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

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Message-ID: <5149C76D.7090409@redhat.com>
Date:	Wed, 20 Mar 2013 10:27:57 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Davidlohr Bueso <davidlohr.bueso@...com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Emmanuel Benisty <benisty.e@...il.com>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	"Low, Jason" <jason.low2@...com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>, aquini@...hat.com,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Larry Woodman <lwoodman@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 4/4] ipc: sem: do not hold ipc lock more than necessary

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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