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

[Chris Snook <csnook@redhat.com>]

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.

	-- Chris
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