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>
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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