Re: [PATCH] sys_time-speedup-small-cleanup

[Oleg Nesterov <oleg@tv-sign.ru>]

On 06/27, Chris Snook wrote:
>
> Oleg Nesterov wrote:
> >On 06/26, Chris Snook wrote:
> >>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. 
> >
> >I can't understand this.
> >
> >Fisrt, smp_rmb() can't help in this case. It can't influence the preceeding
> >LOAD if it was from cache.
> >
> >Even if it could, another CPU can alter the value just after the reading
> >completes, and we have the same situation.
> >
> >Could you please clarify if I am wrong?
> >
> >Oleg.
> >
> 
> You're right, but so is Ingo's patch.  We're not trying to enforce some 
> notion of absolute time, just  make it possible for userspace to guarantee 
> that time cannot be *observed* to travel backwards.  It's still the 
> responsibility of the user to use proper synchronization in multithreaded 
> apps.  Without the smp_rmb() it would be possible on some architectures for 
> the results of the race you describe to leak across other lock-prefixed 
> instructions used to ensure monotonicity in userspace.  Relativity applies 
> to SMP timekeeping, not just space travelers, so if there's no way to prove 
> a race occurred, it doesn't matter whether or not it occurred in some frame 
> of reference.

This doesn't make sense to me, sorry. Could provide more details to explain
the race? Some ascii diagram?

I believe smp_rmb() can't make _any_ difference in this case.

Oleg.