Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Aug 15, 2014 at 04:26:01PM +0200, Oleg Nesterov wrote:
> On 08/15, Frederic Weisbecker wrote:
> >
> > 2014-08-14 16:39 GMT+02:00 Oleg Nesterov <oleg@redhat.com>:
> > > On 08/14, Frederic Weisbecker wrote:
> > >>
> > >> I mean the read side doesn't use a lock with seqlocks. It's only made
> > >> of barriers and sequence numbers to ensure the reader doesn't read
> > >> some half-complete update. But other than that it can as well see the
> > >> update n - 1 since barriers don't enforce latest results.
> > >
> > > Yes, sure, read_seqcount_begin/read_seqcount_retry "right after"
> > > write_seqcount_begin-update-write_seqcount_begin can miss "update" part
> > > along with ->sequence modifications.
> > >
> > > But I still can't understand how this can lead to non-monotonic results,
> > > could you spell?
> >
> > Well lets say clock = T.
> > CPU 0 updates at T + 1.
> > Then I call clock_gettime() from CPU 1 and CPU 2. CPU 1 reads T + 1
> > while CPU 1 still reads T.
> > If I do yet another round of clock_gettime() on CPU 1 and CPU 2, it's
> > possible that CPU 2 still sees T. With the spinlocked version that
> > thing can't happen, the second round would read at least T + 1 for
> > both CPUs.
> 
> But this is fine? And CPU 2 doesn't see a non-monotonic result?
> 
> OK, this could be wrong if, say,
> 
> 	void print_clock(void)
> 	{
> 		lock(SOME_LOCK);
> 		printk(..., clock_gettime());
> 		unlock(SOME_LOCK);
> 	}
> 	
> printed the non-monotonic numbers if print_clock() is called on CPU_1 and
> then on CPU_2. But in this case CPU_2 can't miss the changes on CPU_0 if
> they were already visible to CPU_1 under the same lock. IOW,
> 
> 	int T = 0;	/* can be incremented at any time */
> 
> 	void check_monotony(void)
> 	{
> 		static int t = 0;
> 
> 		lock(SOME_LOCK);
> 		BUG(t > T);
> 		T = t;
> 		unlock(SOME_LOCK);
> 	}
> 
> must work corrrectly (ignoring overflow) even if T is changed without
> SOME_LOCK.
> 
> Otherwise, without some sort of synchronization the different results on
> CPU_1/2 should be fine.
> 
> Or I am still missing your point?

No I think you're right, as long as ordering against something else is involved,
monotonicity is enforced.

Now I'm trying to think about a case where SMP ordering isn't involved.
Perhaps some usecase based on coupling CPU local clocks and clock_gettime()
where a drift between both can appear. Now using a local clock probably only
makes sense in the context of local usecases where the thread clock update
would be local as well. So that's probably not a problem. Now what if somebody
couples multithread process wide clocks with per CPU local clocks. Well that's
probably too foolish to be considered.