From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752521AbcAEQES (ORCPT ); Tue, 5 Jan 2016 11:04:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:46129 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751862AbcAEQEM (ORCPT ); Tue, 5 Jan 2016 11:04:12 -0500 Date: Tue, 5 Jan 2016 18:04:00 +0200 From: "Michael S. Tsirkin" To: Christian Borntraeger Cc: Martin Schwidefsky , Peter Zijlstra , linux-kernel@vger.kernel.org, Arnd Bergmann , linux-arch@vger.kernel.org, Andrew Cooper , virtualization@lists.linux-foundation.org, Stefano Stabellini , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , David Miller , linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-metag@vger.kernel.org, linux-mips@linux-mips.org, x86@kernel.org, user-mode-linux-devel@lists.sourceforge.net, adi-buildroot-devel@lists.sourceforge.net, linux-sh@vger.kernel.org, linux-xtensa@linux-xtensa.org, xen-devel@lists.xenproject.org, Heiko Carstens , Ingo Molnar , Davidlohr Bueso , Andrey Konovalov Subject: Re: [PATCH v2 22/32] s390: define __smp_xxx Message-ID: <20160105180225-mutt-send-email-mst@redhat.com> References: <1451572003-2440-1-git-send-email-mst@redhat.com> <1451572003-2440-23-git-send-email-mst@redhat.com> <20160104134525.GA6344@twins.programming.kicks-ass.net> <20160104221323-mutt-send-email-mst@redhat.com> <20160105091319.59ddefc7@mschwide> <20160105105335-mutt-send-email-mst@redhat.com> <568BE3B9.8020901@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <568BE3B9.8020901@de.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 05, 2016 at 04:39:37PM +0100, Christian Borntraeger wrote: > On 01/05/2016 10:30 AM, Michael S. Tsirkin wrote: > > > > > arch/s390/kernel/vdso.c: smp_mb(); > > > > Looking at > > Author: Christian Borntraeger > > Date: Fri Sep 11 16:23:06 2015 +0200 > > > > s390/vdso: use correct memory barrier > > > > By definition smp_wmb only orders writes against writes. (Finish all > > previous writes, and do not start any future write). To protect the > > vdso init code against early reads on other CPUs, let's use a full > > smp_mb at the end of vdso init. As right now smp_wmb is implemented > > as full serialization, this needs no stable backport, but this change > > will be necessary if we reimplement smp_wmb. > > > > ok from hypervisor point of view, but it's also strange: > > 1. why isn't this paired with another mb somewhere? > > this seems to violate barrier pairing rules. > > 2. how does smp_mb protect against early reads on other CPUs? > > It normally does not: it orders reads from this CPU versus writes > > from same CPU. But init code does not appear to read anything. > > Maybe this is some s390 specific trick? > > > > I could not figure out the above commit. > > It was probably me misreading the code. I change a wmb into a full mb here > since I was changing the defintion of wmb to a compiler barrier. I tried to > fixup all users of wmb that really pair with other code. I assumed that there > must be some reader (as there was a wmb before) but I could not figure out > which. So I just played safe here. > > But it probably can be removed. > > > arch/s390/kvm/kvm-s390.c: smp_mb(); > > This can go. If you have a patch, I can carry that via the kvms390 tree, > or I will spin a new patch with you as suggested-by. > > Christian I have both, will post shortly. -- MST