[PATCH] loop: fput() called in loop_clr_fd() may cause bd_mutexrecursive locking

[Sergey Senozhatsky]

Unmonting mounted with `-o loop' block device causes recursive
bd_mutex locking. fput() calls blkdev_put() for bdev that issued 
disk->fops->release() (loop_clr_fd()) call:

[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


Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>

---

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 1e888c9..b004779 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1028,6 +1028,15 @@ static int loop_clr_fd(struct loop_device *lo)
 	 * lock dependency possibility warning as fput can take
 	 * bd_mutex which is usually taken before lo_ctl_mutex.
 	 */
+	/*
+	 * Need to put file f_op, otherwise fput() may cause
+	 * recursive locking on bd_mutex, calling blkdev_put()
+	 * for bdev that issued disk->fops->release() call.
+	 */
+	if (bdev && bdev == bdev->bd_contains) {
+		fops_put(filp->f_op);
+		filp->f_op = NULL;
+	}
 	fput(filp);
 	return 0;
 }


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