On Thu, Jun 19, 2014 at 09:36:24AM -0400, Tejun Heo wrote:
> Hey, Paul.
>
> On Wed, Jun 18, 2014 at 07:27:08PM -0700, Paul E. McKenney wrote:
> > Yep, smp_load_acquire() orders its load against later loads and stores,
> > so it really does need a memory barrier on weakly ordered systems.
>
> Yeap.
>
> > This is the "publish" operation for dynamically allocated per-CPU
> > references? If so, agreed, you should be able to rely on dependency
> > ordering. Make sure to comment the smp_read_barrier_depends(). ;-)
>
> Definitely, there aren't many things which are more frustrating than
> barriers w/o comments explaining their pairing. I'm pairing
> store_release with read_barrier_depends as that's what RCU is doing.
> Is this the preferred way now? I like the new store_release and
> load_acquire as they document what's being barriered better but as Lai
> suggested in another reply it does seem a bit unbalanced. I wonder
> whether load_acquire_depends would make sense.
If you mean what I think you mean by load_acquire_depends(), it is spelled
"rcu_dereference()" or, in this case, where you are never removing anything
that has been added, "rcu_dereference_raw()". Because you are never
removing anything, you don't need rcu_read_lock() or rcu_read_unlock(),
thus you don't want lockdep yelling at you about not having RCU read-side
critical sections, thus rcu_dereference_raw().
Thanx, Paul
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