linux-kernel - Re: [RFC][PATCH 3/3] ipc/sem: Rework wakeup scheme

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Message-ID: <4E749341.9040101@colorfullife.com>
Date:	Sat, 17 Sep 2011 14:32:01 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC][PATCH 3/3] ipc/sem: Rework wakeup scheme

On 09/16/2011 02:18 PM, Peter Zijlstra wrote:
> On Thu, 2011-09-15 at 19:29 +0200, Manfred Spraul wrote:
>> What is broken?
> So basically sembench was broken and the futex patch is causing spurious
> wakeups.
>
> I've got the below patch to fix up the sem code.
>
> One more question, do the sem wakeups need to be issued in FIFO order?
> There's a comment in there:
>
>   * User space visible behavior:
>   * - FIFO ordering for semop() operations (just FIFO, not starvation
>   *   protection)
>
> that seems to suggest the sem ops processing is in FIFO order, but does
> the user visible effect propagate to the wakeup order?
I'd ask the question the other way arould:
Is the wakeup order user visible?
IMHO: No, the scheduler might reorder the tasks anyway.

>
>   	/*
>   	 * If an interrupt occurred we have to clean up the queue
>   	 */
>   	if (timeout&&  jiffies_left == 0)
>   		error = -EAGAIN;
> +
> +	if (error == -EINTR&&  !signal_pending(current))
> +		goto retry;
> +
>   	unlink_queue(sma,&queue);
>
>   out_unlock_free:
Good solution.
-ERESTARTNOHAND would be even better, but this is definitively better 
than the current code.

--
     Manfred
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