Re: [Xen-devel] [PATCH v2 22/32] s390: define __smp_xxx

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

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

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