Re: Preemption Broken: centrino_target busted under SMP on 2.6.20.4

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

On Thu, 5 Apr 2007 16:50:34 -0400
Dave Jones <davej@redhat.com> wrote:

> On Thu, Apr 05, 2007 at 02:33:03PM -0600, Jeff V. Merkey wrote:
>  > 
>  > BUG: using smp_processor_id() in preemptible [00000001] code: 
>  > kondemand/0/2473
>  > caller is centrino_target+0xfb/0x600
>  > [<401e3646>] debug_smp_processor_id+0x9e/0xb0
>  > [<40112afb>] centrino_target+0xfb/0x600
>  > [<40112a00>] centrino_target+0x0/0x600
>  > [<40305bd9>] __cpufreq_driver_target+0x5c/0x6b
>  > [<f897a537>] do_dbs_timer+0x1bc/0x208 [cpufreq_ondemand]
>  > [<40134a46>] run_workqueue+0x85/0x125
>  > [<40374f7f>] _spin_lock_irqsave+0x18/0x66
>  > [<f897a37b>] do_dbs_timer+0x0/0x208 [cpufreq_ondemand]
>  > [<401353fb>] worker_thread+0xf9/0x124
>  > [<401213b9>] default_wake_function+0x0/0xc
>  > [<40135302>] worker_thread+0x0/0x124
>  > [<40137b37>] kthread+0xb0/0xd9
>  > [<40137a87>] kthread+0x0/0xd9
>  > [<40104b2f>] kernel_thread_helper+0x7/0x10
> 
> Given speedstep-centrino is obsoleted in favour of acpi-cpufreq,
> I'm reluctant to spend too much time turd-polishing.
> This big-hammer diff should fix this..
> 
> 	Dave
> 
> 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.