On Tue, 2014-04-29 at 08:11 -0700, Paul E. McKenney wrote:
> On Mon, Apr 28, 2014 at 05:50:49PM -0700, Tim Chen wrote:
> > On Mon, 2014-04-28 at 16:10 -0700, Paul E. McKenney wrote:
> >
> > > > +#ifdef CONFIG_SMP
> > > > +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> > > > +{
> > > > + int retval;
> > > > + struct task_struct *owner;
> > > > +
> > > > + rcu_read_lock();
> > > > + owner = ACCESS_ONCE(sem->owner);
> > >
> > > OK, I'll bite...
> > >
> > > Why ACCESS_ONCE() instead of rcu_dereference()?
> >
> > We're using it as a speculative check on the sem->owner to see
> > if the owner is running on the cpu. The rcu_read_lock
> > is used for ensuring that the owner->on_cpu memory is
> > still valid.
>
> OK, so if we read complete garbage, all that happens is that we
> lose a bit of performance?
Correct.
> If so, I am OK with it as long as there
> is a comment (which Davidlohr suggested later in this thread).
>
Yes, we should add some comments to clarify things.
Thanks.
Tim
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