From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 652E2C3279B for ; Fri, 6 Jul 2018 11:44:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 251B523E92 for ; Fri, 6 Jul 2018 11:44:41 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 251B523E92 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=c-sky.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932816AbeGFLoi (ORCPT ); Fri, 6 Jul 2018 07:44:38 -0400 Received: from smtp2200-217.mail.aliyun.com ([121.197.200.217]:56843 "EHLO smtp2200-217.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932348AbeGFLog (ORCPT ); Fri, 6 Jul 2018 07:44:36 -0400 X-Alimail-AntiSpam: AC=CONTINUE;BC=0.0747115|-1;CH=green;FP=0|0|0|0|0|-1|-1|-1;HT=e02c03295;MF=ren_guo@c-sky.com;NM=1;PH=DS;RN=12;RT=12;SR=0;TI=SMTPD_---.CN48llI_1530877443; Received: from localhost(mailfrom:ren_guo@c-sky.com fp:SMTPD_---.CN48llI_1530877443) by smtp.aliyun-inc.com(10.147.41.187); Fri, 06 Jul 2018 19:44:03 +0800 Date: Fri, 6 Jul 2018 19:44:03 +0800 From: Guo Ren To: Peter Zijlstra Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, daniel.lezcano@linaro.org, jason@lakedaemon.net, arnd@arndb.de, c-sky_gcc_upstream@c-sky.com, gnu-csky@mentor.com, thomas.petazzoni@bootlin.com, wbx@uclibc-ng.org, green.hu@gmail.com Subject: Re: [PATCH V2 11/19] csky: Atomic operations Message-ID: <20180706114402.GB27148@guoren> References: <860b8db036b33d7b3648cb1f4ec827a53dc1a01b.1530465326.git.ren_guo@c-sky.com> <20180705175902.GF2530@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180705175902.GF2530@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 05, 2018 at 07:59:02PM +0200, Peter Zijlstra wrote: > On Mon, Jul 02, 2018 at 01:30:14AM +0800, Guo Ren wrote: > > > +static inline void arch_spin_lock(arch_spinlock_t *lock) > > +{ > > + unsigned int *p = &lock->lock; > > + unsigned int tmp; > > + > > + asm volatile ( > > + "1: ldex.w %0, (%1) \n" > > + " bnez %0, 1b \n" > > + " movi %0, 1 \n" > > + " stex.w %0, (%1) \n" > > + " bez %0, 1b \n" > > + : "=&r" (tmp) > > + : "r"(p) > > + : "memory"); > > + smp_mb(); > > +} > > Test-and-set with MB acting as ACQUIRE, ok. Em ... Ok, I'll try to use test-and-set function instead of it. > > +static inline void arch_spin_unlock(arch_spinlock_t *lock) > > +{ > > + unsigned int *p = &lock->lock; > > + unsigned int tmp; > > + > > + smp_mb(); > > + asm volatile ( > > + "1: ldex.w %0, (%1) \n" > > + " movi %0, 0 \n" > > + " stex.w %0, (%1) \n" > > + " bez %0, 1b \n" > > + : "=&r" (tmp) > > + : "r"(p) > > + : "memory"); > > +} > > MB acting for RELEASE, but _why_ are you using a LDEX/STEX to clear the > lock word? Would not a normal store work? Normal store is enough, I'll fixup it in next version patch. > Also, the fact that you need MB for release implies your LDEX does not > in fact imply anything and your xchg/cmpxchg implementation is broken. xchg/cmxchg broken without 1th smp_mb()? Why we need protect the instructions flow before the ldex.w? > > +static inline int arch_spin_trylock(arch_spinlock_t *lock) > > +{ > > + unsigned int *p = &lock->lock; > > + unsigned int tmp; > > + > > + asm volatile ( > > + "1: ldex.w %0, (%1) \n" > > + " bnez %0, 2f \n" > > + " movi %0, 1 \n" > > + " stex.w %0, (%1) \n" > > + " bez %0, 1b \n" > > + " movi %0, 0 \n" > > + "2: \n" > > + : "=&r" (tmp) > > + : "r"(p) > > + : "memory"); > > + smp_mb(); > > + > > + return !tmp; > > +} > > Strictly speaking you can avoid the MB on failure. You only need to > provide ACQUIRE semantics on success. > > That said, I would really suggest you implement a ticket lock instead of > a test-and-set lock. They're not really all that complicated and do > provide better worst case behaviour. Ok, I'll try to implement ticket lock in next version patch. > > > > +/****** read lock/unlock/trylock ******/ > > Please have a look at using qrwlock -- esp. if you implement a ticket > lock, then the rwlock comes for 'free'. Ok, I'll try it. Guo Ren