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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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