3.9+ tty lockdep trace.

[Dave Jones]

[ 6760.076083] ======================================================
[ 6760.076084] [ INFO: possible circular locking dependency detected ]
[ 6760.076086] 3.9.0+ #16 Not tainted
[ 6760.076086] -------------------------------------------------------
[ 6760.076087] (agetty)/26163 is trying to acquire lock:
[ 6760.076096] blocked:  ((&buf->work)){+.+...}, instance: ffff88011c8b0020, at: [<ffffffff81062065>] flush_work+0x5/0x2e0
[ 6760.076096] 
but task is already holding lock:
[ 6760.076102] blocked:  (console_lock){+.+.+.}, instance: ffffffff81c2fde0, at: [<ffffffff813bc201>] vt_ioctl+0xb61/0x1230
[ 6760.076103] 
which lock already depends on the new lock.

[ 6760.076103] 
the existing dependency chain (in reverse order) is:
[ 6760.076106] 
-> #1 (console_lock){+.+.+.}:
[ 6760.076111]        [<ffffffff810b3f74>] lock_acquire+0xa4/0x210
[ 6760.076115]        [<ffffffff810416c7>] console_lock+0x77/0x80
[ 6760.076118]        [<ffffffff813c3dcd>] con_flush_chars+0x2d/0x50
[ 6760.076123]        [<ffffffff813b32b2>] n_tty_receive_buf+0x122/0x14d0
[ 6760.076126]        [<ffffffff813b7709>] flush_to_ldisc+0x119/0x170
[ 6760.076130]        [<ffffffff81064381>] process_one_work+0x211/0x700
[ 6760.076133]        [<ffffffff8106498b>] worker_thread+0x11b/0x3a0
[ 6760.076137]        [<ffffffff8106ce5d>] kthread+0xed/0x100
[ 6760.076141]        [<ffffffff81601cac>] ret_from_fork+0x7c/0xb0
[ 6760.076143] 
-> #0 ((&buf->work)){+.+...}:
[ 6760.076146]        [<ffffffff810b349a>] __lock_acquire+0x193a/0x1c00
[ 6760.076149]        [<ffffffff810b3f74>] lock_acquire+0xa4/0x210
[ 6760.076151]        [<ffffffff810620ae>] flush_work+0x4e/0x2e0
[ 6760.076153]        [<ffffffff81065305>] __cancel_work_timer+0x95/0x130
[ 6760.076155]        [<ffffffff810653b0>] cancel_work_sync+0x10/0x20
[ 6760.076157]        [<ffffffff813b8212>] tty_port_destroy+0x12/0x20
[ 6760.076159]        [<ffffffff813c65e8>] vc_deallocate+0xf8/0x110
[ 6760.076162]        [<ffffffff813bc20c>] vt_ioctl+0xb6c/0x1230
[ 6760.076165]        [<ffffffff813b01a5>] tty_ioctl+0x285/0xd50
[ 6760.076168]        [<ffffffff811ba825>] do_vfs_ioctl+0x305/0x530
[ 6760.076170]        [<ffffffff811baad1>] sys_ioctl+0x81/0xa0
[ 6760.076174]        [<ffffffff81601d59>] system_call_fastpath+0x16/0x1b
[ 6760.076174] 
other info that might help us debug this:

[ 6760.076175]  Possible unsafe locking scenario:

[ 6760.076175]        CPU0                    CPU1
[ 6760.076176]        ----                    ----
[ 6760.076177]   lock(console_lock);
[ 6760.076179]                                lock((&buf->work));
[ 6760.076180]                                lock(console_lock);
[ 6760.076181]   lock((&buf->work));
[ 6760.076182] 
 *** DEADLOCK ***

[ 6760.076183] 1 lock on stack by (agetty)/26163:
[ 6760.076188]  #0: blocked:  (console_lock){+.+.+.}, instance: ffffffff81c2fde0, at: [<ffffffff813bc201>] vt_ioctl+0xb61/0x1230
[ 6760.076188] 
stack backtrace:
[ 6760.076190] Pid: 26163, comm: (agetty) Not tainted 3.9.0+ #16
[ 6760.076191] Call Trace:
[ 6760.076196]  [<ffffffff815edb14>] print_circular_bug+0x200/0x20e
[ 6760.076199]  [<ffffffff810b349a>] __lock_acquire+0x193a/0x1c00
[ 6760.076203]  [<ffffffff8100a269>] ? sched_clock+0x9/0x10
[ 6760.076206]  [<ffffffff8100a269>] ? sched_clock+0x9/0x10
[ 6760.076209]  [<ffffffff8100a200>] ? native_sched_clock+0x20/0x80
[ 6760.076213]  [<ffffffff810b3f74>] lock_acquire+0xa4/0x210
[ 6760.076215]  [<ffffffff81062065>] ? flush_work+0x5/0x2e0
[ 6760.076217]  [<ffffffff810620ae>] flush_work+0x4e/0x2e0
[ 6760.076219]  [<ffffffff81062065>] ? flush_work+0x5/0x2e0
[ 6760.076222]  [<ffffffff810b15db>] ? mark_held_locks+0xbb/0x140
[ 6760.076226]  [<ffffffff8113c8a3>] ? __free_pages_ok.part.57+0x93/0xc0
[ 6760.076229]  [<ffffffff810b15db>] ? mark_held_locks+0xbb/0x140
[ 6760.076231]  [<ffffffff810652f2>] ? __cancel_work_timer+0x82/0x130
[ 6760.076234]  [<ffffffff81065305>] __cancel_work_timer+0x95/0x130
[ 6760.076236]  [<ffffffff810653b0>] cancel_work_sync+0x10/0x20
[ 6760.076238]  [<ffffffff813b8212>] tty_port_destroy+0x12/0x20
[ 6760.076240]  [<ffffffff813c65e8>] vc_deallocate+0xf8/0x110
[ 6760.076243]  [<ffffffff813bc20c>] vt_ioctl+0xb6c/0x1230
[ 6760.076246]  [<ffffffff810aec41>] ? lock_release_holdtime.part.30+0xa1/0x170
[ 6760.076250]  [<ffffffff813b01a5>] tty_ioctl+0x285/0xd50
[ 6760.076254]  [<ffffffff812b00f6>] ? inode_has_perm.isra.46.constprop.61+0x56/0x80
[ 6760.076257]  [<ffffffff811ba825>] do_vfs_ioctl+0x305/0x530
[ 6760.076260]  [<ffffffff812b04db>] ? selinux_file_ioctl+0x5b/0x110
[ 6760.076263]  [<ffffffff811baad1>] sys_ioctl+0x81/0xa0
[ 6760.076266]  [<ffffffff81601d59>] system_call_fastpath+0x16/0x1b

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