linux-kernel - Re: [PATCH V4 1/3] cpufreq: Make sure frequency transitions are serialized

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Date:	Fri, 21 Mar 2014 11:05:59 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"ego@...ux.vnet.ibm.com" <ego@...ux.vnet.ibm.com>
Subject: Re: [PATCH V4 1/3] cpufreq: Make sure frequency transitions are
 serialized

On Fri, Mar 21, 2014 at 09:21:02AM +0000, Viresh Kumar wrote:
> @Catalin: We have a problem here and need your expert advice. After changing
> CPU frequency we need to call this code:
> 
> cpufreq_notify_post_transition();
> policy->transition_ongoing = false;
> 
> And the sequence must be like this only. Is this guaranteed without any
> memory barriers? cpufreq_notify_post_transition() isn't touching
> transition_ongoing at all..

The above sequence doesn't say much. As rmk said, the compiler wouldn't
reorder the transition_ongoing write before the function call. I think
most architectures (not sure about Alpha) don't do speculative stores,
so hardware wouldn't reorder them either. However, other stores inside
the cpufreq_notify_post_transition() could be reordered after
transition_ongoing store. The same for memory accesses after the
transition_ongoing update, they could be reordered before.

So what we actually need to know is what are the other relevant memory
accesses that require strict ordering with transition_ongoing.

What I find strange in your patch is that
cpufreq_freq_transition_begin() uses spinlocks around transition_ongoing
update but cpufreq_freq_transition_end() doesn't.

-- 
Catalin
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