From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753091AbbEHO1P (ORCPT ); Fri, 8 May 2015 10:27:15 -0400 Received: from foss.arm.com ([217.140.101.70]:53150 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751860AbbEHO1O (ORCPT ); Fri, 8 May 2015 10:27:14 -0400 Date: Fri, 8 May 2015 15:27:11 +0100 From: Will Deacon To: Peter Zijlstra Cc: Arnaldo Carvalho de Melo , Ingo Molnar , David Ahern , Jiri Olsa , Namhyung Kim , Linux Kernel Mailing List Subject: Re: Question about barriers for ARM on tools/perf/ Message-ID: <20150508142711.GC25587@arm.com> References: <20150508140459.GI7862@kernel.org> <20150508142107.GA25587@arm.com> <20150508142513.GM27504@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150508142513.GM27504@twins.programming.kicks-ass.net> 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 Fri, May 08, 2015 at 03:25:13PM +0100, Peter Zijlstra wrote: > On Fri, May 08, 2015 at 03:21:08PM +0100, Will Deacon wrote: > > Wouldn't it be better to go the other way, and use compiler builtins for > > the memory barriers instead of relying on the kernel? It looks like the > > perf_mmap__{read,write}_head functions are basically just acquire/release > > operations and could therefore be implemented using something like > > __atomic_load_n(&pc->data_head, __ATOMIC_ACQUIRE) and > > __atomic_store_n(&pc->data_tail, tail, __ATOMIC_RELEASE). > > He wants to do smp refcounting, which needs atomic_inc() / > atomic_inc_non_zero() / atomic_dec_return() etc.. Right, of course, but GCC has those too: https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html Will