[PATCH v5 27/44] tty: Drop lock contention stat from ldsem trylocks

[Peter Hurley]

When lockdep is notified of lock contention, lockdep expects its
internal state to indicate the lock is held. Since trylocks cannot
reflect this state unless the lock is actually acquired, contention
stats cannot be collected for trylocks.

Fixes:
[ 1473.912280] =================================
[ 1473.913180] [ BUG: bad contention detected! ]
[ 1473.914071] 3.8.0-next-20130220-sasha-00038-g1ad55df-dirty #8 Tainted: G        W
[ 1473.915684] ---------------------------------
[ 1473.916549] kworker/1:1/361 is trying to contend lock (&tty->ldisc_sem) at:
[ 1473.918031] [<ffffffff81c493df>] tty_ldisc_ref+0x1f/0x60
[ 1473.919060] but there are no locks held!
[ 1473.919813]
[ 1473.919813] other info that might help us debug this:
[ 1473.920044] 2 locks held by kworker/1:1/361:
[ 1473.920044]  #0:  (events){.+.+.+}, at: [<ffffffff811328a8>] process_one_work+0x228/0x6a0
[ 1473.920044]  #1:  ((&buf->work)){+.+...}, at: [<ffffffff811328a8>] process_one_work+0x228/0x6a0
[ 1473.920044]
[ 1473.920044] stack backtrace:
[ 1473.920044] Pid: 361, comm: kworker/1:1 Tainted: G        W    3.8.0-next-20130220-sasha-00038-g1ad55df-dirty #8
[ 1473.920044] Call Trace:
[ 1473.920044]  [<ffffffff81c493df>] ? tty_ldisc_ref+0x1f/0x60
[ 1473.920044]  [<ffffffff81182026>] print_lock_contention_bug+0xf6/0x110
[ 1473.920044]  [<ffffffff81184973>] lock_contended+0x213/0x4e0
[ 1473.920044]  [<ffffffff81c4bb41>] ldsem_down_read_trylock+0xb1/0xc0
[ 1473.920044]  [<ffffffff81c493df>] tty_ldisc_ref+0x1f/0x60
[ 1473.920044]  [<ffffffff81c4a687>] flush_to_ldisc+0x37/0x1a0
[ 1473.920044]  [<ffffffff811329e6>] process_one_work+0x366/0x6a0
[ 1473.920044]  [<ffffffff811328a8>] ? process_one_work+0x228/0x6a0
[ 1473.920044]  [<ffffffff811332a8>] worker_thread+0x238/0x370
[ 1473.920044]  [<ffffffff81133070>] ? rescuer_thread+0x310/0x310
[ 1473.920044]  [<ffffffff8113d873>] kthread+0xe3/0xf0
[ 1473.920044]  [<ffffffff8113d790>] ? flush_kthread_work+0x1f0/0x1f0
[ 1473.920044]  [<ffffffff83dab4fc>] ret_from_fork+0x7c/0xb0
[ 1473.920044]  [<ffffffff8113d790>] ? flush_kthread_work+0x1f0/0x1f0

Reported-by: Sasha Levin <levinsasha928@xxxxxxxxx>
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
 drivers/tty/tty_ldsem.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
index 0ab5b09..c162295 100644
--- a/drivers/tty/tty_ldsem.c
+++ b/drivers/tty/tty_ldsem.c
@@ -419,7 +419,6 @@ int ldsem_down_read_trylock(struct ld_semaphore *sem)
 			return 1;
 		}
 	}
-	lock_stat(sem, contended);
 	return 0;
 }
 
@@ -453,9 +452,7 @@ int ldsem_down_write_trylock(struct ld_semaphore *sem)
 		lock_stat(sem, acquired);
 		return 1;
 	}
-
 	raw_spin_unlock_irq(&sem->wait_lock);
-	lock_stat(sem, contended);
 	return 0;
 }
 
-- 
1.8.1.2

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