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 <borntraeger@xxxxxxxxxx> > > 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
