On Tuesday, February 12, 2013 07:59:54 AM Paul E. McKenney wrote:
> On Mon, Feb 11, 2013 at 08:36:17PM +0100, Rafael J. Wysocki wrote:
> > On Monday, February 11, 2013 05:13:30 PM Nathan Zimmer wrote:
> > > There are some spots that I need to give a much deeper review, cpufreq_register_driver for example.
> > >
> > > But I believe
> > > > @@ -196,7 +195,7 @@ static void __cpufreq_cpu_put(struct cpufreq_policy *data, bool sysfs)
> > > > {
> > > > if (!sysfs)
> > > > kobject_put(&data->kobj);
> > > > - module_put(cpufreq_driver->owner);
> > > > + module_put(rcu_dereference(cpufreq_driver)->owner);
> > > > }
> > > would be ok. In the documentation whatisRCU.txt they give a very similar example.
> >
> > Well, the very same document states the following:
> >
> > Note that the value returned by rcu_dereference() is valid
> > only within the enclosing RCU read-side critical section.
>
> Ah, there is a code sample in that document showing a bug. I added
> comments to the code sample making it clear even to someone skimming
> the document that the code is buggy.
>
> Thanx, Paul
> ------------------------------------------------------------------------
>
> rcu: Make bugginess of code sample more evident
>
> One of the code samples in whatisRCU.txt shows a bug, but someone scanning
> the document quickly might mistake it for a valid use of RCU. Add some
> screaming comments to help keep speed-readers on track.
>
> Reported-by: Nathan Zimmer <nzimmer@xxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
> index 0cc7820..10df0b8 100644
> --- a/Documentation/RCU/whatisRCU.txt
> +++ b/Documentation/RCU/whatisRCU.txt
> @@ -265,9 +265,9 @@ rcu_dereference()
> rcu_read_lock();
> p = rcu_dereference(head.next);
> rcu_read_unlock();
> - x = p->address;
> + x = p->address; /* BUG!!! */
> rcu_read_lock();
> - y = p->data;
> + y = p->data; /* BUG!!! */
> rcu_read_unlock();
>
> Holding a reference from one RCU read-side critical section
>
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--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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