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SubjectRe: [PATCH v2 22/32] s390: define __smp_xxx
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@de.ibm.com>
> > 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


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