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SubjectRe: [PATCH] sys_time-speedup-small-cleanup
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 <oleg@tv-sign.ru>
>
> --- 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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