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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 92D58C282CE for ; Tue, 4 Jun 2019 17:06:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5E279207E0 for ; Tue, 4 Jun 2019 17:06:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727951AbfFDRGP (ORCPT ); Tue, 4 Jun 2019 13:06:15 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35990 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727715AbfFDRGP (ORCPT ); Tue, 4 Jun 2019 13:06:15 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id E7E3281E0A; Tue, 4 Jun 2019 17:06:14 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-85.bos.redhat.com [10.18.17.85]) by smtp.corp.redhat.com (Postfix) with ESMTP id BF20C1001DFD; Tue, 4 Jun 2019 17:06:11 +0000 (UTC) Subject: Re: [PATCH v8 17/19] locking/rwsem: Merge owner into count on x86-64 To: Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , Borislav Petkov , "H. Peter Anvin" , linux-kernel@vger.kernel.org, x86@kernel.org, Davidlohr Bueso , Linus Torvalds , Tim Chen , huang ying References: <20190520205918.22251-1-longman@redhat.com> <20190520205918.22251-18-longman@redhat.com> <20190604094537.GK3402@hirez.programming.kicks-ass.net> <20190604170218.GE3419@hirez.programming.kicks-ass.net> From: Waiman Long Organization: Red Hat Message-ID: <28a6c7b5-c40e-1c89-03e2-688c1135f3b5@redhat.com> Date: Tue, 4 Jun 2019 13:06:11 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190604170218.GE3419@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Tue, 04 Jun 2019 17:06:15 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/4/19 1:02 PM, Peter Zijlstra wrote: > On Tue, Jun 04, 2019 at 11:47:21AM -0400, Waiman Long wrote: >> On 6/4/19 5:45 AM, Peter Zijlstra wrote: >>> On Mon, May 20, 2019 at 04:59:16PM -0400, Waiman Long wrote: >>>> With separate count and owner, there are timing windows where the two >>>> values are inconsistent. That can cause problem when trying to figure >>>> out the exact state of the rwsem. For instance, a RT task will stop >>>> optimistic spinning if the lock is acquired by a writer but the owner >>>> field isn't set yet. That can be solved by combining the count and >>>> owner together in a single atomic value. >>> I just realized we can use cmpxchg_double() here (where available of >>> course). >> Does the 2 doubles need to be 128-bit aligned to use cmpxchg_double()? I >> don't think we can guarantee that unless we explicitly set this alignment. > It does :/ and yes, we'd need to play games with __align(2*sizeof(long)) > and such. So do you want this as an option now as it will be x86 specific? Or we can do that as a follow-up if we want to. Cheers, Longman