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authorSebastian Andrzej Siewior <bigeasy@linutronix.de>2012-12-25 23:02:48 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-04-16 21:48:30 -0700
commit77d0ca8b5d7e7cd48005ddb79b05c79082a68bb5 (patch)
tree3c964724ffd40535d1169a0d1fc92c2f3225497f
parent00c33275b78637fc5423ce05dd5e886836544eb1 (diff)
tty: don't deadlock while flushing workqueue
commit 852e4a8152b427c3f318bb0e1b5e938d64dcdc32 upstream. Since commit 89c8d91e31f2 ("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. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Cc: Alan Cox <alan@linux.intel.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: Bryan O'Donoghue <bryan.odonoghue.lkml@nexus-software.ie> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--drivers/tty/tty_ldisc.c10
1 files changed, 5 insertions, 5 deletions
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index c5782294e532..78f1be2e674b 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -934,17 +934,17 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
* race with the set_ldisc code path.
*/
- tty_lock_pair(tty, o_tty);
tty_ldisc_halt(tty);
- tty_ldisc_flush_works(tty);
- if (o_tty) {
+ if (o_tty)
tty_ldisc_halt(o_tty);
+
+ tty_ldisc_flush_works(tty);
+ if (o_tty)
tty_ldisc_flush_works(o_tty);
- }
+ tty_lock_pair(tty, o_tty);
/* This will need doing differently if we need to lock */
tty_ldisc_kill(tty);
-
if (o_tty)
tty_ldisc_kill(o_tty);