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

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Message-ID: <20090705194646.GF17910@elf.ucw.cz>
Date:	Sun, 5 Jul 2009 21:46:46 +0200
From:	Pavel Machek <pavel@....cz>
To:	Thomas Renninger <trenn@...e.de>
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

On Fri 2009-07-03 12:10:15, Thomas Renninger wrote:
> 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.


Imagine 

dbs_tuners_ins.sampling_down_factor = 0xd0000;
input = 0xabcd;

..then other threads can see 0xdabcd; if they read at "bad"
moment. Not on i386, but this is generic code (right?). Just use
atomic_t.
									Pavel   

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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