Messages in this thread Patch in this message | | | From | Peter Hurley <> | Subject | [PATCH 0/1] Fix hang report | Date | Wed, 11 Dec 2013 21:11:57 -0500 |
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Greg,
I know it's late in the -rc cycle but I'd like to get this fix into 3.13. Although it's only likely to happen at shutdown/reboot, the hang frequency could be as often as 1 in 10000.
The patch fixes the count value returned when the cmpxchg() has successfully changed the count. Only one code path checks the returned count when the cmpxchg() is successful; down_read_failed(). After failed down_read attempt is reversed but before the reader waits for the lock, the new count is checked to ensure _someone_ has the lock:
/* if there are no active locks, wake the new lock owner(s) */ if ((count & LDSEM_ACTIVE_MASK) == 0) __ldsem_wake(sem);
Because ldsem_cmpxchg() was returning the _old_ value on success, this was checking the wrong count value.
No other code paths are impacted by the patch. The equivalent diff below also fixes the problem; however, I feel the intent is less clear.
| diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c | index 22fad8a..29d9e7c 100644 | --- a/drivers/tty/tty_ldsem.c | +++ b/drivers/tty/tty_ldsem.c | @@ -222,7 +222,7 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout) | get_task_struct(tsk); | | /* if there are no active locks, wake the new lock owner(s) */ | - if ((count & LDSEM_ACTIVE_MASK) == 0) | + if ((count + adjust & LDSEM_ACTIVE_MASK) == 0) | __ldsem_wake(sem); | | raw_spin_unlock_irq(&sem->wait_lock);
Regards,
Peter Hurley (1): tty: Fix hang at ldsem_down_read()
drivers/tty/tty_ldsem.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) -- 1.8.1.2
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