[PATCH] x86/vdso/pvclock: Protect STABLE check with the seqcount

[Andy Lutomirski <luto@kernel.org>]

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@redhat.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---

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