On Wed, 2014-06-04 at 13:57 -0400, Andev wrote: > Hi, > > On Tue, Apr 29, 2014 at 11:11 AM, Paul E. McKenney > <[email protected]> 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? If so, I am OK with it as long as there > > is a comment (which Davidlohr suggested later in this thread). > > > > Thanx, Paul > > > > The latest code seems to be missing this comment. Could you please add this?
The comment is there when we declare ->owner in struct rw_semaphore. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
