[dpdk-dev] [PATCH v4 4/4] eventdev: relax smp barriers with C11 atomics

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

The impl_opaque field is shared between the timer arm and cancel
operations. Meanwhile, the state flag acts as a guard variable to
make sure the update of impl_opaque is synchronized. The original
code uses rte_smp barriers to achieve that. This patch uses C11
atomics with an explicit one-way memory barrier instead of full
barriers rte_smp_w/rmb() to avoid the unnecessary barrier on aarch64.

Since compilers can generate the same instructions for volatile and
non-volatile variable in C11 __atomics built-ins, so remain the volatile
keyword in front of state enum to avoid the ABI break issue.

Cc: stable@dpdk.org

Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>
---
v4:
1. Fix typo.
2. Cc to stable release. (Honnappa)

v3:
Fix ABI issue: revert to 'volatile enum rte_event_timer_state type state'.

v2:
1. Removed implementation-specific opaque data cleanup code.
2. Replaced thread fence with atomic ACQURE/RELEASE ordering on state access.

 lib/librte_eventdev/rte_event_timer_adapter.c | 55 ++++++++++++++++++---------
 1 file changed, 37 insertions(+), 18 deletions(-)

diff --git a/lib/librte_eventdev/rte_event_timer_adapter.c b/lib/librte_eventdev/rte_event_timer_adapter.c
index aa01b4d..4c5e49e 100644
--- a/lib/librte_eventdev/rte_event_timer_adapter.c
+++ b/lib/librte_eventdev/rte_event_timer_adapter.c
@@ -629,7 +629,8 @@ swtim_callback(struct rte_timer *tim)
 		sw->expired_timers[sw->n_expired_timers++] = tim;
 		sw->stats.evtim_exp_count++;
 
-		evtim->state = RTE_EVENT_TIMER_NOT_ARMED;
+		__atomic_store_n(&evtim->state, RTE_EVENT_TIMER_NOT_ARMED,
+				__ATOMIC_RELEASE);
 	}
 
 	if (event_buffer_batch_ready(&sw->buffer)) {
@@ -1020,6 +1021,7 @@ __swtim_arm_burst(const struct rte_event_timer_adapter *adapter,
 	int n_lcores;
 	/* Timer list for this lcore is not in use. */
 	uint16_t exp_state = 0;
+	enum rte_event_timer_state n_state;
 
 #ifdef RTE_LIBRTE_EVENTDEV_DEBUG
 	/* Check that the service is running. */
@@ -1060,30 +1062,36 @@ __swtim_arm_burst(const struct rte_event_timer_adapter *adapter,
 	}
 
 	for (i = 0; i < nb_evtims; i++) {
-		/* Don't modify the event timer state in these cases */
-		if (evtims[i]->state == RTE_EVENT_TIMER_ARMED) {
+		n_state = __atomic_load_n(&evtims[i]->state, __ATOMIC_ACQUIRE);
+		if (n_state == RTE_EVENT_TIMER_ARMED) {
 			rte_errno = EALREADY;
 			break;
-		} else if (!(evtims[i]->state == RTE_EVENT_TIMER_NOT_ARMED ||
-			     evtims[i]->state == RTE_EVENT_TIMER_CANCELED)) {
+		} else if (!(n_state == RTE_EVENT_TIMER_NOT_ARMED ||
+			     n_state == RTE_EVENT_TIMER_CANCELED)) {
 			rte_errno = EINVAL;
 			break;
 		}
 
 		ret = check_timeout(evtims[i], adapter);
 		if (unlikely(ret == -1)) {
-			evtims[i]->state = RTE_EVENT_TIMER_ERROR_TOOLATE;
+			__atomic_store_n(&evtims[i]->state,
+					RTE_EVENT_TIMER_ERROR_TOOLATE,
+					__ATOMIC_RELAXED);
 			rte_errno = EINVAL;
 			break;
 		} else if (unlikely(ret == -2)) {
-			evtims[i]->state = RTE_EVENT_TIMER_ERROR_TOOEARLY;
+			__atomic_store_n(&evtims[i]->state,
+					RTE_EVENT_TIMER_ERROR_TOOEARLY,
+					__ATOMIC_RELAXED);
 			rte_errno = EINVAL;
 			break;
 		}
 
 		if (unlikely(check_destination_event_queue(evtims[i],
 							   adapter) < 0)) {
-			evtims[i]->state = RTE_EVENT_TIMER_ERROR;
+			__atomic_store_n(&evtims[i]->state,
+					RTE_EVENT_TIMER_ERROR,
+					__ATOMIC_RELAXED);
 			rte_errno = EINVAL;
 			break;
 		}
@@ -1099,13 +1107,18 @@ __swtim_arm_burst(const struct rte_event_timer_adapter *adapter,
 					  SINGLE, lcore_id, NULL, evtims[i]);
 		if (ret < 0) {
 			/* tim was in RUNNING or CONFIG state */
-			evtims[i]->state = RTE_EVENT_TIMER_ERROR;
+			__atomic_store_n(&evtims[i]->state,
+					RTE_EVENT_TIMER_ERROR,
+					__ATOMIC_RELEASE);
 			break;
 		}
 
-		rte_smp_wmb();
 		EVTIM_LOG_DBG("armed an event timer");
-		evtims[i]->state = RTE_EVENT_TIMER_ARMED;
+		/* RELEASE ordering guarantees the adapter specific value
+		 * changes observed before the update of state.
+		 */
+		__atomic_store_n(&evtims[i]->state, RTE_EVENT_TIMER_ARMED,
+				__ATOMIC_RELEASE);
 	}
 
 	if (i < nb_evtims)
@@ -1132,6 +1145,7 @@ swtim_cancel_burst(const struct rte_event_timer_adapter *adapter,
 	struct rte_timer *timp;
 	uint64_t opaque;
 	struct swtim *sw = swtim_pmd_priv(adapter);
+	enum rte_event_timer_state n_state;
 
 #ifdef RTE_LIBRTE_EVENTDEV_DEBUG
 	/* Check that the service is running. */
@@ -1143,16 +1157,18 @@ swtim_cancel_burst(const struct rte_event_timer_adapter *adapter,
 
 	for (i = 0; i < nb_evtims; i++) {
 		/* Don't modify the event timer state in these cases */
-		if (evtims[i]->state == RTE_EVENT_TIMER_CANCELED) {
+		/* ACQUIRE ordering guarantees the access of implementation
+		 * specific opaque data under the correct state.
+		 */
+		n_state = __atomic_load_n(&evtims[i]->state, __ATOMIC_ACQUIRE);
+		if (n_state == RTE_EVENT_TIMER_CANCELED) {
 			rte_errno = EALREADY;
 			break;
-		} else if (evtims[i]->state != RTE_EVENT_TIMER_ARMED) {
+		} else if (n_state != RTE_EVENT_TIMER_ARMED) {
 			rte_errno = EINVAL;
 			break;
 		}
 
-		rte_smp_rmb();
-
 		opaque = evtims[i]->impl_opaque[0];
 		timp = (struct rte_timer *)(uintptr_t)opaque;
 		RTE_ASSERT(timp != NULL);
@@ -1166,9 +1182,12 @@ swtim_cancel_burst(const struct rte_event_timer_adapter *adapter,
 
 		rte_mempool_put(sw->tim_pool, (void **)timp);
 
-		evtims[i]->state = RTE_EVENT_TIMER_CANCELED;
-
-		rte_smp_wmb();
+		/* The RELEASE ordering here pairs with atomic ordering
+		 * to make sure the state update data observed between
+		 * threads.
+		 */
+		__atomic_store_n(&evtims[i]->state, RTE_EVENT_TIMER_CANCELED,
+				__ATOMIC_RELEASE);
 	}
 
 	return i;
-- 
2.7.4