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