Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756985AbaFSCHc (ORCPT ); Wed, 18 Jun 2014 22:07:32 -0400 Received: from mail-qc0-f170.google.com ([209.85.216.170]:61837 "EHLO mail-qc0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752726AbaFSCHb (ORCPT ); Wed, 18 Jun 2014 22:07:31 -0400 Date: Wed, 18 Jun 2014 22:07:27 -0400 From: Tejun Heo To: Lai Jiangshan Cc: cl@linux-foundation.org, kmo@daterainc.com, linux-kernel@vger.kernel.org, "Paul E. McKenney" Subject: Re: [PATCH 6/6] percpu-refcount: implement percpu_ref_reinit() and percpu_ref_is_zero() Message-ID: <20140619020727.GC20100@mtj.dyndns.org> References: <1403053685-28240-1-git-send-email-tj@kernel.org> <1403053685-28240-7-git-send-email-tj@kernel.org> <53A1097F.3060400@cn.fujitsu.com> <20140618153222.GA11042@htj.dyndns.org> <53A243B8.4010501@cn.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53A243B8.4010501@cn.fujitsu.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 19, 2014 at 09:58:16AM +0800, Lai Jiangshan wrote: > On 06/18/2014 11:32 PM, Tejun Heo wrote: > > 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. > > > > But I searched from the source, smp_load_acquire() is just barrier() in > x86, arm64, ia64, s390, sparc, but it includes memory barrier > instruction in other archs. Hmmm, right, it's a stronger guarantee than the data dependency barrier. This should probably use smp_wmb() and smp_read_barrier_depends(). That's all it needs anyway. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/