linux-kernel - Re: [PATCH linux-next v8] cpufreq: convert the cpufreq

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Message-ID: <20130429213728.GR3780@linux.vnet.ibm.com>
Date:	Mon, 29 Apr 2013 14:37:28 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Nathan Zimmer <nzimmer@....com>, cpufreq@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH linux-next v8] cpufreq: convert the cpufreq_driver to use
 the rcu

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@....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@...aro.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.

							Thanx, Paul

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