eventdev: use C11 atomics for lcore timer armed flag

The in_use flag is a per core variable which is not shared between
lcores in the normal case and the access of this variable should be
ordered on the same core. However, if non-EAL thread pick the highest
lcore to insert timers into, there is the possibility of conflicts
on this flag between threads. Then the atomic compare-and-swap
operation is needed.

Use the C11 atomics instead of the generic rte_atomic operations to
avoid the unnecessary barrier on aarch64.

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>

diff --git a/lib/librte_eventdev/rte_event_timer_adapter.c b/lib/librte_eventdev/rte_event_timer_adapter.c
index 370ea40a745de8395106a755bf08929bdc1bee81..4ed301382126f0b472de75d1a21885d2c9d5a07d 100644 (file)
--- a/lib/librte_eventdev/rte_event_timer_adapter.c
+++ b/lib/librte_eventdev/rte_event_timer_adapter.c
@@ -554,7 +554,7 @@ struct swtim {
        uint32_t timer_data_id;
        /* Track which cores have actually armed a timer */
        struct {
-               rte_atomic16_t v;
+               uint16_t v;
        } __rte_cache_aligned in_use[RTE_MAX_LCORE];
        /* Track which cores' timer lists should be polled */
        unsigned int poll_lcores[RTE_MAX_LCORE];
@@ -606,7 +606,8 @@ swtim_callback(struct rte_timer *tim)
                                      "with immediate expiry value");
                }
 
-               if (unlikely(rte_atomic16_test_and_set(&sw->in_use[lcore].v))) {
+               if (unlikely(sw->in_use[lcore].v == 0)) {
+                       sw->in_use[lcore].v = 1;
                        n_lcores = __atomic_fetch_add(&sw->n_poll_lcores, 1,
                                                     __ATOMIC_RELAXED);
                        __atomic_store_n(&sw->poll_lcores[n_lcores], lcore,
@@ -834,7 +835,7 @@ swtim_init(struct rte_event_timer_adapter *adapter)
 
        /* Initialize the variables that track in-use timer lists */
        for (i = 0; i < RTE_MAX_LCORE; i++)
-               rte_atomic16_init(&sw->in_use[i].v);
+               sw->in_use[i].v = 0;
 
        /* Initialize the timer subsystem and allocate timer data instance */
        ret = rte_timer_subsystem_init();
@@ -1017,6 +1018,8 @@ __swtim_arm_burst(const struct rte_event_timer_adapter *adapter,
        struct rte_timer *tim, *tims[nb_evtims];
        uint64_t cycles;
        int n_lcores;
+       /* Timer list for this lcore is not in use. */
+       uint16_t exp_state = 0;
 
 #ifdef RTE_LIBRTE_EVENTDEV_DEBUG
        /* Check that the service is running. */
@@ -1035,8 +1038,12 @@ __swtim_arm_burst(const struct rte_event_timer_adapter *adapter,
        /* If this is the first time we're arming an event timer on this lcore,
         * mark this lcore as "in use"; this will cause the service
         * function to process the timer list that corresponds to this lcore.
+        * The atomic compare-and-swap operation can prevent the race condition
+        * on in_use flag between multiple non-EAL threads.
         */
-       if (unlikely(rte_atomic16_test_and_set(&sw->in_use[lcore_id].v))) {
+       if (unlikely(__atomic_compare_exchange_n(&sw->in_use[lcore_id].v,
+                       &exp_state, 1, 0,
+                       __ATOMIC_RELAXED, __ATOMIC_RELAXED))) {
                EVTIM_LOG_DBG("Adding lcore id = %u to list of lcores to poll",
                              lcore_id);
                n_lcores = __atomic_fetch_add(&sw->n_poll_lcores, 1,