Add a function to call rcu_barrier once per loop

isc_loop_rcu_barrier() sets a flag so that rcu_barrier() will be
called at the end of the current loop tick. This allows us to minimize
time spent synchronizing between threads.

lib/isc/include/isc/loop.h

@@ -225,4 +225,14 @@ isc_loop_shuttingdown(isc_loop_t *loop);
  *
  * \li 'loop' is a valid loop and the loop tid matches the current tid.
  */
+
+void
+isc_loop_rcu_barrier(isc_loop_t *loop);
+/*%<
+ * Triggers an rcu_barrier() call at the end of the current loop tick.
+ *
+ * Requires:
+ *
+ * \li 'loop' is a valid loop and the loop tid matches the current tid.
+ */
 ISC_LANG_ENDDECLS

lib/isc/loop.c

@@ -236,7 +236,7 @@ loop_init(isc_loop_t *loop, isc_loopmgr_t *loopmgr, uint32_t tid) {
 
 static void
 quiescent_cb(uv_prepare_t *handle) {
-	UNUSED(handle);
+	isc_loop_t *loop = uv_handle_get_data(handle);
 
 #if defined(RCU_QSBR)
 	/* safe memory reclamation */
@@ -247,6 +247,10 @@ quiescent_cb(uv_prepare_t *handle) {
 #else
 	INSIST(!rcu_read_ongoing());
 #endif
+
+	if (loop->rcu_barrier) {
+		rcu_barrier();
+	}
 }
 
 static void
@@ -624,3 +628,11 @@ isc_loop_shuttingdown(isc_loop_t *loop) {
 
 	return (loop->shuttingdown);
 }
+
+void
+isc_loop_rcu_barrier(isc_loop_t *loop) {
+	REQUIRE(VALID_LOOP(loop));
+	REQUIRE(loop->tid == isc_tid());
+
+	loop->rcu_barrier = true;
+}

lib/isc/loop_p.h

@@ -76,6 +76,8 @@ struct isc_loop {
 
 	/* safe memory reclamation */
 	uv_prepare_t quiescent;
+
+	bool rcu_barrier;
 };
 
 /*