Re: [PATCH linux-next v8] cpufreq: convert the cpufreq_driver to use the rcu

Rafael J. Wysocki
2013-04-29T21:39:12Z

On Monday, April 29, 2013 02:37:28 PM Paul E. McKenney wrote:
> On Mon, Apr 29, 2013 at 12:22:32AM +0200, Rafael J. Wysocki wrote:
> > On Thursday, April 04, 2013 09:57:19 PM Viresh Kumar wrote:
> > > On 4 April 2013 20:23, Nathan Zimmer <nzimmer@sgi.com> wrote:
> > > > We eventually would like to remove the rwlock cpufreq_driver_lock or convert
> > > > it back to a spinlock and protect the read sections with RCU. The first step in
> > > > that is moving the cpufreq_driver to use the rcu.
> > > > I don't see an easy wasy to protect the cpufreq_cpu_data structure with the
> > > > RCU, so I am leaving it with the rwlock for now since under certain configs
> > > > __cpufreq_cpu_get is hot spot with 256+ cores.
> > > >
> > > > v5: Go a different way and split up the lock and use the rcu
> > > > v6: use bools instead of checking function pointers
> > > > covert the cpufreq_data_lock to a rwlock
> > > > v7: Rebase to use the already accepted half
> > > > v8: Correct have_governor_per_policy
> > > > Reviewed location of rcu_read_(un)lock in several spots
> > >
> > > Sorry for long delay or too many versions of this patch :)
> > >
> > > Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> >
> > Unfortunately, I had to revert this one, because it is obviously buggy. Why?
> > Because it adds rcu_read_lock()/rcu_read_unlock() around sysfs_create_file()
> > which may sleep due to a GFP_KERNEL memory allocation. Sorry for failing to
> > notice that earlier.
>
> One workaround might be to use SRCU, which allows sleeping in its
> critical sections.

Agreed, but at this point of the cycle I'd just preferred to do the revert and
start over.

Thanks,
Rafael

--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.