> -----Original Message-----
> From: Honnappa Nagarahalli <honnappa.nagaraha...@arm.com>
> Sent: Friday, July 8, 2022 4:16 PM
> To: Van Haaren, Harry <harry.van.haa...@intel.com>; dev@dpdk.org
> Cc: mattias.ronnblom <mattias.ronnb...@ericsson.com>; Morten Brørup
> <m...@smartsharesystems.com>; nd <n...@arm.com>; nd <n...@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?
