tty_lock lockdep warning

[John Stultz]

Hey Alan,
	I've been seeing the following lockdep warning w/ Linus' 3.5-rc HEAD 
from today. This is running x86_64 in a kvm environment.

Let me know if there are any other details you'd like from me.

thanks
-john


=============================================
[ INFO: possible recursive locking detected ]
3.4.0john+ #347 Not tainted
---------------------------------------------
plymouthd/1345 is trying to acquire lock:
 (&tty->legacy_mutex){+.+.+.}, at: [<ffffffff81b10091>] tty_lock+0x61/0xb0

but task is already holding lock:
 (&tty->legacy_mutex){+.+.+.}, at: [<ffffffff81b10091>] tty_lock+0x61/0xb0

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&tty->legacy_mutex);
  lock(&tty->legacy_mutex);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

2 locks held by plymouthd/1345:
 #0:  (tty_mutex){+.+.+.}, at: [<ffffffff81417122>] tty_release+0x1e2/0x600
 #1:  (&tty->legacy_mutex){+.+.+.}, at: [<ffffffff81b10091>] 
tty_lock+0x61/0xb0

stack backtrace:
Pid: 1345, comm: plymouthd Not tainted 3.4.0john+ #347
Call Trace:
 [<ffffffff81044727>] ? vprintk_emit+0x287/0x5d0
 [<ffffffff810b63b6>] validate_chain+0xf36/0x1380
 [<ffffffff8108b433>] ? sched_clock_local+0x43/0xb0
 [<ffffffff8108b6df>] ? local_clock+0x6f/0x80
 [<ffffffff810b0ee9>] ? trace_hardirqs_off_caller+0x29/0x140
 [<ffffffff810b6c3e>] __lock_acquire+0x43e/0xb80
 [<ffffffff810b7431>] lock_acquire+0xb1/0x210
 [<ffffffff81b10091>] ? tty_lock+0x61/0xb0
 [<ffffffff81b0c273>] mutex_lock_nested+0x73/0x4b0
 [<ffffffff81b10091>] ? tty_lock+0x61/0xb0
 [<ffffffff81b10091>] ? tty_lock+0x61/0xb0
 [<ffffffff81b10091>] tty_lock+0x61/0xb0
 [<ffffffff81b1013d>] tty_lock_pair+0x5d/0x64
 [<ffffffff8141712d>] tty_release+0x1ed/0x600
 [<ffffffff811851e8>] fput+0x138/0x2e0
 [<ffffffff81416d18>] tty_ioctl+0xb68/0xd40
 [<ffffffff81b0fee1>] ? _raw_spin_unlock_irqrestore+0x71/0xa0
 [<ffffffff81198d94>] do_vfs_ioctl+0xa4/0x5c0
 [<ffffffff8141d12c>] ? put_ldisc+0x11c/0x190
 [<ffffffff8141d1de>] ? tty_ldisc_deref+0xe/0x10
 [<ffffffff81413887>] ? tty_compat_ioctl+0xc7/0xf0
 [<ffffffff811ded87>] compat_sys_ioctl+0x257/0x1220
 [<ffffffff81b12138>] ? sysenter_dispatch+0x19/0x26
 [<ffffffff81392ffd>] ? trace_hardirqs_off_thunk+0x3a/0x3c
 [<ffffffff81b12126>] sysenter_dispatch+0x7/0x26
 [<ffffffff81392fbe>] ? trace_hardirqs_on_thunk+0x3a/0x3f

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