On Thu, Jan 07, 2016 at 01:13:41PM -0800, Andy Lutomirski wrote:
> On Thu, Jan 7, 2016 at 1:02 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> > On Mon, Jan 04, 2016 at 03:14:28PM -0800, Andy Lutomirski wrote:
> >> If the clock becomes unstable while we're reading it, we need to
> >> bail. We can do this by simply moving the check into the seqcount
> >> loop.
> >>
> >> Reported-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> >> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> >> ---
> >>
> >> Marcelo, how's this?
> >>
> >> arch/x86/entry/vdso/vclock_gettime.c | 12 ++++++------
> >> 1 file changed, 6 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
> >> index 8602f06c759f..1a50e09c945b 100644
> >> --- a/arch/x86/entry/vdso/vclock_gettime.c
> >> +++ b/arch/x86/entry/vdso/vclock_gettime.c
> >> @@ -126,23 +126,23 @@ static notrace cycle_t vread_pvclock(int *mode)
> >> *
> >> * On Xen, we don't appear to have that guarantee, but Xen still
> >> * supplies a valid seqlock using the version field.
> >> -
> >> + *
> >> * We only do pvclock vdso timing at all if
> >> * PVCLOCK_TSC_STABLE_BIT is set, and we interpret that bit to
> >> * mean that all vCPUs have matching pvti and that the TSC is
> >> * synced, so we can just look at vCPU 0's pvti.
> >> */
> >>
> >> - if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
> >> - *mode = VCLOCK_NONE;
> >> - return 0;
> >> - }
> >> -
> >> do {
> >> version = pvti->version;
> >>
> >> smp_rmb();
> >>
> >> + if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
> >> + *mode = VCLOCK_NONE;
> >> + return 0;
> >> + }
> >> +
> >> tsc = rdtsc_ordered();
> >> pvti_tsc_to_system_mul = pvti->tsc_to_system_mul;
> >> pvti_tsc_shift = pvti->tsc_shift;
> >> --
> >> 2.4.3
> >
> > Check it before returning the value (once cleared, it can't be set back
> > to 1), similarly to what was in place before.
> >
> >
>
> I don't understand what you mean.
>
> In the old code (4.3 and 4.4), the vdso checks STABLE_BIT at the end,
> which is correct as long as STABLE_BIT can never change from 0 to 1.
>
> In the -tip code, it's clearly wrong.
>
> In the code in this patch, it should be correct regardless of how
> STABLE_BIT changes as long as the seqcount works. Given that the
> performance cost of doing that is zero, I'd rather keep it that way.
> If we're really paranoid, we could move it after the rest of the pvti
> reads and add a barrier, but is there really any host on which that
> matters?
>
> --Andy
>
> --
> Andy Lutomirski
> AMA Capital Management, LLC
Right, its OK due to version check, thanks.
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