Merge tag 'printk-for-5.10' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:
 "The big new thing is the fully lockless ringbuffer implementation,
  including the support for continuous lines. It will allow to store and
  read messages in any situation wihtout the risk of deadlocks and
  without the need of temporary per-CPU buffers.

  The access is still serialized by logbuf_lock. It synchronizes few
  more operations, for example, temporary buffer for formatting the
  message, syslog and kmsg_dump operations. The lock removal is being
  discussed and should be ready for the next release.

  The continuous lines are handled exactly the same way as before to
  avoid regressions in user space. It means that they are appended to
  the last message when the caller is the same. Only the last message
  can be extended.

  The data ring includes plain text of the messages. Except for an
  integer at the beginning of each message that points back to the
  descriptor ring with other metadata.

  The dictionary has to stay. journalctl uses it to filter the log. It
  allows to show messages related to a given device. The dictionary
  values are stored in the descriptor ring with the other metadata.

  This is the first part of the printk rework as discussed at Plumbers
  2019, see https://lore.kernel.org/r/87k1acz5rx.fsf@linutronix.de. The
  next big step will be handling consoles by kthreads during the normal
  system operation. It will require special handling of situations when
  the kthreads could not get scheduled, for example, early boot,
  suspend, panic.

  Other changes:

   - Add John Ogness as a reviewer for printk subsystem. He is author of
     the rework and is familiar with the code and history.

   - Fix locking in serial8250_do_startup() to prevent lockdep report.

   - Few code cleanups"

* tag 'printk-for-5.10' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux: (27 commits)
  printk: Use fallthrough pseudo-keyword
  printk: reduce setup_text_buf size to LOG_LINE_MAX
  printk: avoid and/or handle record truncation
  printk: remove dict ring
  printk: move dictionary keys to dev_printk_info
  printk: move printk_info into separate array
  printk: reimplement log_cont using record extension
  printk: ringbuffer: add finalization/extension support
  printk: ringbuffer: change representation of states
  printk: ringbuffer: clear initial reserved fields
  printk: ringbuffer: add BLK_DATALESS() macro
  printk: ringbuffer: relocate get_data()
  printk: ringbuffer: avoid memcpy() on state_var
  printk: ringbuffer: fix setting state in desc_read()
  kernel.h: Move oops_in_progress to printk.h
  scripts/gdb: update for lockless printk ringbuffer
  scripts/gdb: add utils.read_ulong()
  docs: vmcoreinfo: add lockless printk ringbuffer vmcoreinfo
  printk: reduce LOG_BUF_SHIFT range for H8300
  printk: ringbuffer: support dataless records
  ...

Documentation/admin-guide/kdump/gdbmacros.txt

@@ -170,57 +170,103 @@ document trapinfo
 	address the kernel panicked.
 end
 
-define dump_log_idx
-	set $idx = $arg0
-	if ($argc > 1)
-		set $prev_flags = $arg1
+define dump_record
+	set var $desc = $arg0
+	set var $info = $arg1
+	if ($argc > 2)
+		set var $prev_flags = $arg2
 	else
-		set $prev_flags = 0
+		set var $prev_flags = 0
 	end
-	set $msg = ((struct printk_log *) (log_buf + $idx))
-	set $prefix = 1
-	set $newline = 1
-	set $log = log_buf + $idx + sizeof(*$msg)
-
-	# prev & LOG_CONT && !(msg->flags & LOG_PREIX)
-	if (($prev_flags & 8) && !($msg->flags & 4))
-		set $prefix = 0
+
+	set var $prefix = 1
+	set var $newline = 1
+
+	set var $begin = $desc->text_blk_lpos.begin % (1U << prb->text_data_ring.size_bits)
+	set var $next = $desc->text_blk_lpos.next % (1U << prb->text_data_ring.size_bits)
+
+	# handle data-less record
+	if ($begin & 1)
+		set var $text_len = 0
+		set var $log = ""
+	else
+		# handle wrapping data block
+		if ($begin > $next)
+			set var $begin = 0
+		end
+
+		# skip over descriptor id
+		set var $begin = $begin + sizeof(long)
+
+		# handle truncated message
+		if ($next - $begin < $info->text_len)
+			set var $text_len = $next - $begin
+		else
+			set var $text_len = $info->text_len
+		end
+
+		set var $log = &prb->text_data_ring.data[$begin]
+	end
+
+	# prev & LOG_CONT && !(info->flags & LOG_PREIX)
+	if (($prev_flags & 8) && !($info->flags & 4))
+		set var $prefix = 0
 	end
 
-	# msg->flags & LOG_CONT
-	if ($msg->flags & 8)
+	# info->flags & LOG_CONT
+	if ($info->flags & 8)
 		# (prev & LOG_CONT && !(prev & LOG_NEWLINE))
 		if (($prev_flags & 8) && !($prev_flags & 2))
-			set $prefix = 0
+			set var $prefix = 0
 		end
-		# (!(msg->flags & LOG_NEWLINE))
-		if (!($msg->flags & 2))
-			set $newline = 0
+		# (!(info->flags & LOG_NEWLINE))
+		if (!($info->flags & 2))
+			set var $newline = 0
 		end
 	end
 
 	if ($prefix)
-		printf "[%5lu.%06lu] ", $msg->ts_nsec / 1000000000, $msg->ts_nsec % 1000000000
+		printf "[%5lu.%06lu] ", $info->ts_nsec / 1000000000, $info->ts_nsec % 1000000000
 	end
-	if ($msg->text_len != 0)
-		eval "printf \"%%%d.%ds\", $log", $msg->text_len, $msg->text_len
+	if ($text_len)
+		eval "printf \"%%%d.%ds\", $log", $text_len, $text_len
 	end
 	if ($newline)
 		printf "\n"
 	end
-	if ($msg->dict_len > 0)
-		set $dict = $log + $msg->text_len
-		set $idx = 0
-		set $line = 1
-		while ($idx < $msg->dict_len)
-			if ($line)
-				printf " "
-				set $line = 0
+
+	# handle dictionary data
+
+	set var $dict = &$info->dev_info.subsystem[0]
+	set var $dict_len = sizeof($info->dev_info.subsystem)
+	if ($dict[0] != '\0')
+		printf " SUBSYSTEM="
+		set var $idx = 0
+		while ($idx < $dict_len)
+			set var $c = $dict[$idx]
+			if ($c == '\0')
+				loop_break
+			else
+				if ($c < ' ' || $c >= 127 || $c == '\\')
+					printf "\\x%02x", $c
+				else
+					printf "%c", $c
+				end
 			end
-			set $c = $dict[$idx]
+			set var $idx = $idx + 1
+		end
+		printf "\n"
+	end
+
+	set var $dict = &$info->dev_info.device[0]
+	set var $dict_len = sizeof($info->dev_info.device)
+	if ($dict[0] != '\0')
+		printf " DEVICE="
+		set var $idx = 0
+		while ($idx < $dict_len)
+			set var $c = $dict[$idx]
 			if ($c == '\0')
-				printf "\n"
-				set $line = 1
+				loop_break
 			else
 				if ($c < ' ' || $c >= 127 || $c == '\\')
 					printf "\\x%02x", $c
@@ -228,33 +274,46 @@ define dump_log_idx
 					printf "%c", $c
 				end
 			end
-			set $idx = $idx + 1
+			set var $idx = $idx + 1
 		end
 		printf "\n"
 	end
 end
-document dump_log_idx
-	Dump a single log given its index in the log buffer.  The first
-	parameter is the index into log_buf, the second is optional and
-	specified the previous log buffer's flags, used for properly
-	formatting continued lines.
+document dump_record
+	Dump a single record. The first parameter is the descriptor,
+	the second parameter is the info, the third parameter is
+	optional and specifies the previous record's flags, used for
+	properly formatting continued lines.
 end
 
 define dmesg
-	set $i = log_first_idx
-	set $end_idx = log_first_idx
-	set $prev_flags = 0
+	# definitions from kernel/printk/printk_ringbuffer.h
+	set var $desc_committed = 1
+	set var $desc_finalized = 2
+	set var $desc_sv_bits = sizeof(long) * 8
+	set var $desc_flags_shift = $desc_sv_bits - 2
+	set var $desc_flags_mask = 3 << $desc_flags_shift
+	set var $id_mask = ~$desc_flags_mask
+
+	set var $desc_count = 1U << prb->desc_ring.count_bits
+	set var $prev_flags = 0
+
+	set var $id = prb->desc_ring.tail_id.counter
+	set var $end_id = prb->desc_ring.head_id.counter
 
 	while (1)
-		set $msg = ((struct printk_log *) (log_buf + $i))
-		if ($msg->len == 0)
-			set $i = 0
-		else
-			dump_log_idx $i $prev_flags
-			set $i = $i + $msg->len
-			set $prev_flags = $msg->flags
+		set var $desc = &prb->desc_ring.descs[$id % $desc_count]
+		set var $info = &prb->desc_ring.infos[$id % $desc_count]
+
+		# skip non-committed record
+		set var $state = 3 & ($desc->state_var.counter >> $desc_flags_shift)
+		if ($state == $desc_committed || $state == $desc_finalized)
+			dump_record $desc $info $prev_flags
+			set var $prev_flags = $info->flags
 		end
-		if ($i == $end_idx)
+
+		set var $id = ($id + 1) & $id_mask
+		if ($id == $end_id)
 			loop_break
 		end
 	end

Documentation/admin-guide/kdump/vmcoreinfo.rst

@@ -189,50 +189,123 @@ from this.
 Free areas descriptor. User-space tools use this value to iterate the
 free_area ranges. MAX_ORDER is used by the zone buddy allocator.
 
-log_first_idx
+prb
+---
+
+A pointer to the printk ringbuffer (struct printk_ringbuffer). This
+may be pointing to the static boot ringbuffer or the dynamically
+allocated ringbuffer, depending on when the the core dump occurred.
+Used by user-space tools to read the active kernel log buffer.
+
+printk_rb_static
+----------------
+
+A pointer to the static boot printk ringbuffer. If @prb has a
+different value, this is useful for viewing the initial boot messages,
+which may have been overwritten in the dynamically allocated
+ringbuffer.
+
+clear_seq
+---------
+
+The sequence number of the printk() record after the last clear
+command. It indicates the first record after the last
+SYSLOG_ACTION_CLEAR, like issued by 'dmesg -c'. Used by user-space
+tools to dump a subset of the dmesg log.
+
+printk_ringbuffer
+-----------------
+
+The size of a printk_ringbuffer structure. This structure contains all
+information required for accessing the various components of the
+kernel log buffer.
+
+(printk_ringbuffer, desc_ring|text_data_ring|dict_data_ring|fail)
+-----------------------------------------------------------------
+
+Offsets for the various components of the printk ringbuffer. Used by
+user-space tools to view the kernel log buffer without requiring the
+declaration of the structure.
+
+prb_desc_ring
 -------------
 
-Index of the first record stored in the buffer log_buf. Used by
-user-space tools to read the strings in the log_buf.
+The size of the prb_desc_ring structure. This structure contains
+information about the set of record descriptors.
 
-log_buf
--------
+(prb_desc_ring, count_bits|descs|head_id|tail_id)
+-------------------------------------------------
+
+Offsets for the fields describing the set of record descriptors. Used
+by user-space tools to be able to traverse the descriptors without
+requiring the declaration of the structure.
+
+prb_desc
+--------
+
+The size of the prb_desc structure. This structure contains
+information about a single record descriptor.
+
+(prb_desc, info|state_var|text_blk_lpos|dict_blk_lpos)
+------------------------------------------------------
+
+Offsets for the fields describing a record descriptors. Used by
+user-space tools to be able to read descriptors without requiring
+the declaration of the structure.
+
+prb_data_blk_lpos
+-----------------
+
+The size of the prb_data_blk_lpos structure. This structure contains
+information about where the text or dictionary data (data block) is
+located within the respective data ring.
+
+(prb_data_blk_lpos, begin|next)
+-------------------------------
 
-Console output is written to the ring buffer log_buf at index
-log_first_idx. Used to get the kernel log.
+Offsets for the fields describing the location of a data block. Used
+by user-space tools to be able to locate data blocks without
+requiring the declaration of the structure.
 
-log_buf_len
+printk_info
 -----------
 
-log_buf's length.
+The size of the printk_info structure. This structure contains all
+the meta-data for a record.
 
-clear_idx
----------
+(printk_info, seq|ts_nsec|text_len|dict_len|caller_id)
+------------------------------------------------------
 
-The index that the next printk() record to read after the last clear
-command. It indicates the first record after the last SYSLOG_ACTION
-_CLEAR, like issued by 'dmesg -c'. Used by user-space tools to dump
-the dmesg log.
+Offsets for the fields providing the meta-data for a record. Used by
+user-space tools to be able to read the information without requiring
+the declaration of the structure.
 
-log_next_idx
-------------
+prb_data_ring
+-------------
 
-The index of the next record to store in the buffer log_buf. Used to
-compute the index of the current buffer position.
+The size of the prb_data_ring structure. This structure contains
+information about a set of data blocks.
 
-printk_log
-----------
+(prb_data_ring, size_bits|data|head_lpos|tail_lpos)
+---------------------------------------------------
 
-The size of a structure printk_log. Used to compute the size of
-messages, and extract dmesg log. It encapsulates header information for
-log_buf, such as timestamp, syslog level, etc.
+Offsets for the fields describing a set of data blocks. Used by
+user-space tools to be able to access the data blocks without
+requiring the declaration of the structure.
 
-(printk_log, ts_nsec|len|text_len|dict_len)
--------------------------------------------
+atomic_long_t
+-------------
+
+The size of the atomic_long_t structure. Used by user-space tools to
+be able to copy the full structure, regardless of its
+architecture-specific implementation.
+
+(atomic_long_t, counter)
+------------------------
 
-It represents field offsets in struct printk_log. User space tools
-parse it and check whether the values of printk_log's members have been
-changed.
+Offset for the long value of an atomic_long_t variable. Used by
+user-space tools to access the long value without requiring the
+architecture-specific declaration.
 
 (free_area.free_list, MIGRATE_TYPES)
 ------------------------------------

MAINTAINERS

@@ -13970,6 +13970,7 @@ PRINTK
 M:	Petr Mladek <pmladek@suse.com>
 M:	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
 R:	Steven Rostedt <rostedt@goodmis.org>
+R:	John Ogness <john.ogness@linutronix.de>
 S:	Maintained
 F:	include/linux/printk.h
 F:	kernel/printk/

drivers/base/core.c

@@ -4061,22 +4061,21 @@ void device_shutdown(void)
  */
 
 #ifdef CONFIG_PRINTK
-static int
-create_syslog_header(const struct device *dev, char *hdr, size_t hdrlen)
+static void
+set_dev_info(const struct device *dev, struct dev_printk_info *dev_info)
 {
 	const char *subsys;
-	size_t pos = 0;
+
+	memset(dev_info, 0, sizeof(*dev_info));
 
 	if (dev->class)
 		subsys = dev->class->name;
 	else if (dev->bus)
 		subsys = dev->bus->name;
 	else
-		return 0;
+		return;
 
-	pos += snprintf(hdr + pos, hdrlen - pos, "SUBSYSTEM=%s", subsys);
-	if (pos >= hdrlen)
-		goto overflow;
+	strscpy(dev_info->subsystem, subsys, sizeof(dev_info->subsystem));
 
 	/*
 	 * Add device identifier DEVICE=:
@@ -4092,41 +4091,28 @@ create_syslog_header(const struct device *dev, char *hdr, size_t hdrlen)
 			c = 'b';
 		else
 			c = 'c';
-		pos++;
-		pos += snprintf(hdr + pos, hdrlen - pos,
-				"DEVICE=%c%u:%u",
-				c, MAJOR(dev->devt), MINOR(dev->devt));
+
+		snprintf(dev_info->device, sizeof(dev_info->device),
+			 "%c%u:%u", c, MAJOR(dev->devt), MINOR(dev->devt));
 	} else if (strcmp(subsys, "net") == 0) {
 		struct net_device *net = to_net_dev(dev);
 
-		pos++;
-		pos += snprintf(hdr + pos, hdrlen - pos,
-				"DEVICE=n%u", net->ifindex);
+		snprintf(dev_info->device, sizeof(dev_info->device),
+			 "n%u", net->ifindex);
 	} else {
-		pos++;
-		pos += snprintf(hdr + pos, hdrlen - pos,
-				"DEVICE=+%s:%s", subsys, dev_name(dev));
+		snprintf(dev_info->device, sizeof(dev_info->device),
+			 "+%s:%s", subsys, dev_name(dev));
 	}
-
-	if (pos >= hdrlen)
-		goto overflow;
-
-	return pos;
-
-overflow:
-	dev_WARN(dev, "device/subsystem name too long");
-	return 0;
 }
 
 int dev_vprintk_emit(int level, const struct device *dev,
 		     const char *fmt, va_list args)
 {
-	char hdr[128];
-	size_t hdrlen;
+	struct dev_printk_info dev_info;
 
-	hdrlen = create_syslog_header(dev, hdr, sizeof(hdr));
+	set_dev_info(dev, &dev_info);
 
-	return vprintk_emit(0, level, hdrlen ? hdr : NULL, hdrlen, fmt, args);
+	return vprintk_emit(0, level, &dev_info, fmt, args);
 }
 EXPORT_SYMBOL(dev_vprintk_emit);
 

include/linux/crash_core.h

@@ -55,6 +55,9 @@ phys_addr_t paddr_vmcoreinfo_note(void);
 #define VMCOREINFO_OFFSET(name, field) \
 	vmcoreinfo_append_str("OFFSET(%s.%s)=%lu\n", #name, #field, \
 			      (unsigned long)offsetof(struct name, field))
+#define VMCOREINFO_TYPE_OFFSET(name, field) \
+	vmcoreinfo_append_str("OFFSET(%s.%s)=%lu\n", #name, #field, \
+			      (unsigned long)offsetof(name, field))
 #define VMCOREINFO_LENGTH(name, value) \
 	vmcoreinfo_append_str("LENGTH(%s)=%lu\n", #name, (unsigned long)value)
 #define VMCOREINFO_NUMBER(name) \

include/linux/debug_locks.h

@@ -2,9 +2,9 @@
 #ifndef __LINUX_DEBUG_LOCKING_H
 #define __LINUX_DEBUG_LOCKING_H
 
-#include <linux/kernel.h>
 #include <linux/atomic.h>
 #include <linux/bug.h>
+#include <linux/printk.h>
 
 struct task_struct;
 

include/linux/dev_printk.h

@@ -21,6 +21,14 @@
 
 struct device;
 
+#define PRINTK_INFO_SUBSYSTEM_LEN	16
+#define PRINTK_INFO_DEVICE_LEN		48
+
+struct dev_printk_info {
+	char subsystem[PRINTK_INFO_SUBSYSTEM_LEN];
+	char device[PRINTK_INFO_DEVICE_LEN];
+};
+
 #ifdef CONFIG_PRINTK
 
 __printf(3, 0) __cold

include/linux/kernel.h

@@ -526,7 +526,6 @@ extern unsigned int sysctl_oops_all_cpu_backtrace;
 #endif /* CONFIG_SMP */
 
 extern void bust_spinlocks(int yes);
-extern int oops_in_progress;		/* If set, an oops, panic(), BUG() or die() is in progress */
 extern int panic_timeout;
 extern unsigned long panic_print;
 extern int panic_on_oops;

include/linux/printk.h

@@ -12,6 +12,8 @@
 extern const char linux_banner[];
 extern const char linux_proc_banner[];
 
+extern int oops_in_progress;	/* If set, an oops, panic(), BUG() or die() is in progress */
+
 #define PRINTK_MAX_SINGLE_HEADER_LEN 2
 
 static inline int printk_get_level(const char *buffer)
@@ -159,10 +161,12 @@ static inline void printk_nmi_direct_enter(void) { }
 static inline void printk_nmi_direct_exit(void) { }
 #endif /* PRINTK_NMI */
 
+struct dev_printk_info;
+
 #ifdef CONFIG_PRINTK
-asmlinkage __printf(5, 0)
+asmlinkage __printf(4, 0)
 int vprintk_emit(int facility, int level,
-		 const char *dict, size_t dictlen,
+		 const struct dev_printk_info *dev_info,
 		 const char *fmt, va_list args);
 
 asmlinkage __printf(1, 0)

init/Kconfig

@@ -682,7 +682,8 @@ config IKHEADERS
 
 config LOG_BUF_SHIFT
 	int "Kernel log buffer size (16 => 64KB, 17 => 128KB)"
-	range 12 25
+	range 12 25 if !H8300
+	range 12 19 if H8300
 	default 17
 	depends on PRINTK
 	help

kernel/printk/Makefile

@@ -2,3 +2,4 @@
 obj-y	= printk.o
 obj-$(CONFIG_PRINTK)	+= printk_safe.o
 obj-$(CONFIG_A11Y_BRAILLE_CONSOLE)	+= braille.o
+obj-$(CONFIG_PRINTK)	+= printk_ringbuffer.o

kernel/printk/internal.h

@@ -14,9 +14,9 @@
 
 extern raw_spinlock_t logbuf_lock;
 
-__printf(5, 0)
+__printf(4, 0)
 int vprintk_store(int facility, int level,
-		  const char *dict, size_t dictlen,
+		  const struct dev_printk_info *dev_info,
 		  const char *fmt, va_list args);
 
 __printf(1, 0) int vprintk_default(const char *fmt, va_list args);

kernel/printk/printk_safe.c

@@ -375,7 +375,7 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
 	    raw_spin_trylock(&logbuf_lock)) {
 		int len;
 
-		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args);
+		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
 		raw_spin_unlock(&logbuf_lock);
 		defer_console_output();
 		return len;

scripts/gdb/linux/dmesg.py

@@ -16,8 +16,13 @@ import sys
 
 from linux import utils
 
-printk_log_type = utils.CachedType("struct printk_log")
-
+printk_info_type = utils.CachedType("struct printk_info")
+prb_data_blk_lpos_type = utils.CachedType("struct prb_data_blk_lpos")
+prb_desc_type = utils.CachedType("struct prb_desc")
+prb_desc_ring_type = utils.CachedType("struct prb_desc_ring")
+prb_data_ring_type = utils.CachedType("struct prb_data_ring")
+printk_ringbuffer_type = utils.CachedType("struct printk_ringbuffer")
+atomic_long_type = utils.CachedType("atomic_long_t")
 
 class LxDmesg(gdb.Command):
     """Print Linux kernel log buffer."""
@@ -26,44 +31,110 @@ class LxDmesg(gdb.Command):
         super(LxDmesg, self).__init__("lx-dmesg", gdb.COMMAND_DATA)
 
     def invoke(self, arg, from_tty):
-        log_buf_addr = int(str(gdb.parse_and_eval(
-            "(void *)'printk.c'::log_buf")).split()[0], 16)
-        log_first_idx = int(gdb.parse_and_eval("'printk.c'::log_first_idx"))
-        log_next_idx = int(gdb.parse_and_eval("'printk.c'::log_next_idx"))
-        log_buf_len = int(gdb.parse_and_eval("'printk.c'::log_buf_len"))
-
         inf = gdb.inferiors()[0]
-        start = log_buf_addr + log_first_idx
-        if log_first_idx < log_next_idx:
-            log_buf_2nd_half = -1
-            length = log_next_idx - log_first_idx
-            log_buf = utils.read_memoryview(inf, start, length).tobytes()
-        else:
-            log_buf_2nd_half = log_buf_len - log_first_idx
-            a = utils.read_memoryview(inf, start, log_buf_2nd_half)
-            b = utils.read_memoryview(inf, log_buf_addr, log_next_idx)
-            log_buf = a.tobytes() + b.tobytes()
-
-        length_offset = printk_log_type.get_type()['len'].bitpos // 8
-        text_len_offset = printk_log_type.get_type()['text_len'].bitpos // 8
-        time_stamp_offset = printk_log_type.get_type()['ts_nsec'].bitpos // 8
-        text_offset = printk_log_type.get_type().sizeof
-
-        pos = 0
-        while pos < log_buf.__len__():
-            length = utils.read_u16(log_buf, pos + length_offset)
-            if length == 0:
-                if log_buf_2nd_half == -1:
-                    gdb.write("Corrupted log buffer!\n")
+
+        # read in prb structure
+        prb_addr = int(str(gdb.parse_and_eval("(void *)'printk.c'::prb")).split()[0], 16)
+        sz = printk_ringbuffer_type.get_type().sizeof
+        prb = utils.read_memoryview(inf, prb_addr, sz).tobytes()
+
+        # read in descriptor ring structure
+        off = printk_ringbuffer_type.get_type()['desc_ring'].bitpos // 8
+        addr = prb_addr + off
+        sz = prb_desc_ring_type.get_type().sizeof
+        desc_ring = utils.read_memoryview(inf, addr, sz).tobytes()
+
+        # read in descriptor array
+        off = prb_desc_ring_type.get_type()['count_bits'].bitpos // 8
+        desc_ring_count = 1 << utils.read_u32(desc_ring, off)
+        desc_sz = prb_desc_type.get_type().sizeof
+        off = prb_desc_ring_type.get_type()['descs'].bitpos // 8
+        addr = utils.read_ulong(desc_ring, off)
+        descs = utils.read_memoryview(inf, addr, desc_sz * desc_ring_count).tobytes()
+
+        # read in info array
+        info_sz = printk_info_type.get_type().sizeof
+        off = prb_desc_ring_type.get_type()['infos'].bitpos // 8
+        addr = utils.read_ulong(desc_ring, off)
+        infos = utils.read_memoryview(inf, addr, info_sz * desc_ring_count).tobytes()
+
+        # read in text data ring structure
+        off = printk_ringbuffer_type.get_type()['text_data_ring'].bitpos // 8
+        addr = prb_addr + off
+        sz = prb_data_ring_type.get_type().sizeof
+        text_data_ring = utils.read_memoryview(inf, addr, sz).tobytes()
+
+        # read in text data
+        off = prb_data_ring_type.get_type()['size_bits'].bitpos // 8
+        text_data_sz = 1 << utils.read_u32(text_data_ring, off)
+        off = prb_data_ring_type.get_type()['data'].bitpos // 8
+        addr = utils.read_ulong(text_data_ring, off)
+        text_data = utils.read_memoryview(inf, addr, text_data_sz).tobytes()
+
+        counter_off = atomic_long_type.get_type()['counter'].bitpos // 8
+
+        sv_off = prb_desc_type.get_type()['state_var'].bitpos // 8
+
+        off = prb_desc_type.get_type()['text_blk_lpos'].bitpos // 8
+        begin_off = off + (prb_data_blk_lpos_type.get_type()['begin'].bitpos // 8)
+        next_off = off + (prb_data_blk_lpos_type.get_type()['next'].bitpos // 8)
+
+        ts_off = printk_info_type.get_type()['ts_nsec'].bitpos // 8
+        len_off = printk_info_type.get_type()['text_len'].bitpos // 8
+
+        # definitions from kernel/printk/printk_ringbuffer.h
+        desc_committed = 1
+        desc_finalized = 2
+        desc_sv_bits = utils.get_long_type().sizeof * 8
+        desc_flags_shift = desc_sv_bits - 2
+        desc_flags_mask = 3 << desc_flags_shift
+        desc_id_mask = ~desc_flags_mask
+
+        # read in tail and head descriptor ids
+        off = prb_desc_ring_type.get_type()['tail_id'].bitpos // 8
+        tail_id = utils.read_u64(desc_ring, off + counter_off)
+        off = prb_desc_ring_type.get_type()['head_id'].bitpos // 8
+        head_id = utils.read_u64(desc_ring, off + counter_off)
+
+        did = tail_id
+        while True:
+            ind = did % desc_ring_count
+            desc_off = desc_sz * ind
+            info_off = info_sz * ind
+
+            # skip non-committed record
+            state = 3 & (utils.read_u64(descs, desc_off + sv_off +
+                                        counter_off) >> desc_flags_shift)
+            if state != desc_committed and state != desc_finalized:
+                if did == head_id:
                     break
-                pos = log_buf_2nd_half
+                did = (did + 1) & desc_id_mask
                 continue
 
-            text_len = utils.read_u16(log_buf, pos + text_len_offset)
-            text_start = pos + text_offset
-            text = log_buf[text_start:text_start + text_len].decode(
-                encoding='utf8', errors='replace')
-            time_stamp = utils.read_u64(log_buf, pos + time_stamp_offset)
+            begin = utils.read_ulong(descs, desc_off + begin_off) % text_data_sz
+            end = utils.read_ulong(descs, desc_off + next_off) % text_data_sz
+
+            # handle data-less record
+            if begin & 1 == 1:
+                text = ""
+            else:
+                # handle wrapping data block
+                if begin > end:
+                    begin = 0
+
+                # skip over descriptor id
+                text_start = begin + utils.get_long_type().sizeof
+
+                text_len = utils.read_u16(infos, info_off + len_off)
+
+                # handle truncated message
+                if end - text_start < text_len:
+                    text_len = end - text_start
+
+                text = text_data[text_start:text_start + text_len].decode(
+                    encoding='utf8', errors='replace')
+
+            time_stamp = utils.read_u64(infos, info_off + ts_off)
 
             for line in text.splitlines():
                 msg = u"[{time:12.6f}] {line}\n".format(
@@ -75,7 +146,9 @@ class LxDmesg(gdb.Command):
                     msg = msg.encode(encoding='utf8', errors='replace')
                 gdb.write(msg)
 
-            pos += length
+            if did == head_id:
+                break
+            did = (did + 1) & desc_id_mask
 
 
 LxDmesg()

scripts/gdb/linux/utils.py

@@ -123,6 +123,13 @@ def read_u64(buffer, offset):
         return read_u32(buffer, offset + 4) + (read_u32(buffer, offset) << 32)
 
 
+def read_ulong(buffer, offset):
+    if get_long_type().sizeof == 8:
+        return read_u64(buffer, offset)
+    else:
+        return read_u32(buffer, offset)
+
+
 target_arch = None