From owner-svn-src-head@freebsd.org Fri Apr 26 07:38:45 2019 Return-Path: Delivered-To: svn-src-head@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 88DA0158BDED; Fri, 26 Apr 2019 07:38:45 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id BC1C287335; Fri, 26 Apr 2019 07:38:44 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.15.2/8.15.2) with ESMTPS id x3Q7cbQP044429 (version=TLSv1.2 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO); Fri, 26 Apr 2019 10:38:40 +0300 (EEST) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua x3Q7cbQP044429 Received: (from kostik@localhost) by tom.home (8.15.2/8.15.2/Submit) id x3Q7caLf044428; Fri, 26 Apr 2019 10:38:36 +0300 (EEST) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Fri, 26 Apr 2019 10:38:36 +0300 From: Konstantin Belousov To: Mark Johnston Cc: Wojciech Macek , Wojciech Macek , src-committers , svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r346593 - head/sys/sys Message-ID: <20190426073836.GP12936@kib.kiev.ua> References: <201904230636.x3N6aWQK057863@repo.freebsd.org> <20190425040817.GA3789@spy> <20190425082222.GJ12936@kib.kiev.ua> <20190426060456.GA59853@spy> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190426060456.GA59853@spy> User-Agent: Mutt/1.11.4 (2019-03-13) X-Spam-Status: No, score=-1.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FORGED_GMAIL_RCVD,FREEMAIL_FROM, NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on tom.home X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 26 Apr 2019 07:38:45 -0000 On Fri, Apr 26, 2019 at 02:04:56AM -0400, Mark Johnston wrote: > On Thu, Apr 25, 2019 at 11:22:22AM +0300, Konstantin Belousov wrote: > > On Thu, Apr 25, 2019 at 07:38:21AM +0200, Wojciech Macek wrote: > > > Intel does not reorder reads against the condition "if" here. I know for > > > sure that ARM does, but therestill might be some other architectures that > > > also suffers such behavior - I just don't have any means to verify. > > > I remember the discussion for rS302292 where we agreed that this kind of > > > patches should be the least impacting in perfomrance as possible. Adding > > > unconditional memory barrier causes significant performance drop on Intel, > > > where in fact, the issue was never seen. > > > > > Atomic_thread_fence_acq() is nop on x86, or rather, it is compiler memory > > barrier. If you need read/read fence on some architectures, I am sure > > that you need compiler barrier on all. > > To add a bit, one reason to prefer atomic(9) to explicit fences is > precisely because it issues fences only when required by a given > CPU architecture. There is no "unconditional memory barrier" added by > the diff even without the #ifdef. Well, atomic_thread_fence_acq() is the explicit fence. And on x86 it does add unconditional compiler memory barrier.