[PATCH 2/5] x86/pvclock: no need to use strong read barriers in pvclock_get_time_values

Jeremy Fitzhardinge
2009-10-06T00:51:01Z

The time values are guaranteed to be only updated by the pcpu that the
vcpu is currently running on (preempting the guest), so there's never
any risk of cross-pcpu races. Therefore, we only need a barrier to
prevent compiler reordering.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
arch/x86/kernel/pvclock.c | 4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index 963f349..5ecce7f 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -86,12 +86,12 @@ static unsigned pvclock_get_time_values(struct pvclock_shadow_time *dst,
{
do {
dst->version = src->version;
- rmb(); /* fetch version before data */
+ barrier(); /* fetch version before data */
dst->tsc_timestamp = src->tsc_timestamp;
dst->system_timestamp = src->system_time;
dst->tsc_to_nsec_mul = src->tsc_to_system_mul;
dst->tsc_shift = src->tsc_shift;
- rmb(); /* test version after fetching data */
+ barrier(); /* test version after fetching data */
} while ((src->version & 1) || (dst->version != src->version));

return dst->version;
--
1.6.2.5