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

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Message-ID: <5149D7ED.4040702@redhat.com>
Date:	Wed, 20 Mar 2013 11:38:21 -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 03/20/2013 10:27 AM, Rik van Riel wrote:
> 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.

Half a billion semaphore operations later, I am pretty sure
the above was the cause of the semaphore hangups we both
observed :)

I am currently building a kernel with the cleaned up patch
series I put together while building the previous test kernel.

If all goes well, expect a patch series after lunch...

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