X-Git-Url: https://git.liburcu.org/?p=urcu.git;a=blobdiff_plain;f=urcu-qsbr.c;h=0e1252fdbba707e4515cbbabe8c8a837a6408c79;hp=1ac553faefdf20ba1f775fe3af1fd49fd2e698f2;hb=0ecb3fde04135d76545ce55d80abea9aef465b54;hpb=6abb4bd53503c325dc94b0c0f60b44b9550b462f

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index 1ac553f..0e1252f 100644
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -106,14 +106,35 @@ static void wait_gp(void)
 		      NULL, NULL, 0);
 }
 
-static void wait_for_quiescent_state(void)
+static void update_counter_and_wait(void)
 {
 	LIST_HEAD(qsreaders);
 	int wait_loops = 0;
 	struct rcu_reader *index, *tmp;
 
-	if (list_empty(&registry))
-		return;
+#if (BITS_PER_LONG < 64)
+	/* Switch parity: 0 -> 1, 1 -> 0 */
+	STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
+#else	/* !(BITS_PER_LONG < 64) */
+	/* Increment current G.P. */
+	STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
+#endif	/* !(BITS_PER_LONG < 64) */
+
+	/*
+	 * Must commit rcu_gp_ctr update to memory before waiting for quiescent
+	 * state. Failure to do so could result in the writer waiting forever
+	 * while new readers are always accessing data (no progress). Enforce
+	 * compiler-order of store to rcu_gp_ctr before load rcu_reader ctr.
+	 */
+	barrier();
+
+	/*
+	 * Adding a smp_mb() which is _not_ formally required, but makes the
+	 * model easier to understand. It does not have a big performance impact
+	 * anyway, given this is the write-side.
+	 */
+	smp_mb();
+
 	/*
 	 * Wait for each thread rcu_reader_qs_gp count to become 0.
 	 */
@@ -125,9 +146,9 @@ static void wait_for_quiescent_state(void)
 			smp_mb();
 		}
 
-		list_for_each_entry_safe(index, tmp, &registry, head) {
+		list_for_each_entry_safe(index, tmp, &registry, node) {
 			if (!rcu_gp_ongoing(&index->ctr))
-				list_move(&index->head, &qsreaders);
+				list_move(&index->node, &qsreaders);
 		}
 
 		if (list_empty(&registry)) {
@@ -159,14 +180,6 @@ static void wait_for_quiescent_state(void)
  */
 
 #if (BITS_PER_LONG < 64)
-/*
- * called with rcu_gp_lock held.
- */
-static void switch_next_rcu_qparity(void)
-{
-	STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
-}
-
 void synchronize_rcu(void)
 {
 	unsigned long was_online;
@@ -189,44 +202,35 @@ void synchronize_rcu(void)
 
 	mutex_lock(&rcu_gp_lock);
 
-	switch_next_rcu_qparity();	/* 0 -> 1 */
-
-	/*
-	 * Must commit qparity update to memory before waiting for parity
-	 * 0 quiescent state. Failure to do so could result in the writer
-	 * waiting forever while new readers are always accessing data (no
-	 * progress).
-	 * Ensured by STORE_SHARED and LOAD_SHARED.
-	 */
+	if (list_empty(&registry))
+		goto out;
 
 	/*
 	 * Wait for previous parity to be empty of readers.
 	 */
-	wait_for_quiescent_state();	/* Wait readers in parity 0 */
+	update_counter_and_wait();	/* 0 -> 1, wait readers in parity 0 */
 
 	/*
 	 * Must finish waiting for quiescent state for parity 0 before
-	 * committing qparity update to memory. Failure to do so could result in
-	 * the writer waiting forever while new readers are always accessing
-	 * data (no progress).
-	 * Ensured by STORE_SHARED and LOAD_SHARED.
+	 * committing next rcu_gp_ctr update to memory. Failure to do so could
+	 * result in the writer waiting forever while new readers are always
+	 * accessing data (no progress).  Enforce compiler-order of load
+	 * rcu_reader ctr before store to rcu_gp_ctr.
 	 */
-
-	switch_next_rcu_qparity();	/* 1 -> 0 */
+	barrier();
 
 	/*
-	 * Must commit qparity update to memory before waiting for parity
-	 * 1 quiescent state. Failure to do so could result in the writer
-	 * waiting forever while new readers are always accessing data (no
-	 * progress).
-	 * Ensured by STORE_SHARED and LOAD_SHARED.
+	 * Adding a smp_mb() which is _not_ formally required, but makes the
+	 * model easier to understand. It does not have a big performance impact
+	 * anyway, given this is the write-side.
 	 */
+	smp_mb();
 
 	/*
 	 * Wait for previous parity to be empty of readers.
 	 */
-	wait_for_quiescent_state();	/* Wait readers in parity 1 */
-
+	update_counter_and_wait();	/* 1 -> 0, wait readers in parity 1 */
+out:
 	mutex_unlock(&rcu_gp_lock);
 
 	/*
@@ -254,8 +258,10 @@ void synchronize_rcu(void)
 		STORE_SHARED(rcu_reader.ctr, 0);
 
 	mutex_lock(&rcu_gp_lock);
-	STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
-	wait_for_quiescent_state();
+	if (list_empty(&registry))
+		goto out;
+	update_counter_and_wait();
+out:
 	mutex_unlock(&rcu_gp_lock);
 
 	if (was_online)
@@ -299,7 +305,7 @@ void rcu_register_thread(void)
 	assert(rcu_reader.ctr == 0);
 
 	mutex_lock(&rcu_gp_lock);
-	list_add(&rcu_reader.head, &registry);
+	list_add(&rcu_reader.node, &registry);
 	mutex_unlock(&rcu_gp_lock);
 	_rcu_thread_online();
 }
@@ -312,7 +318,7 @@ void rcu_unregister_thread(void)
 	 */
 	_rcu_thread_offline();
 	mutex_lock(&rcu_gp_lock);
-	list_del(&rcu_reader.head);
+	list_del(&rcu_reader.node);
 	mutex_unlock(&rcu_gp_lock);
 }