From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751672AbcFXQqL (ORCPT ); Fri, 24 Jun 2016 12:46:11 -0400 Received: from bedivere.hansenpartnership.com ([66.63.167.143]:60584 "EHLO bedivere.hansenpartnership.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751132AbcFXQqI (ORCPT ); Fri, 24 Jun 2016 12:46:08 -0400 Message-ID: <1466786765.2343.37.camel@HansenPartnership.com> Subject: Re: [PATCH -tip 00/12] locking/atomics: Add and use inc,dec calls for FETCH-OP flavors From: James Bottomley To: Davidlohr Bueso , peterz@infradead.org, mingo@kernel.org Cc: davem@davemloft.net, cw00.choi@samsung.com, dougthompson@xmission.com, bp@alien8.de, mchehab@osg.samsung.com, gregkh@linuxfoundation.org, pfg@sgi.com, jikos@kernel.org, hans.verkuil@cisco.com, awalls@md.metrocast.net, dledford@redhat.com, sean.hefty@intel.com, kys@microsoft.com, heiko.carstens@de.ibm.com, sumit.semwal@linaro.org, schwidefsky@de.ibm.com, linux-kernel@vger.kernel.org Date: Fri, 24 Jun 2016 09:46:05 -0700 In-Reply-To: <1466453164-13185-1-git-send-email-dave@stgolabs.net> References: <1466453164-13185-1-git-send-email-dave@stgolabs.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.5 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2016-06-20 at 13:05 -0700, Davidlohr Bueso wrote: > Hi, > > The series is really straightforward and based on Peter's work that > introduces[1] the atomic_fetch_$op machinery. Only patch 1 implements > the actual atomic_fetch_{inc,dec} calls based on > atomic_fetch_{add,sub}. Could I just ask why? atomic_inc_return(x) - 1 seems a reasonable thing to do to me. Is it because on architectures where atomics are implemented in asm, it costs us one more CPU instruction to do the extra decrement which gcc can't optimise? If that's it, I'm not sure the added complexity justifies the cycle savings. James