linux-kernel - Re: possible circular locking dependency detected

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Message-Id: <20100521141445.bae41292.akpm@linux-foundation.org>
Date:	Fri, 21 May 2010 14:14:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ciprian Docan <docan@...n.rutgers.edu>
Cc:	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	Tejun Heo <tj@...nel.org>
Subject: Re: possible circular locking dependency detected

On Thu, 20 May 2010 12:34:00 -0400 (EDT)
Ciprian Docan <docan@...n.rutgers.edu> wrote:

> 
> Hi,
> 
> I got the following in the dmesg:
> 
> --------------------------------------------------------------------
> [ INFO: possible circular locking dependency detected ]
> 2.6.33-rc8 #4
> -------------------------------------------------------
> fdisk/29231 is trying to acquire lock:
>   (&type->s_umount_key#47){++++..}, at: [<ffffffff810fb13c>] 
> get_super+0x5c/0xaf
> 
> but task is already holding lock:
>   (&bdev->bd_mutex){+.+.+.}, at: [<ffffffff811f2df0>] 
> blkdev_ioctl+0x5c5/0x6b1
> 
> which lock already depends on the new lock.
> 
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #1 (&bdev->bd_mutex){+.+.+.}:
>         [<ffffffff8106e65b>] __lock_acquire+0xb5d/0xd05
>         [<ffffffff8106e8cf>] lock_acquire+0xcc/0xe9
>         [<ffffffff81402d09>] __mutex_lock_common+0x4c/0x348
>         [<ffffffff814030c9>] mutex_lock_nested+0x3e/0x43
>         [<ffffffff8111f4a9>] __blkdev_put+0x34/0x16c
>         [<ffffffff8111f5f1>] blkdev_put+0x10/0x12
>         [<ffffffff8112063b>] close_bdev_exclusive+0x24/0x2d
>         [<ffffffff810fbcaa>] get_sb_bdev+0xef/0x1a1
>         [<ffffffffa0114189>] vfat_get_sb+0x18/0x1a [vfat]
>         [<ffffffff810fb8bc>] vfs_kern_mount+0xa9/0x168
>         [<ffffffff810fb9e3>] do_kern_mount+0x4d/0xed
>         [<ffffffff81110f54>] do_mount+0x72f/0x7a6
>         [<ffffffff81111053>] sys_mount+0x88/0xc2
>         [<ffffffff8100236b>] system_call_fastpath+0x16/0x1b

vfs_kern_mount() holds s_umount.  My brain isn't large enough to work
out where that lock was taken, yet it's so obvious that no code
comments were needed.  Sigh.  Might be down under sget().

vfs_kern_mount() ends up calling into __blkdev_put(), which takes
bd_mutex.

> -> #0 (&type->s_umount_key#47){++++..}:
>         [<ffffffff8106e505>] __lock_acquire+0xa07/0xd05
>         [<ffffffff8106e8cf>] lock_acquire+0xcc/0xe9
>         [<ffffffff81403450>] down_read+0x51/0x84
>         [<ffffffff810fb13c>] get_super+0x5c/0xaf
>         [<ffffffff8111facd>] fsync_bdev+0x18/0x48
>         [<ffffffff811f433c>] invalidate_partition+0x25/0x42
>         [<ffffffff8114bda2>] rescan_partitions+0x37/0x3a7
>         [<ffffffff811f2dff>] blkdev_ioctl+0x5d4/0x6b1
>         [<ffffffff8111eca4>] block_ioctl+0x37/0x3b
>         [<ffffffff811060d0>] vfs_ioctl+0x32/0xa6
>         [<ffffffff81106650>] do_vfs_ioctl+0x490/0x4d6
>         [<ffffffff811066ec>] sys_ioctl+0x56/0x79
>         [<ffffffff8100236b>] system_call_fastpath+0x16/0x1b

blkdev_reread_part() takes bd_mutex then does
	rescan_partitions
	->invalidate_partition
	  ->fsync_bdev
	    ->get_super  (takes s_umount for reading)

> other info that might help us debug this:
> 
> 1 lock held by fdisk/29231:
>   #0:  (&bdev->bd_mutex){+.+.+.}, at: [<ffffffff811f2df0>] 
> blkdev_ioctl+0x5c5/0x6b1
> 
> stack backtrace:
> Pid: 29231, comm: fdisk Not tainted 2.6.33-rc8 #4
> Call Trace:
>   [<ffffffff8106d6dc>] print_circular_bug+0xa8/0xb6
>   [<ffffffff8106e505>] __lock_acquire+0xa07/0xd05
>   [<ffffffff81062009>] ? sched_clock_local+0x1c/0x82
>   [<ffffffff8106e8cf>] lock_acquire+0xcc/0xe9
>   [<ffffffff810fb13c>] ? get_super+0x5c/0xaf
>   [<ffffffff8106b936>] ? lock_release_holdtime+0x2c/0xdb
>   [<ffffffff81403450>] down_read+0x51/0x84
>   [<ffffffff810fb13c>] ? get_super+0x5c/0xaf
>   [<ffffffff810fb13c>] get_super+0x5c/0xaf
>   [<ffffffff8111facd>] fsync_bdev+0x18/0x48
>   [<ffffffff811f433c>] invalidate_partition+0x25/0x42
>   [<ffffffff81402c8e>] ? mutex_trylock+0x12a/0x159
>   [<ffffffff8114bda2>] rescan_partitions+0x37/0x3a7
>   [<ffffffff8106d0c9>] ? trace_hardirqs_on+0xd/0xf
>   [<ffffffff811f2df0>] ? blkdev_ioctl+0x5c5/0x6b1
>   [<ffffffff811f2dff>] blkdev_ioctl+0x5d4/0x6b1
>   [<ffffffff8106d098>] ? trace_hardirqs_on_caller+0x118/0x13c
>   [<ffffffff8111eca4>] block_ioctl+0x37/0x3b
>   [<ffffffff811060d0>] vfs_ioctl+0x32/0xa6
>   [<ffffffff81106650>] do_vfs_ioctl+0x490/0x4d6
>   [<ffffffff811066ec>] sys_ioctl+0x56/0x79
>   [<ffffffff8102f9bd>] ? __wake_up+0x22/0x4d
>   [<ffffffff8100236b>] system_call_fastpath+0x16/0x1b
> -------------------------------------------------------------------------
> 
> Kernel version used: 2.6.33-rc8 #4. I do not remember the exact steps, but 
> I was trying to format an USB stick using the fdisk. Please let me know if 
> you need additional informations. Thank you.
> 

So yup, that's ab/ba deadlockable.  I cannot immediately see any change
which might have caused that.  Tejun has been mucking with the
partitions code recently but nothing leaps out at me.

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