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: sta...@dpdk.org
Signed-off-by: Phil Yang <phil.y...@arm.com>
Reviewed-by: Dharmik Thakkar <dharmik.thak...@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.w...@arm.com>
Acked-by: Erik Gabriel Carrillo <erik.g.carri...@intel.com>
---
v4:
1.Change 'CAS' to 'compare-and-swap' to pass the coding style check.
2.Cc to stable release. (Honnappa)
v2:
1. Make the code comments more accurate. (Erik)
2. Define the in_use flag as an unsigned type. (Stephen)
lib/librte_eventdev/rte_event_timer_adapter.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)
diff --git a/lib/librte_eventdev/rte_event_timer_adapter.c
b/lib/librte_eventdev/rte_event_timer_adapter.c
index 370ea40..4ed3013 100644
--- 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,
--
2.7.4
