linux-kernel - Re: [PATCH v2 1/2] cpufreq: governor: Fix race in dbs_update_util

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Message-ID: <20160222134507.GQ28226@vireshk-i7>
Date:	Mon, 22 Feb 2016 19:15:07 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] cpufreq: governor: Fix race in
 dbs_update_util_handler()

On 22-02-16, 14:14, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> There is a scenario that may lead to undesired results in
> dbs_update_util_handler().  Namely, if two CPUs sharing a policy
> enter the funtion at the same time, pass the sample delay check
> and then one of them is stalled until dbs_work_handler() (queued
> up by the other CPU) clears the work counter, it may update the
> work counter and queue up another work item prematurely.
> 
> To prevent that from happening, use the observation that the CPU
> queuing up a work item in dbs_update_util_handler() updates the
> last sample time.  This means that if another CPU was stalling after
> passing the sample delay check and now successfully updated the work
> counter as a result of the race described above, it will see the new
> value of the last sample time which is different from what it used in
> the sample delay check before.  If that happens, the sample delay
> check passed previously is not valid any more, so the CPU should not
> continue.
> 
> Fixes: f17cbb53783c (cpufreq: governor: Avoid atomic operations in hot paths)
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> 
> Changes from v1:
> - Typo in the changelog fixed.
> - READ_ONCE() used instead of ACCESS_ONCE().
> - If the race is detected, return instead of looping.

Acked-by: Viresh Kumar <viresh.kumar@...aro.org>

-- 
viresh