linux-kernel - Re: [PATCH v2 6/6] percpu-refcount: implement percpu_ref_reinit() and percpu_ref_is

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Date:	Thu, 19 Jun 2014 09:55:02 -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 v2 6/6] percpu-refcount: implement percpu_ref_reinit()
 and percpu_ref_is_zero()

On Thu, Jun 19, 2014 at 09:31:04AM -0400, Tejun Heo wrote:
> On Thu, Jun 19, 2014 at 11:01:26AM +0800, Lai Jiangshan wrote:
> > > +	/*
> > > +	 * Restore per-cpu operation.  smp_store_release() is paired with
> > > +	 * smp_load_acquire() in __pcpu_ref_alive() and guarantees that the
> > 
> > s/smp_load_acquire()/smp_read_barrier_depends()/
> 
> Will update.
> 
> > s/smp_store_release()/smp_mb()/  if you accept my next comment.
> >
> > > +	 * zeroing is visible to all percpu accesses which can see the
> > > +	 * following PCPU_REF_DEAD clearing.
> > > +	 */
> > > +	for_each_possible_cpu(cpu)
> > > +		*per_cpu_ptr(pcpu_count, cpu) = 0;
> > > +
> > > +	smp_store_release(&ref->pcpu_count_ptr,
> > > +			  ref->pcpu_count_ptr & ~PCPU_REF_DEAD);
> > 
> > I think it would be better if smp_mb() is used.
> 
> smp_wmb() would be better here.  We don't need the reader side.
> 
> > it is documented that smp_read_barrier_depends() and smp_mb() are paired.
> > Not smp_read_barrier_depends() and smp_store_release().

Well, sounds like the documentation needs an update, then.  ;-)

For example, current rcu_assign_pointer() is a wrapper around
smp_store_release().

> I don't know.  I thought about doing that but the RCU accessors are
> pairing store_release with read_barrier_depends, so I don't think the
> particular paring is problematic and store_release is better at
> documenting what's being barriered.

Which Tejun noted as well.

							Thanx, Paul

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