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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/