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

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Message-ID: <20140618153222.GA11042@htj.dyndns.org>
Date:	Wed, 18 Jun 2014 11:32:22 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	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 Wed, Jun 18, 2014 at 11:37:35AM +0800, Lai Jiangshan wrote:
> > @@ -97,7 +98,10 @@ static inline void percpu_ref_kill(struct percpu_ref *ref)
> >  static inline bool __pcpu_ref_alive(struct percpu_ref *ref,
> >  				    unsigned __percpu **pcpu_countp)
> >  {
> > -	unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr);
> > +	unsigned long pcpu_ptr;
> > +
> > +	/* paired with smp_store_release() in percpu_ref_reinit() */
> > +	pcpu_ptr = smp_load_acquire(&ref->pcpu_count_ptr);
> 
> 
> Does "smp_load_acquire()" hurts the performance of percpu_ref_get/put()
> in non-x86 system?

It's equivalent to data dependency barrier.  The only arch which needs
something more than barrier() is alpha.  It isn't an issue.

Thanks.

-- 
tejun
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