Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

From: Viresh Kumar <viresh.kumar@linaro.org>
To: Stephen Boyd <sboyd@codeaurora.org>
Cc: Prarit Bhargava <prarit@redhat.com>,
	Saravana Kannan <skannan@codeaurora.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Lenny Szubowicz <lszubowi@redhat.com>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>
Subject: Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]
Date: Tue, 5 Aug 2014 13:16:48 +0530	[thread overview]
Message-ID: <CAKohpombG3Gj4mbm02vPNnAxzaRvQP7rb=iQ14dCFH4_+71H3w@mail.gmail.com> (raw)
In-Reply-To: <53DBEC27.7050803@codeaurora.org>

On 2 August 2014 01:06, Stephen Boyd <sboyd@codeaurora.org> wrote:
> I have the same options. The difference is that my driver has a governor
> per policy. That's set with the CPUFREQ_HAVE_GOVERNOR_PER_POLICY flag.

You may call me stupid but I got a bit confused after looking into the code
again. Why does the crash dump depends on this flag?

We *always* remove the governor specific directory while switching governors
(Ofcourse only if its updated for All CPUs). And so on a dual core platform,
where both CPU 0 & 1 share a clock line, switching of governors should result
in this crash dump?

I may know the answer to the stupid question I had, but not sure why that is a
problem. The only (and quite significant) difference that this flag makes
is the location of governor-specific directory:
- w/o this flag: /sys/devices/system/cpu/cpufreq/<here>
- w/ this flag: /sys/devices/system/cpu/cpu*/cpufreq/<here>

So, is there some issue with the sysfs lock for <cpu*/cpufreq/> node as while
switching governor we change  <cpu*/cpufreq/scaling_governor> at the same
location?

--
viresh

--
viresh