Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751680AbdLEVRw (ORCPT ); Tue, 5 Dec 2017 16:17:52 -0500 Received: from bombadil.infradead.org ([65.50.211.133]:51740 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751066AbdLEVRt (ORCPT ); Tue, 5 Dec 2017 16:17:49 -0500 Date: Tue, 5 Dec 2017 22:17:35 +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: <20171205211735.GT3165@worktop.lehotels.local> References: <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> <20171205191733.GQ3165@worktop.lehotels.local> <20171205212053-mutt-send-email-mst@kernel.org> <20171205193339.GP7829@linux.vnet.ibm.com> <20171205215020-mutt-send-email-mst@kernel.org> <20171205195752.GS3165@worktop.lehotels.local> <20171205222757-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171205222757-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 Content-Length: 805 Lines: 21 On Tue, Dec 05, 2017 at 10:28:38PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 08:57:52PM +0100, Peter Zijlstra wrote: > > On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote: > > > > > WRITE_ONCE(obj->val, 1); > > > > > smp_wmb(); > > > > > WRITE_ONCE(*foo, obj); > > > > > > > > I believe Peter was instead suggesting: > > > > > > > > WRITE_ONCE(obj->val, 1); > > > > smp_store_release(foo, obj); > > > > > > Isn't that more expensive though? > > > > Depends on the architecture. The only architecture where it is more > > expensive and people actually still care about is ARM I think. > > Right. Why should I use the more expensive smp_store_release then? Because it makes more sense. Memory ordering is hard enough, don't make it harder still if you don't have to.