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Subject: lockdep spew from tty
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linux Kernel list <linux-kernel@vger.kernel.org>
Date: Tue, 21 May 2013 11:22:00 +1000
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Hi Greg !

Caught that on a console today running some 3.10-almost-rc2
(based on ec50f2a97a4a7098a81b40030e0bfe28bdc43740). Right now I don't
have the bandwidth to investigate but I though you might be
interested :-)

I'll take another peek if it happens again.

======================================================
[ INFO: possible circular locking dependency detected ]
3.10.0-rc1-test #19 Not tainted
-------------------------------------------------------
kworker/24:1/1089 is trying to acquire lock:
 (&ldata->output_lock){+.+...}, at: [<c00000000037aa0c>] .process_echoes+0x34/0x2ec

but task is already holding lock:
 ((&buf->work)){+.+...}, at: [<c0000000000778b4>] .process_one_work+0x1f8/0x43c

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 ((&buf->work)){+.+...}:
       [<c00000000007714c>] .flush_work+0x38/0x258
       [<c0000000000781e4>] .__cancel_work_timer+0xe0/0x140
       [<c0000000003820c4>] .tty_port_destroy+0x14/0x2c
       [<c000000000390d88>] .vc_deallocate+0xfc/0x128
       [<c000000000385d8c>] .vt_ioctl+0xae4/0x13a4
       [<c00000000037a218>] .tty_ioctl+0xd1c/0xe68
       [<c0000000001403b0>] .vfs_ioctl+0x44/0x6c
       [<c000000000140e54>] .do_vfs_ioctl+0x614/0x6ac
       [<c000000000140f30>] .SyS_ioctl+0x44/0x70
       [<c000000000009ca4>] syscall_exit+0x0/0x98

-> #1 (console_lock){+.+.+.}:
       [<c00000000005aeec>] .console_lock+0x80/0x98
       [<c00000000039180c>] .do_con_write.part.16+0x3c/0x1fb0
       [<c0000000003937ec>] .con_write+0x28/0x40
       [<c00000000037b344>] .n_tty_write+0x28c/0x424
       [<c000000000377c84>] .tty_write+0x184/0x238
       [<c00000000012f0ec>] .vfs_write+0xd4/0x1cc
       [<c00000000012f5d0>] .SyS_write+0x48/0x7c
       [<c000000000009ca4>] syscall_exit+0x0/0x98

-> #0 (&ldata->output_lock){+.+...}:
       [<c0000000000a4dc4>] .lock_acquire+0x54/0x70
       [<c000000000705780>] .mutex_lock_nested+0x9c/0x4d4
       [<c00000000037aa0c>] .process_echoes+0x34/0x2ec
       [<c00000000037cc04>] .n_tty_receive_buf+0xc64/0xf90
       [<c000000000380d3c>] .flush_to_ldisc+0x110/0x1ac
       [<c00000000007793c>] .process_one_work+0x280/0x43c
       [<c000000000077d10>] .worker_thread+0x1e0/0x324
       [<c00000000007e360>] .kthread+0xc8/0xd4
       [<c000000000009fac>] .ret_from_kernel_thread+0x5c/0xb0

other info that might help us debug this:

Chain exists of:
  &ldata->output_lock --> console_lock --> (&buf->work)

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock((&buf->work));
                               lock(console_lock);
                               lock((&buf->work));
  lock(&ldata->output_lock);

 *** DEADLOCK ***

2 locks held by kworker/24:1/1089:
 #0:  (events){.+.+.+}, at: [<c0000000000778b4>] .process_one_work+0x1f8/0x43c
 #1:  ((&buf->work)){+.+...}, at: [<c0000000000778b4>] .process_one_work+0x1f8/0x43c

stack backtrace:
CPU: 24 PID: 1089 Comm: kworker/24:1 Not tainted 3.10.0-rc1-test #19
Workqueue: events .flush_to_ldisc
Call Trace:
[c000003ed7c37350] [c000000000011b18] .show_stack+0x50/0x14c (unreliable)
[c000003ed7c37420] [c00000000070eb90] .dump_stack+0x28/0x3c
[c000003ed7c37490] [c00000000070b16c] .print_circular_bug+0x364/0x374
[c000003ed7c37540] [c0000000000a4088] .__lock_acquire+0x14d8/0x1d08
[c000003ed7c37690] [c0000000000a4dc4] .lock_acquire+0x54/0x70
[c000003ed7c37720] [c000000000705780] .mutex_lock_nested+0x9c/0x4d4
[c000003ed7c37830] [c00000000037aa0c] .process_echoes+0x34/0x2ec
[c000003ed7c378f0] [c00000000037cc04] .n_tty_receive_buf+0xc64/0xf90
[c000003ed7c37aa0] [c000000000380d3c] .flush_to_ldisc+0x110/0x1ac
[c000003ed7c37b60] [c00000000007793c] .process_one_work+0x280/0x43c
[c000003ed7c37c20] [c000000000077d10] .worker_thread+0x1e0/0x324
[c000003ed7c37cd0] [c00000000007e360] .kthread+0xc8/0xd4

Cheers,
Ben.