Re: [PATCH v2 4/4] ipc: sem: do not hold ipc lock more than necessary

[Rik van Riel <riel@redhat.com>]

On 03/05/2013 04:36 AM, Davidlohr Bueso wrote:

> @@ -1476,8 +1539,8 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
>   	queue.sleeper = current;
>
>   sleep_again:
> -	current->state = TASK_INTERRUPTIBLE;
>   	sem_unlock(sma);
> +	current->state = TASK_INTERRUPTIBLE;
>
>   	if (timeout)
>   		jiffies_left = schedule_timeout(jiffies_left);

After modifying my test case to start with a semaphore value of 1 on
every semaphore, and do down followed by up (to have only one process
take each semaphore at a time), I started seeing lost wakeups and the
test case being stuck.

I believe the change above is the cause of that issue.

By unlocking before setting current->state to TASK_INTERRUPTIBLE,
there is a small window where the next lock holder can grab the
lock and wake us up, before we set ourselves to TASK_INTERRUPTIBLE
and go to sleep.

I have reverted your change in my code and am building a test kernel
now.

If things work, I'll clean up the whole patch series for a re-posting
today.

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