linux-kernel - [PATCH 0/1] Fix hang report

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Date:	Wed, 11 Dec 2013 21:11:57 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Jiri Slaby <jslaby@...e.cz>, linux-kernel@...r.kernel.org,
	Sami Farin <sami.farin@...il.com>,
	Peter Hurley <peter@...leysoftware.com>
Subject: [PATCH 0/1] Fix hang report

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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