Re: [PATCH v2] rwsem: Support optimistic spinning

[Tim Chen]

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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