From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752872AbdLES5x (ORCPT ); Tue, 5 Dec 2017 13:57:53 -0500 Received: from mx1.redhat.com ([209.132.183.28]:49296 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752126AbdLES5v (ORCPT ); Tue, 5 Dec 2017 13:57:51 -0500 Date: Tue, 5 Dec 2017 20:57:46 +0200 From: "Michael S. Tsirkin" To: Peter Zijlstra 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: <20171205204928-mutt-send-email-mst@kernel.org> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171205183946.GP3165@worktop.lehotels.local> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Tue, 05 Dec 2017 18:57:51 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 05, 2017 at 07:39:46PM +0100, Peter Zijlstra wrote: > On Tue, Dec 05, 2017 at 08:31:20PM +0200, Michael S. Tsirkin wrote: > > > Apropos, READ_ONCE is now asymmetrical with WRITE_ONCE. > > > > I can read a pointer with READ_ONCE and be sure the value > > is sane, but only if I also remember to put in smp_wmb before > > WRITE_ONCE. Otherwise the pointer is ok but no guarantees > > about the data pointed to. > > That was already the case on everything except Alpha. And the canonical > match do the data dependency is store_release, not wmb. Oh, interesting static __always_inline void __write_once_size(volatile void *p, void *res, int size) { switch (size) { case 1: *(volatile __u8 *)p = *(__u8 *)res; break; case 2: *(volatile __u16 *)p = *(__u16 *)res; break; case 4: *(volatile __u32 *)p = *(__u32 *)res; break; case 8: *(volatile __u64 *)p = *(__u64 *)res; break; default: barrier(); __builtin_memcpy((void *)p, (const void *)res, size); barrier(); } } #define WRITE_ONCE(x, val) \ ({ \ union { typeof(x) __val; char __c[1]; } __u = \ { .__val = (__force typeof(x)) (val) }; \ __write_once_size(&(x), __u.__c, sizeof(x)); \ __u.__val; \ }) I don't see WRITE_ONCE inserting any barriers, release or write. So it seems that on an architecture where writes can be reordered, if I do *pointer = 0xa; WRITE_ONCE(array[x], pointer); array write might bypass the pointer write, and readers will read a stale value. -- MST