Re: cpufreq cleanups - .30 vs .31

Mathieu Desnoyers
2009-07-27T16:37:44Z

* Pallipadi, Venkatesh (venkatesh.pallipadi@intel.com) wrote:
> On Mon, 2009-07-27 at 07:25 -0700, Mathieu Desnoyers wrote:
> > * Dave Jones (davej@redhat.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
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