From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752077AbdLETSC (ORCPT ); Tue, 5 Dec 2017 14:18:02 -0500 Received: from merlin.infradead.org ([205.233.59.134]:39614 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751208AbdLETR6 (ORCPT ); Tue, 5 Dec 2017 14:17:58 -0500 Date: Tue, 5 Dec 2017 20:17:33 +0100 From: Peter Zijlstra To: "Michael S. Tsirkin" Cc: "Paul E. McKenney" , linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, Jason Wang , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends() Message-ID: <20171205191733.GQ3165@worktop.lehotels.local> References: <20171201195053.GA23494@linux.vnet.ibm.com> <1512157876-24665-21-git-send-email-paulmck@linux.vnet.ibm.com> <20171205202928-mutt-send-email-mst@kernel.org> <20171205183946.GP3165@worktop.lehotels.local> <20171205204928-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171205204928-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 05, 2017 at 08:57:46PM +0200, Michael S. Tsirkin wrote: > I don't see WRITE_ONCE inserting any barriers, release or > write. Correct, never claimed there was. Just saying that: obj = READ_ONCE(*foo); val = READ_ONCE(obj->val); Never needs a barrier (except on Alpha and we want to make that go away). Simply because a CPU needs to complete the load of @obj before it can compute the address &obj->val. Thus the second load _must_ come after the first load and we get LOAD-LOAD ordering. Alpha messing that up is a royal pain, and Alpha not being an active/living architecture is just not worth the pain of keeping this in the generic model.