From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Date: Tue, 05 Jan 2016 15:39:37 +0000 Subject: Re: [PATCH v2 22/32] s390: define __smp_xxx Message-Id: <568BE3B9.8020901@de.ibm.com> List-Id: 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> In-Reply-To: <20160105105335-mutt-send-email-mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "Michael S. Tsirkin" , Martin Schwidefsky Cc: Peter Zijlstra , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Arnd Bergmann , linux-arch-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Andrew Cooper , virtualization-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Stefano Stabellini , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , David Miller , linux-ia64-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linuxppc-dev-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org, linux-s390-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, sparclinux-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, linux-metag-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mips-6z/3iImG2C8G8FEW9MqTrA@public.gmane.org, x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, user-mode-linux-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, adi-buildroot-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, linux-sh-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-xtensa-PjhNF2WwrV/0Sa2dR60CXw@public.gmane.org, xen-devel-GuqFBffKawtpuQazS67q72D2FQJk+8+b@public.gmane.org, Heiko Carstens , Ingo Molnar 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