linux-kernel - Re: [PATCH 3/4] PM / OPP: take RCU lock in dev_pm_opp_get_opp

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Message-ID: <3741028.CtOSUWyiaT@dtor-glaptop>
Date:	Wed, 17 Dec 2014 09:28:42 -0800
From:	Dmitry Torokhov <dtor@...omium.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Stefan Wahren <stefan.wahren@...e.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Nishanth Menon <nm@...com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] PM / OPP: take RCU lock in dev_pm_opp_get_opp_count

On Wednesday, December 17, 2014 10:06:17 AM Viresh Kumar wrote:
> On 17 December 2014 at 04:39, Dmitry Torokhov <dtor@...omium.org> wrote:
> > A lot of callers are missing the fact that dev_pm_opp_get_opp_count
> > needs to be called under RCU lock. Given that RCU locks can safely be
> > nested, instead of providing *_locked() API, let's take RCU lock inside
> 
> Hmm, I asked for a *_locked() API because many users of
> dev_pm_opp_get_opp_count() are already calling it from rcu read side
> critical sections.
> 
> Now, there are two questions:
> - Can rcu-read side critical sections be nested ?
> 
> Yes, this is what the comment over rcu_read_lock() says
> 
>  * RCU read-side critical sections may be nested.  Any deferred actions
>  * will be deferred until the outermost RCU read-side critical section
>  * completes.
> 
> - Would it be better to drop these double rcu_read_locks() ? i.e. either
> get a *_locked() API or fix the callers of dev_pm_opp_get_opp_count().
> 
> @Paul: What do you say ?
>

FWIW the change is a stop-gap; I hope we'll get away from using 
dev_pm_opp_get_opp_count() in cpufreq drivers and then we can revert the 
change. I just did not want to touch cpufreq drivers unless necessary.

Thanks,
Dmitry
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