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Message-Id: <1245963285.4534.20542.camel@localhost.localdomain>
Date: Thu, 25 Jun 2009 13:54:45 -0700
From: "Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc: Dave Jones <davej@...hat.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
"kernel-testers@...r.kernel.org" <kernel-testers@...r.kernel.org>,
Ingo Molnar <mingo@...e.hu>, "Rafael J. Wysocki" <rjw@...k.pl>,
Dave Young <hidave.darkstar@...il.com>,
Pekka Enberg <penberg@...helsinki.fi>,
Thomas Renninger <trenn@...e.de>
Subject: Re: [patch 2/3] cpufreq: Define dbs_mutex purpose and cleanup its
usage
On Thu, 2009-06-25 at 12:46 -0700, Mathieu Desnoyers wrote:
> * venkatesh.pallipadi@...el.com (venkatesh.pallipadi@...el.com) wrote:
> > Commit b14893a62c73af0eca414cfed505b8c09efc613c although it was very
> > much needed to cleanup ondemand timer cleanly, openup a can of worms
> > related to locking dependencies in cpufreq.
> >
> > Patch here defines the need for dbs_mutex and cleans up its usage in
> > ondemand governor. This also resolves the lockdep warnings reported here
> >
> > http://lkml.indiana.edu/hypermail/linux/kernel/0906.1/01925.html
> >
> > @@ -598,14 +593,16 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
> > max(min_sampling_rate,
> > latency * LATENCY_MULTIPLIER);
> > }
> > + mutex_unlock(&dbs_mutex);
> > +
> > dbs_timer_init(this_dbs_info);
> >
> > - mutex_unlock(&dbs_mutex);
> > break;
> >
> > case CPUFREQ_GOV_STOP:
> > - mutex_lock(&dbs_mutex);
> > dbs_timer_exit(this_dbs_info);
>
> Hrm, so.. how do we protect against concurrent :
>
> CPUFREQ_GOV_START/CPUFREQ_GOV_STOP now ?
concurrent _START _STOP across CPUs does not matter for timer_init and
timer_exit. On same CPU, there cannot be two concurrent _START as upper
level cpufreq will have policy_rwsem in write mode. I cannot think of a
flow where _START and _STOP on same CPU is possible.
However two concurrent _STOP for same CPU is still possible, as we are
releasing the rwsem lock before STOP callback. "Back to drawing board"
time to figure this all out..
Thanks,
Venki
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