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

[urcu.git] / urcu-qsbr.c

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index a239be036d416f68259c0f586f5d0ce7248385e9..8d8a9cf05ced0fd18ef9943110dc22a2e6e05e87 100644 (file)
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -150,26 +150,33 @@ static void update_counter_and_wait(void)
         */
        for (;;) {
                wait_loops++;
-               if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
-                       uatomic_dec(&gp_futex);
+               if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
+                       uatomic_set(&gp_futex, -1);
+                       /*
+                        * Write futex before write waiting (the other side
+                        * reads them in the opposite order).
+                        */
+                       cmm_smp_wmb();
+                       cds_list_for_each_entry(index, &registry, node) {
+                               _CMM_STORE_SHARED(index->waiting, 1);
+                       }
                        /* Write futex before read reader_gp */
                        cmm_smp_mb();
                }
-
                cds_list_for_each_entry_safe(index, tmp, &registry, node) {
                        if (!rcu_gp_ongoing(&index->ctr))
                                cds_list_move(&index->node, &qsreaders);
                }
 
                if (cds_list_empty(&registry)) {
-                       if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
+                       if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
                                /* Read reader_gp before write futex */
                                cmm_smp_mb();
                                uatomic_set(&gp_futex, 0);
                        }
                        break;
                } else {
-                       if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS) {
+                       if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
                                wait_gp();
                        } else {
 #ifndef HAS_INCOHERENT_CACHES