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.
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