Re: Preemption Broken: centrino_target busted under SMP on 2.6.20.4

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

On Mon, 9 Apr 2007 23:08:23 -0400 Dave Jones <davej@redhat.com> wrote:

>  > 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).
> 
> One last try...
> (I didn't think too long about this, so this might be equally busted,
>  but if so, see comment above).
> 
> 	Dave
> 
> diff --git a/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c b/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c
> index f43b987..38e31ce 100644
> --- a/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c
> +++ b/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c
> @@ -720,11 +720,13 @@ static int centrino_target (struct cpufreq_policy *policy,
>  			cpu_set(j, set_mask);
>  
>  		set_cpus_allowed(current, set_mask);
> +		preempt_disable();
>  		if (unlikely(!cpu_isset(smp_processor_id(), set_mask))) {
>  			dprintk("couldn't limit to CPUs in this domain\n");
>  			retval = -EAGAIN;
>  			if (first_cpu) {
>  				/* We haven't started the transition yet. */
> +				preempt_enable();
>  				goto migrate_end;
>  			}
>  			break;
> @@ -765,6 +767,7 @@ static int centrino_target (struct cpufreq_policy *policy,
>  			break;
>  
>  		cpu_set(j, covered_cpus);
> +		preempt_enable();
>  	}
>  

Yes, I expect that should squish the warnings.  It looks all racy wrt cpu hotplug
and against async set_cpus_allowed(), but if those are our worst problems, we're
good.