linux-kernel - Re: [PATCH] loop: fput() called in loop_clr_fd() may cause bd

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Date:	Sun, 18 Dec 2011 01:19:28 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kay Sievers <kay.sievers@...y.org>,
	Namhyung Kim <namhyung@...il.com>,
	Lukas Czerner <lczerner@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] loop: fput() called in loop_clr_fd() may cause bd_mutex
 recursive locking

On (12/17/11 22:12), Al Viro wrote:
> > 
> > [23044.654647] umount/24442 is trying to acquire lock:
> > [23044.654652]  (&bdev->bd_mutex){+.+.+.}, at: [<ffffffff81144311>] blkdev_put+0x1f/0x131
> > [23044.654670] 
> > [23044.654672] but task is already holding lock:
> > [23044.654677]  (&bdev->bd_mutex){+.+.+.}, at: [<ffffffff811441a1>] __blkdev_put+0x33/0x184
> > [23044.654690] 
> > [23044.654692] other info that might help us debug this:
> > [23044.654697]  Possible unsafe locking scenario:
> > [23044.654727] 
> > [23044.654731] 1 lock held by umount/24442:
> > [23044.654735]  #0:  (&bdev->bd_mutex){+.+.+.}, at: [<ffffffff811441a1>] __blkdev_put+0x33/0x184
> > [23044.654748] 
> > [23044.654762] Call Trace:
> > [23044.654773]  [<ffffffff81075611>] __lock_acquire+0x15bf/0x1659
> > [23044.654784]  [<ffffffff8114b3e3>] ? inotify_free_group_priv+0x4f/0x4f
> > [23044.654792]  [<ffffffff81144311>] ? blkdev_put+0x1f/0x131
> > [23044.654799]  [<ffffffff81075c6a>] lock_acquire+0x138/0x1b3
> > [23044.654807]  [<ffffffff81144311>] ? blkdev_put+0x1f/0x131
> > [23044.654814]  [<ffffffff81144311>] ? blkdev_put+0x1f/0x131
> > [23044.654824]  [<ffffffff8147ce67>] mutex_lock_nested+0x5e/0x325
> > [23044.654831]  [<ffffffff81144311>] ? blkdev_put+0x1f/0x131
> > [23044.654838]  [<ffffffff81149963>] ? fsnotify+0x441/0x459
> > [23044.654846]  [<ffffffff81144311>] blkdev_put+0x1f/0x131
> > [23044.654853]  [<ffffffff81144443>] blkdev_close+0x20/0x22
> > [23044.654863]  [<ffffffff81116b21>] fput+0x117/0x1cf
> > [23044.654874]  [<ffffffffa016eb71>] loop_clr_fd+0x1f2/0x201 [loop]
> > [23044.654882]  [<ffffffffa016f861>] lo_release+0x40/0x6f [loop]
> > [23044.654890]  [<ffffffff81144244>] __blkdev_put+0xd6/0x184
> > [23044.654898]  [<ffffffff8114441a>] blkdev_put+0x128/0x131
> > [23044.654906]  [<ffffffff8111704e>] kill_block_super+0x60/0x65
> > [23044.654914]  [<ffffffff81117366>] deactivate_locked_super+0x32/0x63
> > [23044.654922]  [<ffffffff81117cc9>] deactivate_super+0x3a/0x3e
> > [23044.654931]  [<ffffffff8112fc5d>] mntput_no_expire+0xbf/0xc4
> > [23044.654939]  [<ffffffff811309c7>] sys_umount+0x2c5/0x2f3
> > [23044.654949]  [<ffffffff81484b12>] system_call_fastpath+0x16/0x1b
[..]
> 
> NAK - you've "fixed" a false positive from lock checker by failing to close
> the underlying device.
> 

Sorry, why is that a false positive?

blkdev_put() calls lo_release() while holding bd_mutex,
lo_release() calls loop_clr_fd() -> fput(). fput() once again
attempts to grub already held bd_mutex calling blkdev_put().
Looks like a recursion to me.


	Sergey
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