[PATCH] lockdep: fix blkdev_open() warning

[PATCH] lockdep: fix blkdev_open() warning

On Wed, 2006-08-09 at 07:57 +0200, Rolf Eike Beer wrote:
> =============================================
> [ INFO: possible recursive locking detected ]
> ---------------------------------------------
> parted/7929 is trying to acquire lock:
>  (&bdev->bd_mutex){--..}, at: [<c105eb8d>] __blkdev_put+0x1e/0x13c
>
> but task is already holding lock:
>  (&bdev->bd_mutex){--..}, at: [<c105eec6>] do_open+0x72/0x3a8
>
> other info that might help us debug this:
> 1 lock held by parted/7929:
>  #0:  (&bdev->bd_mutex){--..}, at: [<c105eec6>] do_open+0x72/0x3a8
> stack backtrace:
>  [<c1003aad>] show_trace_log_lvl+0x58/0x15b
>  [<c100495f>] show_trace+0xd/0x10
>  [<c1004979>] dump_stack+0x17/0x1a
>  [<c102dee5>] __lock_acquire+0x753/0x99c
>  [<c102e3b0>] lock_acquire+0x4a/0x6a
>  [<c1204501>] mutex_lock_nested+0xc8/0x20c
>  [<c105eb8d>] __blkdev_put+0x1e/0x13c
>  [<c105ecc4>] blkdev_put+0xa/0xc
>  [<c105f18a>] do_open+0x336/0x3a8
>  [<c105f21b>] blkdev_open+0x1f/0x4c
>  [<c1057b40>] __dentry_open+0xc7/0x1aa
>  [<c1057c91>] nameidata_to_filp+0x1c/0x2e
>  [<c1057cd1>] do_filp_open+0x2e/0x35
>  [<c1057dd7>] do_sys_open+0x38/0x68
>  [<c1057e33>] sys_open+0x16/0x18
>  [<c1002845>] sysenter_past_esp+0x56/0x8d

OK, I'm having a look here; its all new to me so bear with me.

blkdev_open() calls
  do_open(bdev, ...,BD_MUTEX_NORMAL) and takes
    mutex_lock_nested(&bdev->bd_mutex, BD_MUTEX_NORMAL)

then something fails, and we're thrown to:

out_first: where
    if (bdev != bdev->bd_contains)
      blkdev_put(bdev->bd_contains) which is
        __blkdev_put(bdev->bd_contains, BD_MUTEX_NORMAL) which does
          mutex_lock_nested(&bdev->bd_contains->bd_mutex, BD_MUTEX_NORMAL) <--- lockdep trigger

When going to out_first, dbev->bd_contains is either bdev or whole, and
since we take the branch it must be whole. So it seems to me the
following patch would be the right one:

[akpm@osdl.org: compile fix]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>