linux-kernel - Re: [PATCH 1/2] CPUFREQ: Remove unneeded dbs_mutexes from ondemand and conservative governors

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Date:	Fri, 3 Jul 2009 12:10:15 +0200
From:	Thomas Renninger <trenn@...e.de>
To:	Pavel Machek <pavel@....cz>
Cc:	kernel@...ble.org, cpufreq@...r.kernel.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu, rjw@...k.pl,
	hidave.darkstar@...il.com, penberg@...helsinki.fi,
	kernel-testers@...r.kernel.org, davej@...hat.com,
	mathieu.desnoyers@...ymtl.ca,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
Subject: Re: [PATCH 1/2] CPUFREQ: Remove unneeded dbs_mutexes from ondemand and conservative governors

Hi Pavel,

On Tuesday 30 June 2009 08:33:39 Pavel Machek wrote:
> On Thu 2009-06-25 16:01:24, Thomas Renninger wrote:
> > Comment from Venkatesh:
> > ...
> > This mutex is just serializing the changes to those variables. I could't
> > think of any functionality issues of not having the lock as such.
> > 
> > -> rip it out.
> > 
> > CC: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
> > Signed-off-by: Thomas Renninger <trenn@...e.de>
> 
> >  static struct dbs_tuners {
> > @@ -236,10 +222,7 @@ static ssize_t store_sampling_down_factor(struct cpufreq_policy *unused,
> >  	if (ret != 1 || input > MAX_SAMPLING_DOWN_FACTOR || input < 1)
> >  		return -EINVAL;
> >  
> > -	mutex_lock(&dbs_mutex);
> >  	dbs_tuners_ins.sampling_down_factor = input;
> > -	mutex_unlock(&dbs_mutex);
> > -
> 
> You'd need to make s_down_factor atomic_t for this to work....
Can you provide a userspace scenario (or tell which kind of event must
happen in between), that this would cause problems, please.

Thanks,

  Thomas
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