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Message-ID: <20140619133104.GH11042@htj.dyndns.org>
Date: Thu, 19 Jun 2014 09:31:04 -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,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 6/6] percpu-refcount: implement percpu_ref_reinit()
and percpu_ref_is_zero()
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().
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.
Thanks.
--
tejun
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