Re: Preemption Broken: centrino_target busted under SMP on 2.6.20.4

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 9 Apr 2007 22:31:08 -0400 Dave Jones <davej@redhat.com> wrote:

> On Mon, Apr 09, 2007 at 05:26:51PM -0700, Andrew Morton wrote:
>  > On Thu, 5 Apr 2007 16:50:34 -0400
>  > Dave Jones <davej@redhat.com> wrote:
>  > 
>  > > diff --git a/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c b/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c
>  > > index f43b987..824d0a2 100644
>  > > --- a/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c
>  > > +++ b/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c
>  > > @@ -708,6 +708,7 @@ static int centrino_target (struct cpufreq_policy *policy,
>  > >  	saved_mask = current->cpus_allowed;
>  > >  	first_cpu = 1;
>  > >  	cpus_clear(covered_cpus);
>  > > +	preempt_disable();
>  > >  	for_each_cpu_mask(j, online_policy_cpus) {
>  > >  		/*
>  > >  		 * Support for SMP systems.
>  > > @@ -798,6 +799,7 @@ static int centrino_target (struct cpufreq_policy *policy,
>  > >  	}
>  > >  
>  > >  migrate_end:
>  > > +	preempt_enable();
>  > >  	set_cpus_allowed(current, saved_mask);
>  > >  	return 0;
>  > >  }
>  > 
>  > 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.

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.

> [which may be 'the obvious', you wouldn't believe the evening I've had
>  involving gas leaks and noxious fumes. Wheee, floaty head.]

Yeah, I get a lot of patches like that.