Oleg Nesterov wrote:
on top of sys_time-speedup.patch
Ingo Molnar wrote:
asmlinkage long sys_time(time_t __user * tloc)
{
- time_t i;
- struct timeval tv;
+ /*
+ * We read xtime.tv_sec atomically - it's updated
+ * atomically by update_wall_time(), so no need to
+ * even read-lock the xtime seqlock:
+ */
+ time_t i = xtime.tv_sec;
- do_gettimeofday(&tv);
- i = tv.tv_sec;
+ smp_rmb(); /* sys_time() results are coherent */
Why do we need this barrier? My guess it is needed to prevent
the reading of xtime.tv_sec twice, yes? In that case a simple
barrier() should be enough.
Without the smp_rmb, you can potentially have a situation where one CPU is still
reading an old value from cache while another has the new value. It's generally
a rather small race window on most architectures, but very bad things can happen
if time ever goes backwards, so it's worth the overhead of maintaining coherence
on smp.
-- Chris
Signed-off-by: Oleg Nesterov <[email protected]>
--- t/kernel/time.c~ 2007-06-26 16:28:59.000000000 +0400
+++ t/kernel/time.c 2007-06-26 16:32:09.000000000 +0400
@@ -64,7 +64,7 @@ asmlinkage long sys_time(time_t __user *
*/
time_t i = xtime.tv_sec;
- smp_rmb(); /* sys_time() results are coherent */
+ barrier(); /* sys_time() results are coherent */
if (tloc) {
if (put_user(i, tloc))
-
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