linux-kernel - Re: [PATCH v2] CPU hotplug, freezer: Fix bugs in CPU hotplug call path

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Date:	Tue, 04 Oct 2011 15:37:20 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, pavel@....cz,
	len.brown@...el.com, mingo@...e.hu, akpm@...ux-foundation.org,
	suresh.b.siddha@...el.com, lucas.demarchi@...fusion.mobi,
	linux-pm@...ts.linux-foundation.org, rusty@...tcorp.com.au,
	vatsa@...ux.vnet.ibm.com, ashok.raj@...el.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] CPU hotplug, freezer: Fix bugs in CPU hotplug call
 path

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?
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