linux-kernel - Re: cpufreq cleanups

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Date:	Mon, 27 Jul 2009 12:37:37 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	Dave Jones <davej@...hat.com>, Thomas Renninger <trenn@...e.de>,
	"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>
Subject: Re: cpufreq cleanups - .30 vs .31

* Pallipadi, Venkatesh (venkatesh.pallipadi@...el.com) wrote:
> On Mon, 2009-07-27 at 07:25 -0700, Mathieu Desnoyers wrote:
> > * Dave Jones (davej@...hat.com) wrote:
> > > On Mon, Jul 06, 2009 at 01:18:18PM +0200, Thomas Renninger wrote:
> > > 
> > >  > So if not find too intrusive, I'd say:
> > >  > Venkatesh's whole series of:
> > >  > [patch 0/4] Take care of cpufreq lockdep issues (take 2)
> > >  > should be seen in .31.
> > >  >  ...
> > >  > The one patch from Mathieu:
> > >  > [patch 2.6.30 2/4] CPUFREQ: fix (utter) cpufreq_add_dev mess
> > >  > is a separate, general cleanup which should show up in .31.
> > > 
> > > I came to the same conclusion after reading the thread, and looking
> > > over the patches.  I merged the above, and sent Linus a pull request
> > > a few minutes ago.
> > > 
> > > Thanks Mathieu and Venki for chasing this down.
> > > 
> > > 	Dave
> > 
> > Given I never got an answer to this question, I'm re-asking a question I
> > asked in a previous thread about Venki's patchset:
> > 
> > [CPUFREQ] Cleanup locking in ondemand governor
> > commit	5a75c82828e7c088ca6e7b4827911dc29cc8e774
> > 
> > From the earlier thread:
> > Subject: Re: [patch 2.6.30 3/4] cpufreq add gov mutex
> > 
> > I am worried about potential races between add_dev/remove_dev, which
> > currently lock the rwsem as mean of protection, and execution of timer
> > handler that would not take the rwsem to protect itself anymore, due to
> > your changes.
> > 
> > I'm especially worried about the call to
> > 
> >               __cpufreq_driver_target(dbs_info->cur_policy,
> >                         dbs_info->freq_lo, CPUFREQ_RELATION_H);
> > 
> > which seems to depend on policy-level information, protected at the
> > rwsem-level.
> > 
> > By removing the rwsem from the timer handler, I don't see how you plan
> > to protect this information from add_dev/remove_dev execution.
> > 
> 
> Sorry I missed the question earlier.
> 
> The invariant here is that the timer routine will not be running while
> policy is inconsistent due to add/remove. The cpufreq layer calls START
> at the end of add_dev when all policy stuff has been setup, which starts
> the timer. And STOP along remove_dev before cleaning up policy which
> stops the timer.
> 
> If you are thinking of races with other cpufreq sysfs interfaces, they
> go through the per cpu rwsem along with add/remove.
> 

Great, that makes sense.

Thanks !

Mathieu

> Thanks,
> Venki
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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