linux-kernel - Re: [PATCH 2/5] cpufreq, fix locking around CPUFREQ_GOV_POLICY

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Date:	Mon, 10 Nov 2014 16:14:57 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Robert Schöne <robert.schoene@...dresden.de>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 2/5] cpufreq, fix locking around CPUFREQ_GOV_POLICY_EXIT calls

On 5 November 2014 20:23, Prarit Bhargava <prarit@...hat.com> wrote:
> commit 955ef4833574636819cd269cfbae12f79cbde63a (" cpufreq: Drop rwsem
> lock around CPUFREQ_GOV_POLICY_EXIT") opens up a hole in the locking
> scheme for cpufreq.
>
> Simple tests such as rapidly switching the governor between ondemand and
> performance or attempting to read policy values while a governor switch occurs
> now fail with very NULL pointer warnings, sysfs namespace collisions, and
> system hangs.  In short, the locking that policy->rwsem is supposed to provide
> is currently broken.
>
> The identified commit attempts to resolve a lockdep warning by removing
> a lock around a section of code which does a shutdown of the
> existing policy.  The problem is that this is part of the _critical_ section of
> code that switches the governors and must be protected by the lock; without
> locking readers may access now NULL or stale data, and writes may collide with
> each other.
>
> With the previous patch, which now returns -EBUSY during times of
> contention the deadlock reported in
> 955ef4833574636819cd269cfbae12f79cbde63a (" cpufreq: Drop rwsem lock
> around CPUFREQ_GOV_POLICY_EXIT") cannot occur, so adding the locks back
> into this section of code is possible.

I still fail to understand why ? What will the _trylock() change ?
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