urcu-defer: Add fast path for empty queues

defer_barrier does not need to call synchronize_rcu() when all queues are empty.
Skip the G.P. if this is the case.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>

diff --git a/urcu-defer.c b/urcu-defer.c
index cd939c7ac85b4bd03bb3d24f769cbb8ef8f70604..412eb30477908b2a60acff9dde794d9ecfc793f8 100644 (file)
--- a/urcu-defer.c
+++ b/urcu-defer.c
@@ -133,9 +133,12 @@ static void rcu_defer_barrier_queue(struct defer_queue *queue,
 
 static void _rcu_defer_barrier_thread(void)
 {
-       unsigned long head;
+       unsigned long head, num_items;
 
        head = defer_queue.head;
+       num_items = head - defer_queue.tail;
+       if (unlikely(!num_items))
+               return;
        synchronize_rcu();
        rcu_defer_barrier_queue(&defer_queue, head);
 }
@@ -148,20 +151,44 @@ void rcu_defer_barrier_thread(void)
        internal_urcu_unlock(&urcu_defer_mutex);
 }
 
+/*
+ * rcu_defer_barrier - Execute all queued rcu callbacks.
+ *
+ * Execute all RCU callbacks queued before rcu_defer_barrier() execution.
+ * All callbacks queued on the local thread prior to a rcu_defer_barrier() call
+ * are guaranteed to be executed.
+ * Callbacks queued by other threads concurrently with rcu_defer_barrier()
+ * execution are not guaranteed to be executed in the current batch (could
+ * be left for the next batch). These callbacks queued by other threads are only
+ * guaranteed to be executed if there is explicit synchronization between
+ * the thread adding to the queue and the thread issuing the defer_barrier call.
+ */
+
 void rcu_defer_barrier(void)
 {
        struct deferer_registry *index;
+       unsigned long num_items = 0;
 
        if (!registry)
                return;
 
        internal_urcu_lock(&urcu_defer_mutex);
-       for (index = registry; index < registry + num_deferers; index++)
+       for (index = registry; index < registry + num_deferers; index++) {
                index->last_head = LOAD_SHARED(index->defer_queue->head);
+               num_items += index->last_head - index->defer_queue->tail;
+       }
+       if (likely(!num_items)) {
+               /*
+                * We skip the grace period because there are no queued
+                * callbacks to execute.
+                */
+               goto end;
+       }
        synchronize_rcu();
        for (index = registry; index < registry + num_deferers; index++)
                rcu_defer_barrier_queue(index->defer_queue,
                                          index->last_head);
+end:
        internal_urcu_unlock(&urcu_defer_mutex);
 }