linux-kernel - Re: [PATCH 2/3] ipc/sem.c: handle spurious wakeups

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Message-ID: <4E97332C.8020202@colorfullife.com>
Date:	Thu, 13 Oct 2011 20:51:24 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 2/3] ipc/sem.c: handle spurious wakeups

On 10/11/2011 11:54 PM, Andrew Morton wrote:
> On Sat, 24 Sep 2011 19:37:11 +0200
> Manfred Spraul<manfred@...orfullife.com>  wrote:
>
>> semtimedop() does not handle spurious wakeups, it returns -EINTR to user space.
>> Most other schedule() users would just loop and not return to user space.
>> The patch adds such a loop to semtimedop()
> What is a "spurious wakeup" and how can a process receive one?
A spurious wakeup means that someone calls wake_up_process() without a 
proper reason.
The most common case would be a wake_up_process() that was somehow delayed.

Peter's patch made such delayed wakeups very common, this is how we 
found the issue.

The "standard" kernel primitives handle such wakeups, ipc/sem.c doesn't 
handle that.

> I'm wondering about the userspace-visible effects of this change, and
> any compatibility issues?
This change has no userspace visible effects.

--
     Manfred
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