On Thu, Mar 26, 2015 at 09:59:24PM +0100, Radim Krčmář wrote:
> 2015-03-23 20:21-0300, Marcelo Tosatti:
> >
> > The following point:
> >
> > 2. per-CPU pvclock time info is updated if the
> > underlying CPU changes.
> >
> > Is not true anymore since "KVM: x86: update pvclock area conditionally,
> > on cpu migration".
> >
> > Add task migration notification back.
> >
> > Problem noticed by Andy Lutomirski.
> >
> > Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> > CC: stable@xxxxxxxxxx # 3.11+
>
> Revert contains a bug that got pointed out in the discussion:
>
> > diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
> > do {
> > cpu = __getcpu() & VGETCPU_CPU_MASK;
> >
> > pvti = get_pvti(cpu);
>
> We can migrate to 'other cpu' here.
>
> > + migrate_count = pvti->migrate_count;
> > +
> > version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
>
> And migrate back to 'cpu' here.
Migrating back will increase pvti->migrate_count, right ?
> rdtsc was executed on different cpu, so pvti and tsc might not be in
> sync, but migrate_count hasn't changed.
>
> > cpu1 = __getcpu() & VGETCPU_CPU_MASK;
>
> (Reading cpuid here is useless.)
>
> > } while (unlikely(cpu != cpu1 ||
> > (pvti->pvti.version & 1) ||
> > - pvti->pvti.version != version));
> > + pvti->pvti.version != version ||
> > + pvti->migrate_count != migrate_count));
>
> We can workaround the bug with,
>
> cpu = __getcpu() & VGETCPU_CPU_MASK;
> pvti = get_pvti(cpu);
> migrate_count = pvti->migrate_count;
> if (cpu != (__getcpu() & VGETCPU_CPU_MASK))
> continue;
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