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Subject: Re: [PATCH v2] CPU hotplug, freezer: Fix bugs in CPU hotplug call
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From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>, pavel@ucw.cz, len.brown@intel.com,
        mingo@elte.hu, akpm@linux-foundation.org, suresh.b.siddha@intel.com,
        lucas.demarchi@profusion.mobi, linux-pm@lists.linux-foundation.org,
        rusty@rustcorp.com.au, vatsa@linux.vnet.ibm.com, ashok.raj@intel.com,
        linux-kernel@vger.kernel.org
Date: Tue, 04 Oct 2011 15:37:20 +0200
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On Tue, 2011-10-04 at 18:58 +0530, Srivatsa S. Bhat wrote:
> +static int tasks_frozen;
> +
> +void set_tasks_frozen_flag(void)
> +{
> +       tasks_frozen = 1;
> +       smp_mb();
> +}
> +
> +void clear_tasks_frozen_flag(void)
> +{
> +       tasks_frozen = 0;
> +       smp_mb();
> +}
> +
> +int tasks_are_frozen(void)
> +{
> +       return tasks_frozen;
> +} 

See Documentation/memory-barriers.txt, memory barriers always come in
pairs, furthermore memory barriers always should have a comment
explaining the ordering and referring to the pair match.

I think you want at least an smp_rmb() before reading tasks_frozen,
possible you also want to use ACCESS_ONCE() to force the compiler to
emit the read.

Furthermore, do you really need this? isn't it both set and read from
the same task context, all under pm_mutex?