urcu-qsbr: avoid useless futex wakeups and burning CPU for long grace periods

[urcu.git] / urcu / static / urcu-qsbr.h

diff --git a/urcu/static/urcu-qsbr.h b/urcu/static/urcu-qsbr.h
index 5b7adac658bfe1b64264d3adcf2b06e0a00c2430..489abb0c399cc1026b387f37960530dfb9400bdf 100644 (file)
--- a/urcu/static/urcu-qsbr.h
+++ b/urcu/static/urcu-qsbr.h
@@ -124,6 +124,7 @@ struct rcu_reader {
        unsigned long ctr;
        /* Data used for registry */
        struct cds_list_head node __attribute__((aligned(CAA_CACHE_LINE_SIZE)));
+       int waiting;
        pthread_t tid;
 };
 
@@ -136,7 +137,11 @@ extern int32_t gp_futex;
  */
 static inline void wake_up_gp(void)
 {
-       if (unlikely(uatomic_read(&gp_futex) == -1)) {
+       if (unlikely(_CMM_LOAD_SHARED(rcu_reader.waiting))) {
+               _CMM_STORE_SHARED(rcu_reader.waiting, 0);
+               cmm_smp_mb();
+               if (uatomic_read(&gp_futex) != -1)
+                       return;
                uatomic_set(&gp_futex, 0);
                futex_noasync(&gp_futex, FUTEX_WAKE, 1,
                      NULL, NULL, 0);