• Linus Torvalds's avatar
    devpts: clean up interface to pty drivers · 67245ff3
    Linus Torvalds authored
    This gets rid of the horrible notion of having that
    
        struct inode *ptmx_inode
    
    be the linchpin of the interface between the pty code and devpts.
    
    By de-emphasizing the ptmx inode, a lot of things actually get cleaner,
    and we will have a much saner way forward.  In particular, this will
    allow us to associate with any particular devpts instance at open-time,
    and not be artificially tied to one particular ptmx inode.
    
    The patch itself is actually fairly straightforward, and apart from some
    locking and return path cleanups it's pretty mechanical:
    
     - the interfaces that devpts exposes all take "struct pts_fs_info *"
       instead of "struct inode *ptmx_inode" now.
    
       NOTE! The "struct pts_fs_info" thing is a completely opaque structure
       as far as the pty driver is concerned: it's still declared entirely
       internally to devpts. So the pty code can't actually access it in any
       way, just pass it as a "cookie" to the devpts code.
    
     - the "look up the pts fs info" is now a single clear operation, that
       also does the reference count increment on the pts superblock.
    
       So "devpts_add/del_ref()" is gone, and replaced by a "lookup and get
       ref" operation (devpts_get_ref(inode)), along with a "put ref" op
       (devpts_put_ref()).
    
     - the pty master "tty->driver_data" field now contains the pts_fs_info,
       not the ptmx inode.
    
     - because we don't care about the ptmx inode any more as some kind of
       base index, the ref counting can now drop the inode games - it just
       gets the ref on the superblock.
    
     - the pts_fs_info now has a back-pointer to the super_block. That's so
       that we can easily look up the information we actually need. Although
       quite often, the pts fs info was actually all we wanted, and not having
       to look it up based on some magical inode makes things more
       straightforward.
    
    In particular, now that "devpts_get_ref(inode)" operation should really
    be the *only* place we need to look up what devpts instance we're
    associated with, and we do it exactly once, at ptmx_open() time.
    
    The other side of this is that one ptmx node could now be associated
    with multiple different devpts instances - you could have a single
    /dev/ptmx node, and then have multiple mount namespaces with their own
    instances of devpts mounted on /dev/pts/.  And that's all perfectly sane
    in a model where we just look up the pts instance at open time.
    
    This will eventually allow us to get rid of our odd single-vs-multiple
    pts instance model, but this patch in itself changes no semantics, only
    an internal binding model.
    
    Cc: Eric Biederman <ebiederm@xmission.com>
    Cc: Peter Anvin <hpa@zytor.com>
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Peter Hurley <peter@hurleysoftware.com>
    Cc: Serge Hallyn <serge.hallyn@ubuntu.com>
    Cc: Willy Tarreau <w@1wt.eu>
    Cc: Aurelien Jarno <aurelien@aurel32.net>
    Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
    Cc: Jann Horn <jann@thejh.net>
    Cc: Greg KH <greg@kroah.com>
    Cc: Jiri Slaby <jslaby@suse.com>
    Cc: Florian Weimer <fw@deneb.enyo.de>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    67245ff3
pty.c 22.4 KB