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

[Harry van Haaren]

This commit fixes a potential racey-add that could occur if
multiple service-lcores were executing the same MT-safe service
at the same time, with service statistics collection enabled.

Because multiple threads can run and execute the service, the
stats values can have multiple writer threads, resulting in the
requirement of using atomic addition for correctness.

Note that when a MT unsafe service is executed, a spinlock is
held, so the stats increments are protected. This fact is used
to avoid executing atomic add instructions when not required.

This patch causes a 1.25x increase in cycle-cost for polling a
MT safe service when statistics are enabled. No change was seen
for MT unsafe services, or when statistics are disabled.

Reported-by: Mattias Rönnblom <mattias.ronnblom at ericsson.com>
Suggested-by: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>
Suggested-by: Morten Brørup <mb at smartsharesystems.com>
Signed-off-by: Harry van Haaren <harry.van.haaren at intel.com>

---
---
 lib/eal/common/rte_service.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

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++;
+		}
 	} else
 		s->spec.callback(userdata);
 }
-- 
2.32.0