From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends() Date: Tue, 5 Dec 2017 23:42:44 +0200 Message-ID: <20171205232516-mutt-send-email-mst@kernel.org> References: <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> <20171205211735.GT3165@worktop.lehotels.local> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii 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 To: Peter Zijlstra Return-path: Content-Disposition: inline In-Reply-To: <20171205211735.GT3165@worktop.lehotels.local> Sender: kvm-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, Dec 05, 2017 at 10:17:35PM +0100, Peter Zijlstra wrote: > 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. I suspect I have to - ptr_ring is a very low level construct used by netowrking on data path so making it a bit more complicated for a bit of performance is probably justified. -- MST