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

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Date:	Wed, 12 Oct 2011 09:09:35 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 2/3] ipc/sem.c: handle spurious wakeups

On Tue, 2011-10-11 at 14:54 -0700, 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?

Its a wakeup unrelated to the condition its waiting for. They shouldn't
happen (often) but all wait loops should deal with them. Sadly of course
most out of core wait loops don't :-(

> I'm wondering about the userspace-visible effects of this change, and
> any compatibility issues?

For this particular case it would be returning to userspace with -EINTR
without a signal having been raised what so ever. Not a big deal as
userspace it supposed to be able to deal with -EINTR and retry.

Also, these spurious wakeups hardly ever happen in the current kernel, I
only noticed it because I made them slightly more likely with a patch
currently on the back-burner until I figure out a sane way to audit all
1400+ schedule() and co. callsites.
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