linux-kernel - Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

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Date:	Tue, 8 Dec 2015 22:04:02 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	linux-pm@...r.kernel.org, linaro-kernel@...ts.linaro.org,
	ashwin.chaugule@...aro.org,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable
 for synchronization

On 08-12-15, 17:42, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Subject: [PATCH] cpufreq: governor: Use lockless timer function
> 
> It is possible to get rid of the timer_lock spinlock used by the
> governor timer function for synchronization, but a couple of races
> need to be avoided.
> 
> The first race is between multiple dbs_timer_handler() instances
> that may be running in parallel with each other on different
> CPUs.  Namely, one of them has to queue up the work item, but it
> cannot be queued up more than once.  To achieve that,
> atomic_inc_return() can be used on the skip_work field of
> struct cpu_common_dbs_info.
> 
> The second race is between an already running dbs_timer_handler()
> and gov_cancel_work().  In that case the dbs_timer_handler() might
> not notice the skip_work incrementation in gov_cancel_work() and
> it might queue up its work item after gov_cancel_work() had
> returned (and that work item would corrupt skip_work going
> forward).  To prevent that from happening, gov_cancel_work()
> can be made wait for the timer function to complete (on all CPUs)
> right after skip_work has been incremented.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/cpufreq/cpufreq_governor.c |   49 ++++++++++++++++---------------------
>  drivers/cpufreq/cpufreq_governor.h |    9 +-----
>  2 files changed, 24 insertions(+), 34 deletions(-)

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

-- 
viresh
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