linux-kernel - Re: [PATCH 6/6] percpu-refcount: implement percpu_ref_reinit() and percpu_ref_is

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Date:	Thu, 19 Jun 2014 10:05:49 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>, cl@...ux-foundation.org,
	kmo@...erainc.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] percpu-refcount: implement percpu_ref_reinit() and
 percpu_ref_is_zero()

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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