Re: [PATCH 2/2] service: fix potential stats race-condition on MT services

[Bruce Richardson <bruce.richardson@intel.com>]

On Fri, Jul 08, 2022 at 03:31:01PM +0000, Van Haaren, Harry wrote:
> > -----Original Message-----
> > From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> > Sent: Friday, July 8, 2022 4:16 PM
> > To: Van Haaren, Harry <harry.van.haaren@intel.com>; dev@dpdk.org
> > Cc: mattias.ronnblom <mattias.ronnblom@ericsson.com>; Morten Brørup
> > <mb@smartsharesystems.com>; nd <nd@arm.com>; nd <nd@arm.com>
> > Subject: RE: [PATCH 2/2] service: fix potential stats race-condition on MT services
> 
> <snip previous discussions>
> 
> > > diff --git a/lib/eal/common/rte_service.c b/lib/eal/common/rte_service.c
> > > index ef31b1f63c..f045e74ef3 100644
> > > --- a/lib/eal/common/rte_service.c
> > > +++ b/lib/eal/common/rte_service.c
> > > @@ -363,9 +363,15 @@ service_runner_do_callback(struct
> > > rte_service_spec_impl *s,
> > >  		uint64_t start = rte_rdtsc();
> > >  		s->spec.callback(userdata);
> > >  		uint64_t end = rte_rdtsc();
> > > -		s->cycles_spent += end - start;
> > > +		uint64_t cycles = end - start;
> > >  		cs->calls_per_service[service_idx]++;
> > > -		s->calls++;
> > > +		if (service_mt_safe(s)) {
> > > +			__atomic_fetch_add(&s->cycles_spent, cycles,
> > > __ATOMIC_RELAXED);
> > > +			__atomic_fetch_add(&s->calls, 1,
> > > __ATOMIC_RELAXED);
> > > +		} else {
> > > +			s->cycles_spent += cycles;
> > > +			s->calls++;
> > This is still a problem from a reader perspective. It is possible that the writes could be
> > split while a reader is reading the stats. These need to be atomic adds.
> 
> Thanks for pointing out; I do "think" in x86 in terms of load/store tearing; and on x86
> naturally aligned load/stores will not tear. Apologies for missing the ARM angle here.
> 
> I'm not sure how to best encode the difference between tearing & "locked instructions"
> to make things multi-writer safe. But they're not the same thing, and I'd prefer not pay
> the penalty for LOCK instructions (multi-writer) only to satisfy the non-tearing requirements.
> 
> Is there an rte_atomic-* type that is guaranteed as non-tearing?
> 
> In that case, changing the type of the calls/cycles_spent variables to such a type to ensure "non-tearing"
> single-reader, single-writer behaviour is enough, instead of forcing __atomic_fetch_add() everywhere?

Regular read, increment and then atomic store should work without locks
where alignment is correct on most architectures, and doing the store
atomically should prevent any tearing.