linux-kernel - Re: [PATCH] cpufreq: fix governor start/stop race condition

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Date:	Fri, 24 May 2013 11:01:35 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Xiaoguang Chen <chenxg@...vell.com>
Cc:	"rjw@...k.pl" <rjw@...k.pl>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ning Jiang <njiang1@...vell.com>, Yilu Mao <ylmao@...vell.com>,
	Zhoujie Wu <zjwu@...vell.com>
Subject: Re: [PATCH] cpufreq: fix governor start/stop race condition

On 23 May 2013 08:14, Xiaoguang Chen <chenxg@...vell.com> wrote:
> Do you mean my patch will cause deadlock? I once tried to add another lock
> to protect the GOV_STOP/START sequence instead of using the rwsem in this
> patch.
> But I saw deadlock indeed.
> In cpufreq_add_policy_cpu, the lock has to be added before the rwsem since
> GOV_STOP is
> before lock_policy_rwsem_write, but in cpufreq_update_policy, it will first
> get the rwsem, and then
> call __cpufreq_set_policy which will contain GOV_STOP again, if we add the
> new lock before this GOV_STOP,
> then we may get deadlock in below sequence:
> 1) hotplug in one cpu by calling cpufreq_add_policy_cpu in which new lock is
> locked first then rwsem is locked.
> 2) governor change in cpufreq_update_policy in which rwsem is locked first
> then new lock is locked.
> this is a deadlock issue if above two steps interleaves

Check this patch.

https://patchwork-mail.kernel.org/patch/2575231/
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