From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends() Date: Tue, 5 Dec 2017 11:33:39 -0800 Message-ID: <20171205193339.GP7829@linux.vnet.ibm.com> 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> <20171205191733.GQ3165@worktop.lehotels.local> <20171205212053-mutt-send-email-mst@kernel.org> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20171205212053-mutt-send-email-mst@kernel.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: "Michael S. Tsirkin" Cc: tglx@linutronix.de, kvm@vger.kernel.org, Peter Zijlstra , fweisbec@gmail.com, jiangshanlai@gmail.com, linux-kernel@vger.kernel.org, rostedt@goodmis.org, josh@joshtriplett.org, dhowells@redhat.com, edumazet@google.com, netdev@vger.kernel.org, mathieu.desnoyers@efficios.com, oleg@redhat.com, dipankar@in.ibm.com, akpm@linux-foundation.org, virtualization@lists.linux-foundation.org, mingo@kernel.org List-Id: virtualization@lists.linuxfoundation.org On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 08:17:33PM +0100, Peter Zijlstra wrote: > > 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. > > > > Right. What I am saying is that for writes you need > > 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); > and this barrier is no longer paired with anything until > you realize there's a dependency barrier within READ_ONCE. > > Barrier pairing was a useful tool to check code validity, > maybe there are other, better tools now. There are quite a few people who say that smp_store_release() is easier for the tools to analyze than is smp_wmb(). My experience with smp_read_barrier_depends() and rcu_dereference() leads me to believe that they are correct. Thanx, Paul