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Subjectmq: INFO: possible circular locking dependency detected
Hi all,

While fuzzing with trinity inside a KVM tools guest, using latest -next kernel, I've stumbled on the dump below.

I think this is the result of commit 765927b2 ("switch dentry_open() to struct path, make it grab references itself").

[ 62.090519] ======================================================
[ 62.091016] [ INFO: possible circular locking dependency detected ]
[ 62.091016] 3.6.0-rc1-next-20120803-sasha #544 Tainted: G W
[ 62.091016] -------------------------------------------------------
[ 62.091016] trinity-child0/6077 is trying to acquire lock:
[ 62.091016] (&sb->s_type->i_mutex_key#14){+.+.+.}, at: [<ffffffff8127c074>] vfs_unlink+0x54/0x100
[ 62.091016]
[ 62.091016] but task is already holding lock:
sb_writers#8){.+.+.+}, at: [<ffffffff812900bf>] mnt_want_write+0x1f/0x50
[ 62.097920]
[ 62.097920] which lock already depends on the new lock.
[ 62.097920]
[ 62.097920]
[ 62.097920] the existing dependency chain (in reverse order) is:
[ 62.097920]
-> #1 (sb_writers#8){.+.+.+}:
[ 62.097920] [<ffffffff8117b58e>] validate_chain+0x69e/0x790
[ 62.097920] [<ffffffff8117baa3>] __lock_acquire+0x423/0x4c0
[ 62.097920] [<ffffffff8117bcca>] lock_acquire+0x18a/0x1e0
[ 62.097920] [<ffffffff81271282>] __sb_start_write+0x192/0x1f0
[ 62.097920] [<ffffffff812900bf>] mnt_want_write+0x1f/0x50
[ 62.097920] [<ffffffff818de4f8>] do_create+0xe8/0x160
[ 62.097920] [<ffffffff818de79b>] sys_mq_open+0x1ab/0x2a0
[ 62.097920] [<ffffffff83749379>] system_call_fastpath+0x16/0x1b
[ 62.097920]
-> #0 (&sb->s_type->i_mutex_key#14){+.+.+.}:
[ 62.097920] [<ffffffff8117ab3f>] check_prev_add+0x11f/0x4d0
[ 62.097920] [<ffffffff8117b58e>] validate_chain+0x69e/0x790
[ 62.097920] [<ffffffff8117baa3>] __lock_acquire+0x423/0x4c0
[ 62.097920] [<ffffffff8117bcca>] lock_acquire+0x18a/0x1e0
[ 62.097920] [<ffffffff83744db0>] __mutex_lock_common+0x60/0x590
[ 62.097920] [<ffffffff83745410>] mutex_lock_nested+0x40/0x50
[ 62.097920] [<ffffffff8127c074>] vfs_unlink+0x54/0x100
[ 62.097920] [<ffffffff818de3ab>] sys_mq_unlink+0xfb/0x160
[ 62.097920] [<ffffffff83749379>] system_call_fastpath+0x16/0x1b
[ 62.097920]
[ 62.097920] other info that might help us debug this:
[ 62.097920]
[ 62.097920] Possible unsafe locking scenario:
[ 62.097920]
[ 62.097920] CPU0 CPU1
[ 62.097920] ---- ----
[ 62.097920] lock(sb_writers#8);
[ 62.097920] lock(&sb->s_type->i_mutex_key#14);
[ 62.097920] lock(sb_writers#8);
[ 62.097920] lock(&sb->s_type->i_mutex_key#14);
[ 62.097920]
[ 62.097920] *** DEADLOCK ***
[ 62.097920]
[ 62.097920] 2 locks held by trinity-child0/6077:
[ 62.097920] #0: (&sb->s_type->i_mutex_key#13/1){+.+.+.}, at: [<ffffffff818de31f>] sys_mq_unlink+0x6f/0x160
[ 62.097920] #1: (sb_writers#8){.+.+.+}, at: [<ffffffff812900bf>] mnt_want_write+0x1f/0x50
[ 62.097920]
[ 62.097920] stack backtrace:
[ 62.097920] Pid: 6077, comm: trinity-child0 Tainted: G W 3.6.0-rc1-next-20120803-sasha #544
[ 62.097920] Call Trace:
[ 62.097920] [<ffffffff81178b25>] print_circular_bug+0x105/0x120
[ 62.097920] [<ffffffff8117ab3f>] check_prev_add+0x11f/0x4d0
[ 62.097920] [<ffffffff8117b58e>] validate_chain+0x69e/0x790
[ 62.097920] [<ffffffff8114ed58>] ? sched_clock_cpu+0x108/0x120
[ 62.097920] [<ffffffff8117baa3>] __lock_acquire+0x423/0x4c0
[ 62.097920] [<ffffffff8117bcca>] lock_acquire+0x18a/0x1e0
[ 62.097920] [<ffffffff8127c074>] ? vfs_unlink+0x54/0x100
[ 62.097920] [<ffffffff83744db0>] __mutex_lock_common+0x60/0x590
[ 62.097920] [<ffffffff8127c074>] ? vfs_unlink+0x54/0x100
[ 62.097920] [<ffffffff81271296>] ? __sb_start_write+0x1a6/0x1f0
[ 62.097920] [<ffffffff8127b2ad>] ? generic_permission+0x2d/0x140
[ 62.097920] [<ffffffff8127c074>] ? vfs_unlink+0x54/0x100
[ 62.097920] [<ffffffff83745410>] mutex_lock_nested+0x40/0x50
[ 62.097920] [<ffffffff8127c074>] vfs_unlink+0x54/0x100
[ 62.097920] [<ffffffff818de3ab>] sys_mq_unlink+0xfb/0x160
[ 62.097920] [<ffffffff83749379>] system_call_fastpath+0x16/0x1b


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