in_purpose = 0; my $in_purpose = 0;
my $initial_section_counter = $section_counter; my $initial_section_counter = $section_counter;
my ($orig_file) = @_;
if (defined($ENV{'SRCTREE'})) { if (defined($ENV{'SRCTREE'})) {
$file = "$ENV{'SRCTREE'}" . "/" . "@_"; $file = "$ENV{'SRCTREE'}" . "/" . $orig_file;
} }
else { else {
$file = "@_"; $file = $orig_file;
} }
if (defined($source_map{$file})) { if (defined($source_map{$file})) {
$file = $source_map{$file}; $file = $source_map{$file};
...@@ -2665,7 +2680,7 @@ sub process_file($) { ...@@ -2665,7 +2680,7 @@ sub process_file($) {
print "<refentry>\n"; print "<refentry>\n";
print " <refnamediv>\n"; print " <refnamediv>\n";
print " <refname>\n"; print " <refname>\n";
print " ${file}\n"; print " ${orig_file}\n";
print " </refname>\n"; print " </refname>\n";
print " <refpurpose>\n"; print " <refpurpose>\n";
print " Document generation inconsistency\n"; print " Document generation inconsistency\n";
...@@ -2679,7 +2694,7 @@ sub process_file($) { ...@@ -2679,7 +2694,7 @@ sub process_file($) {
print " <para>\n"; print " <para>\n";
print " The template for this document tried to insert\n"; print " The template for this document tried to insert\n";
print " the structured comment from the file\n"; print " the structured comment from the file\n";
print " <filename>${file}</filename> at this point,\n"; print " <filename>${orig_file}</filename> at this point,\n";
print " but none was found.\n"; print " but none was found.\n";
print " This dummy section is inserted to allow\n"; print " This dummy section is inserted to allow\n";
print " generation to continue.\n"; print " generation to continue.\n";
...@@ -2696,9 +2711,11 @@ $kernelversion = get_kernel_version(); ...@@ -2696,9 +2711,11 @@ $kernelversion = get_kernel_version();
# generate a sequence of code that will splice in highlighting information # generate a sequence of code that will splice in highlighting information
# using the s// operator. # using the s// operator.
foreach my $pattern (sort keys %highlights) { foreach my $k (keys @highlights) {
# print STDERR "scanning pattern:$pattern, highlight:($highlights{$pattern})\n"; my $pattern = $highlights[$k][0];
$dohighlight .= "\$contents =~ s:$pattern:$highlights{$pattern}:gs;\n"; my $result = $highlights[$k][1];
# print STDERR "scanning pattern:$pattern, highlight:($result)\n";
$dohighlight .= "\$contents =~ s:$pattern:$result:gs;\n";
} }
# Read the file that maps relative names to absolute names for # Read the file that maps relative names to absolute names for
......
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