1 files changed, 5 insertions, 4 deletions

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index db697b81b8b1..1e6244202612 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -769,13 +769,14 @@ static cycle_t __vsyscall_fn vread_tsc(void)
         cycle_t ret;
 
         /*
-         * Surround the RDTSC by barriers, to make sure it's not
-         * speculated to outside the seqlock critical section and
-         * does not cause time warps:
+         * Empirically, a fence (of type that depends on the CPU)
+         * before rdtsc is enough to ensure that rdtsc is ordered
+         * with respect to loads.  The various CPU manuals are unclear
+         * as to whether rdtsc can be reordered with later loads,
+         * but no one has ever seen it happen.
          */
         rdtsc_barrier();
         ret = (cycle_t)vget_cycles();
-        rdtsc_barrier();
 
         return ret >= VVAR(vsyscall_gtod_data).clock.cycle_last ?
                 ret : VVAR(vsyscall_gtod_data).clock.cycle_last;