From mboxrd@z Thu Jan 1 00:00:00 1970 From: mst@redhat.com (Michael S. Tsirkin) Date: Tue, 5 Jan 2016 18:04:00 +0200 Subject: [PATCH v2 22/32] s390: define __smp_xxx In-Reply-To: <568BE3B9.8020901@de.ibm.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> Message-ID: <20160105180225-mutt-send-email-mst@redhat.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.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