linux-kernel - Re: [PATCH 2/2] cpufreq: serialize calls to __cpufreq

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Date:	Thu, 05 Sep 2013 02:26:10 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] cpufreq: serialize calls to __cpufreq_governor()

On Wednesday, September 04, 2013 04:50:01 PM Stephen Boyd wrote:
> On 09/04/13 16:55, Rafael J. Wysocki wrote:
> >
> > Well, I'm not sure when Viresh is going to be back.
> >
> > Srivatsa, can you please resend this patch with a proper changelog?
> >
> 
> I haven't had a chance to try this out yet, but I was just thinking
> about this patch. How is it going to work? If one task opens the file
> and another task is taking down the CPU wouldn't we deadlock in the
> CPU_DOWN notifier waiting for the kobject to be released? Task 1 will
> grab the kobject reference and sleep on the hotplug mutex and task 2
> will put the kobject and wait for the completion, but it won't happen.
> At least I think that's what would happen.

Do you mean the completion in sysfs_deactivate()?  Yes, we can deadlock
there.

Well, I guess the Srivatsa's patch may be salvaged by making it do a "trylock"
version of get_online_cpus(), but then I wonder if there's no better way.

Thanks,
Rafael

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