linux-kernel - Re: [PATCH 13/13] cpufreq: make sure frequency transitions are serialized

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Message-ID: <2081161.dnl1xTqcUT@vostro.rjw.lan>
Date:	Wed, 26 Jun 2013 23:57:51 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, robin.randhawa@....com,
	Steve.Bannister@....com, Liviu.Dudau@....com,
	charles.garcia-tobin@....com, arvind.chauhan@....com,
	dave.martin@....com
Subject: Re: [PATCH 13/13] cpufreq: make sure frequency transitions are serialized

On Monday, June 24, 2013 07:01:59 PM Viresh Kumar wrote:
> On 24 June 2013 19:03, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > Looks OK, but since transition_ongoing is either 0 or 1 now, as far as I can
> > say, it would be better to make it a bool and use = true/false instead of
> > ++/-- I suppose.
> 
> Another fixup:
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 6ca7eac..49d942a 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -108,7 +108,7 @@ static BLOCKING_NOTIFIER_HEAD(cpufreq_policy_notifier_list);
>  static struct srcu_notifier_head cpufreq_transition_notifier_list;
> 
>  /* Tracks status of transition */
> -static int transition_ongoing;
> +static bool transition_ongoing;
> 
>  static bool init_cpufreq_transition_notifier_list_called;
>  static int __init init_cpufreq_transition_notifier_list(void)
> @@ -271,7 +271,7 @@ void __cpufreq_notify_transition(struct
> cpufreq_policy *policy,
>                                 "In middle of another frequency transition\n"))
>                         return;
> 
> -               transition_ongoing++;
> +               transition_ongoing = true;
> 
>                 /* detect if the driver reported a value as "old frequency"
>                  * which is not equal to what the cpufreq core thinks is
> @@ -296,7 +296,7 @@ void __cpufreq_notify_transition(struct
> cpufreq_policy *policy,
>                                 "No frequency transition in progress\n"))
>                         return;
> 
> -               transition_ongoing--;
> +               transition_ongoing = false;
> 
>                 adjust_jiffies(CPUFREQ_POSTCHANGE, freqs);
>                 pr_debug("FREQ: %lu - CPU: %lu", (unsigned long)freqs->new,

Well, now, seeing that the locking around this seems to be kind of haphazard,
I'm wondering what prevents two different threads from doing CPUFREQ_PRECHANGE
concurrently in such a way that thread A will check transition_ongoing
and thread B will check transition_ongoing and then both will set it if it
was 'false' before.  And then one of them will trigger the WARN() in
CPUFREQ_POSTCHANGE.

Is there any protection in place and if so then how does it work?

Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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