Re: [PATCH v2] rwsem: Support optimistic spinning

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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?  If so, I am OK with it as long as there
is a comment (which Davidlohr suggested later in this thread).

							Thanx, Paul

> > (My first question was "where is the update side", but this is covered
> > by task_struct allocation and deallocation.)
> 
> Tim
>