Since commit 89c8d91e31 ("tty: localise the lock") I see a dead lock
in one of my dummy_hcd + g_nokia test cases. The first run was usually
okay, the second often resulted in a splat by lockdep and the third was
usually a dead lock.
Lockdep complained about tty->hangup_work and tty->legacy_mutex taken
both ways:
| ======================================================
| [ INFO: possible circular locking dependency detected ]
| 3.7.0-rc6+ #204 Not tainted
| -------------------------------------------------------
| kworker/2:1/35 is trying to acquire lock:
| (&tty->legacy_mutex){+.+.+.}, at: [<c14051e6>] tty_lock_nested+0x36/0x80
|
| but task is already holding lock:
| ((&tty->hangup_work)){+.+...}, at: [<c104f6e4>] process_one_work+0x124/0x5e0
|
| which lock already depends on the new lock.
|
| the existing dependency chain (in reverse order) is:
|
| -> #2 ((&tty->hangup_work)){+.+...}:
| [<c107fe74>] lock_acquire+0x84/0x190
| [<c104d82d>] flush_work+0x3d/0x240
| [<c12e6986>] tty_ldisc_flush_works+0x16/0x30
| [<c12e7861>] tty_ldisc_release+0x21/0x70
| [<c12e0dfc>] tty_release+0x35c/0x470
| [<c1105e28>] __fput+0xd8/0x270
| [<c1105fcd>] ____fput+0xd/0x10
| [<c1051dd9>] task_work_run+0xb9/0xf0
| [<c1002a51>] do_notify_resume+0x51/0x80
| [<c140550a>] work_notifysig+0x35/0x3b
|
| -> #1 (&tty->legacy_mutex/1){+.+...}:
| [<c107fe74>] lock_acquire+0x84/0x190
| [<c140276c>] mutex_lock_nested+0x6c/0x2f0
| [<c14051e6>] tty_lock_nested+0x36/0x80
| [<c1405279>] tty_lock_pair+0x29/0x70
| [<c12e0bb8>] tty_release+0x118/0x470
| [<c1105e28>] __fput+0xd8/0x270
| [<c1105fcd>] ____fput+0xd/0x10
| [<c1051dd9>] task_work_run+0xb9/0xf0
| [<c1002a51>] do_notify_resume+0x51/0x80
| [<c140550a>] work_notifysig+0x35/0x3b
|
| -> #0 (&tty->legacy_mutex){+.+.+.}:
| [<c107f3c9>] __lock_acquire+0x1189/0x16a0
| [<c107fe74>] lock_acquire+0x84/0x190
| [<c140276c>] mutex_lock_nested+0x6c/0x2f0
| [<c14051e6>] tty_lock_nested+0x36/0x80
| [<c140523f>] tty_lock+0xf/0x20
| [<c12df8e4>] __tty_hangup+0x54/0x410
| [<c12dfcb2>] do_tty_hangup+0x12/0x20
| [<c104f763>] process_one_work+0x1a3/0x5e0
| [<c104fec9>] worker_thread+0x119/0x3a0
| [<c1055084>] kthread+0x94/0xa0
| [<c140ca37>] ret_from_kernel_thread+0x1b/0x28
|
|other info that might help us debug this:
|
|Chain exists of:
| &tty->legacy_mutex --> &tty->legacy_mutex/1 --> (&tty->hangup_work)
|
| Possible unsafe locking scenario:
|
| CPU0 CPU1
| ---- ----
| lock((&tty->hangup_work));
| lock(&tty->legacy_mutex/1);
| lock((&tty->hangup_work));
| lock(&tty->legacy_mutex);
|
| *** DEADLOCK ***
Before the path mentioned tty_ldisc_release() look like this:
| tty_ldisc_halt(tty);
| tty_ldisc_flush_works(tty);
| tty_lock();
As it can be seen, it first flushes the workqueue and then grabs the
tty_lock. Now we grab the lock first:
| tty_lock_pair(tty, o_tty);
| tty_ldisc_halt(tty);
| tty_ldisc_flush_works(tty);
so lockdep's complaint seems valid.
The earlier version of this patch took the ldisc_mutex since the other
user of tty_ldisc_flush_works() (tty_set_ldisc()) did this.
Peter Hurley then said that it is should not be requried. Since it
wasn't done earlier, I dropped this part.
The code under tty_ldisc_kill() was executed earlier with the tty lock
taken so it is taken again.
I was able to reproduce the deadlock on v3.8-rc1, this patch fixes the
problem in my testcase. I didn't notice any problems so far.
Cc: Alan Cox <alan@linux.intel.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
852e4a8152