linux-kernel - Re: Preemption Broken: centrino_target busted under SMP on 2.6.20.4

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Message-ID: <20070410030500.GA15509@redhat.com>
Date:	Mon, 9 Apr 2007 23:05:00 -0400
From:	Dave Jones <davej@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Jeff V. Merkey" <jmerkey@...fmountaingroup.com>,
	Linux kernel <linux-kernel@...r.kernel.org>,
	Venki Pallipadi <venkatesh.pallipadi@...el.com>
Subject: Re: Preemption Broken:  centrino_target busted under SMP on 2.6.20.4

On Mon, Apr 09, 2007 at 07:41:42PM -0700, Andrew Morton wrote:

 > >  > This means we'll call set_cpus_allowed() while in atomic state, but
 > >  > set_cpus_allowed() does sleepy stuff.
 > > 
 > > Puzzled. This diff shouldn't change anything about the context we're in
 > > when we call set_cpus_allowed, and as we're not seeing warnings now,
 > > I'm not sure what I'm missing?
 > 
 > set_cpus_allowed() will only sleep in special circumstances: when we're
 > telling the target task that it is not allwed to run on a CPU upon which it
 > is presently executing.  So it needs to be synchronously migrated off that
 > CPU, which requires that the set_cpus_allowed() caller block.
 > 
 > You're probably just not hitting that case.

Oh, now I see it. The set_cpus_allowed that was inside the preempt stuff
I was adding. (that the diff elided).  Yeah, that's a problem. Bugger.

 > Probably we should have a might_sleep() in set_cpus_allowed(), although
 > there might be callers who are guaranteeed to never hit that case and who
 > might legitimately want special treatment to avoid the warning.

This whole file is going away in .22, and we have a viable alternative in
.21 (acpi-cpufreq), so I'm not overly worried about fixing this up
given it only shows up in debug kernels, especially at this stage in -rc.

(Yeah, it's a cop-out, but unless someone with more interest in this problem
 steps up, I've bigger fishes to fry).

	Dave

-- 
http://www.codemonkey.org.uk
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