[dpdk-dev] [PATCH v2 6/6] service: relax barriers with C11 atomics

[Phil Yang <phil.yang@arm.com>]

The runstate, comp_runstate and app_runstate are used as guard variables
in the service core lib. To guarantee the inter-threads visibility of
these guard variables, it uses rte_smp_r/wmb. This patch use c11 atomic
built-ins to relax these barriers.

Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
---
 lib/librte_eal/common/rte_service.c | 115 ++++++++++++++++++++++++++----------
 1 file changed, 84 insertions(+), 31 deletions(-)

diff --git a/lib/librte_eal/common/rte_service.c b/lib/librte_eal/common/rte_service.c
index 8cac265..dbb8211 100644
--- a/lib/librte_eal/common/rte_service.c
+++ b/lib/librte_eal/common/rte_service.c
@@ -265,7 +265,6 @@ rte_service_component_register(const struct rte_service_spec *spec,
 	s->spec = *spec;
 	s->internal_flags |= SERVICE_F_REGISTERED | SERVICE_F_START_CHECK;
 
-	rte_smp_wmb();
 	rte_service_count++;
 
 	if (id_ptr)
@@ -282,7 +281,6 @@ rte_service_component_unregister(uint32_t id)
 	SERVICE_VALID_GET_OR_ERR_RET(id, s, -EINVAL);
 
 	rte_service_count--;
-	rte_smp_wmb();
 
 	s->internal_flags &= ~(SERVICE_F_REGISTERED);
 
@@ -301,12 +299,17 @@ rte_service_component_runstate_set(uint32_t id, uint32_t runstate)
 	struct rte_service_spec_impl *s;
 	SERVICE_VALID_GET_OR_ERR_RET(id, s, -EINVAL);
 
+	/* comp_runstate act as the guard variable. Use store-release
+	 * memory order. This synchronizes with load-acquire in
+	 * service_run and service_runstate_get function.
+	 */
 	if (runstate)
-		s->comp_runstate = RUNSTATE_RUNNING;
+		__atomic_store_n(&s->comp_runstate, RUNSTATE_RUNNING,
+				__ATOMIC_RELEASE);
 	else
-		s->comp_runstate = RUNSTATE_STOPPED;
+		__atomic_store_n(&s->comp_runstate, RUNSTATE_STOPPED,
+				__ATOMIC_RELEASE);
 
-	rte_smp_wmb();
 	return 0;
 }
 
@@ -316,12 +319,17 @@ rte_service_runstate_set(uint32_t id, uint32_t runstate)
 	struct rte_service_spec_impl *s;
 	SERVICE_VALID_GET_OR_ERR_RET(id, s, -EINVAL);
 
+	/* app_runstate act as the guard variable. Use store-release
+	 * memory order. This synchronizes with load-acquire in
+	 * service_run runstate_get function.
+	 */
 	if (runstate)
-		s->app_runstate = RUNSTATE_RUNNING;
+		__atomic_store_n(&s->app_runstate, RUNSTATE_RUNNING,
+				__ATOMIC_RELEASE);
 	else
-		s->app_runstate = RUNSTATE_STOPPED;
+		__atomic_store_n(&s->app_runstate, RUNSTATE_STOPPED,
+				__ATOMIC_RELEASE);
 
-	rte_smp_wmb();
 	return 0;
 }
 
@@ -330,15 +338,24 @@ rte_service_runstate_get(uint32_t id)
 {
 	struct rte_service_spec_impl *s;
 	SERVICE_VALID_GET_OR_ERR_RET(id, s, -EINVAL);
-	rte_smp_rmb();
 
-	int check_disabled = !(s->internal_flags & SERVICE_F_START_CHECK);
-	int lcore_mapped = (__atomic_load_n(&s->num_mapped_cores,
+	/* comp_runstate and app_runstate act as the guard variables.
+	 * Use load-acquire memory order. This synchronizes with
+	 * store-release in service state set functions.
+	 */
+	if (__atomic_load_n(&s->comp_runstate,
+			__ATOMIC_ACQUIRE) == RUNSTATE_RUNNING &&
+		 __atomic_load_n(&s->app_runstate,
+			__ATOMIC_ACQUIRE) == RUNSTATE_RUNNING) {
+		int check_disabled = !(s->internal_flags &
+					SERVICE_F_START_CHECK);
+		int lcore_mapped = (__atomic_load_n(&s->num_mapped_cores,
 					    __ATOMIC_RELAXED) > 0);
 
-	return (s->app_runstate == RUNSTATE_RUNNING) &&
-		(s->comp_runstate == RUNSTATE_RUNNING) &&
-		(check_disabled | lcore_mapped);
+		return (check_disabled | lcore_mapped);
+	} else
+		return 0;
+
 }
 
 static inline void
@@ -367,9 +384,15 @@ service_run(uint32_t i, struct core_state *cs, uint64_t service_mask,
 	if (!s)
 		return -EINVAL;
 
-	if (s->comp_runstate != RUNSTATE_RUNNING ||
-			s->app_runstate != RUNSTATE_RUNNING ||
-			!(service_mask & (UINT64_C(1) << i))) {
+	/* comp_runstate and app_runstate act as the guard variables.
+	 * Use load-acquire memory order. This synchronizes with
+	 * store-release in service state set functions.
+	 */
+	if (__atomic_load_n(&s->comp_runstate,
+			__ATOMIC_ACQUIRE) != RUNSTATE_RUNNING ||
+		 __atomic_load_n(&s->app_runstate,
+			__ATOMIC_ACQUIRE) != RUNSTATE_RUNNING ||
+		!(service_mask & (UINT64_C(1) << i))) {
 		cs->service_active_on_lcore[i] = 0;
 		return -ENOEXEC;
 	}
@@ -434,7 +457,12 @@ service_runner_func(void *arg)
 	const int lcore = rte_lcore_id();
 	struct core_state *cs = &lcore_states[lcore];
 
-	while (cs->runstate == RUNSTATE_RUNNING) {
+	/* runstate act as the guard variable. Use load-acquire
+	 * memory order here to synchronize with store-release
+	 * in runstate update functions.
+	 */
+	while (__atomic_load_n(&cs->runstate,
+			__ATOMIC_ACQUIRE) == RUNSTATE_RUNNING) {
 		const uint64_t service_mask = cs->service_mask;
 
 		for (i = 0; i < RTE_SERVICE_NUM_MAX; i++) {
@@ -445,8 +473,6 @@ service_runner_func(void *arg)
 		}
 
 		cs->loops++;
-
-		rte_smp_rmb();
 	}
 
 	lcore_config[lcore].state = WAIT;
@@ -614,15 +640,18 @@ rte_service_lcore_reset_all(void)
 		if (lcore_states[i].is_service_core) {
 			lcore_states[i].service_mask = 0;
 			set_lcore_state(i, ROLE_RTE);
-			lcore_states[i].runstate = RUNSTATE_STOPPED;
+			/* runstate act as guard variable Use
+			 * store-release memory order here to synchronize
+			 * with load-acquire in runstate read functions.
+			 */
+			__atomic_store_n(&lcore_states[i].runstate,
+				RUNSTATE_STOPPED, __ATOMIC_RELEASE);
 		}
 	}
 	for (i = 0; i < RTE_SERVICE_NUM_MAX; i++)
 		__atomic_store_n(&rte_services[i].num_mapped_cores, 0,
 				    __ATOMIC_RELAXED);
 
-	rte_smp_wmb();
-
 	return 0;
 }
 
@@ -638,9 +667,11 @@ rte_service_lcore_add(uint32_t lcore)
 
 	/* ensure that after adding a core the mask and state are defaults */
 	lcore_states[lcore].service_mask = 0;
-	lcore_states[lcore].runstate = RUNSTATE_STOPPED;
-
-	rte_smp_wmb();
+	/* Use store-release memory order here to synchronize with
+	 * load-acquire in runstate read functions.
+	 */
+	__atomic_store_n(&lcore_states[lcore].runstate, RUNSTATE_STOPPED,
+		__ATOMIC_RELEASE);
 
 	return rte_eal_wait_lcore(lcore);
 }
@@ -655,7 +686,12 @@ rte_service_lcore_del(uint32_t lcore)
 	if (!cs->is_service_core)
 		return -EINVAL;
 
-	if (cs->runstate != RUNSTATE_STOPPED)
+	/* runstate act as the guard variable. Use load-acquire
+	 * memory order here to synchronize with store-release
+	 * in runstate update functions.
+	 */
+	if (__atomic_load_n(&cs->runstate,
+			__ATOMIC_ACQUIRE) != RUNSTATE_STOPPED)
 		return -EBUSY;
 
 	set_lcore_state(lcore, ROLE_RTE);
@@ -674,13 +710,21 @@ rte_service_lcore_start(uint32_t lcore)
 	if (!cs->is_service_core)
 		return -EINVAL;
 
-	if (cs->runstate == RUNSTATE_RUNNING)
+	/* runstate act as the guard variable. Use load-acquire
+	 * memory order here to synchronize with store-release
+	 * in runstate update functions.
+	 */
+	if (__atomic_load_n(&cs->runstate,
+			__ATOMIC_ACQUIRE) == RUNSTATE_RUNNING)
 		return -EALREADY;
 
 	/* set core to run state first, and then launch otherwise it will
 	 * return immediately as runstate keeps it in the service poll loop
 	 */
-	cs->runstate = RUNSTATE_RUNNING;
+	/* Use load-acquire memory order here to synchronize with
+	 * store-release in runstate update functions.
+	 */
+	__atomic_store_n(&cs->runstate, RUNSTATE_RUNNING, __ATOMIC_RELEASE);
 
 	int ret = rte_eal_remote_launch(service_runner_func, 0, lcore);
 	/* returns -EBUSY if the core is already launched, 0 on success */
@@ -693,7 +737,12 @@ rte_service_lcore_stop(uint32_t lcore)
 	if (lcore >= RTE_MAX_LCORE)
 		return -EINVAL;
 
-	if (lcore_states[lcore].runstate == RUNSTATE_STOPPED)
+	/* runstate act as the guard variable. Use load-acquire
+	 * memory order here to synchronize with store-release
+	 * in runstate update functions.
+	 */
+	if (__atomic_load_n(&lcore_states[lcore].runstate,
+			__ATOMIC_ACQUIRE) == RUNSTATE_STOPPED)
 		return -EALREADY;
 
 	uint32_t i;
@@ -713,7 +762,11 @@ rte_service_lcore_stop(uint32_t lcore)
 			return -EBUSY;
 	}
 
-	lcore_states[lcore].runstate = RUNSTATE_STOPPED;
+	/* Use store-release memory order here to synchronize with
+	 * load-acquire in runstate read functions.
+	 */
+	__atomic_store_n(&lcore_states[lcore].runstate, RUNSTATE_STOPPED,
+		__ATOMIC_RELEASE);
 
 	return 0;
 }
-- 
2.7.4