linux-kernel - Re: x86: disable preemption in delay

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Message-Id: <20071115195234.2dc8406d.akpm@linux-foundation.org>
Date:	Thu, 15 Nov 2007 19:52:34 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	mingo@...e.hu
Subject: Re: x86: disable preemption in delay_tsc()

On Thu, 15 Nov 2007 19:41:16 -0800 Arjan van de Ven <arjan@...radead.org> wrote:

> On Thu, 15 Nov 2007 04:00:47 GMT
> Linux Kernel Mailing List <linux-kernel@...r.kernel.org> wrote:
> 
> > Gitweb:
> > http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=35d5d08a085c56f153458c3f5d8ce24123617faf
> > Commit:     35d5d08a085c56f153458c3f5d8ce24123617faf Parent:
> > 7eea436433b7b18045f272562e256976f593f7c0 Author:     Andrew Morton
> > <akpm@...ux-foundation.org> AuthorDate: Wed Nov 14 17:00:41 2007 -0800
> > Committer:  Linus Torvalds <torvalds@...dy.linux-foundation.org>
> > CommitDate: Wed Nov 14 18:45:44 2007 -0800
> > 
> >     x86: disable preemption in delay_tsc()
> >     
> >     Marin Mitov points out that delay_tsc() can misbehave if it is
> > preempted and rescheduled on a different CPU which has a skewed TSC.
> > Fix it by disabling preemption.
> >  
> 
> this worries me.. this appears to effectively disable preemption during
> udelay() and mdelay() loops... which are very obvious latency inducers.
> 
> Now you can argue that if you're preemptible you should have used
> msleep() and co, and I'll totally buy that. 
> 
> 
> Maybe we should just check if we're still on the same cpu or something,
> or have a cheap way to pin a process to a cpu.... but both are longer
> term solutions.
> 

Yes, we can do better.

But this bug can cause very rare failures in probably a large number of
device drivers on a minorty of machines.  Ugly.  So I felt it best to
plug it fast while people think about more sophisticated fixes.

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