From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Date: Wed, 16 Sep 2009 16:40:27 +0000 (-0400)
Subject: Ensure LOAD_SHARED/STORE_SHARED semantic is used in QSBR RCU
X-Git-Tag: v0.1~83
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=4e560c1794fe68ca77ae9bcaa60c5e4f5ed0856e;p=userspace-rcu.git

Ensure LOAD_SHARED/STORE_SHARED semantic is used in QSBR RCU

Ensures all volatile accesses are indeed volatile.
On architectures without coherent caches, will perform the proper cache flushes
when needed.

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

diff --git a/urcu-qsbr-static.h b/urcu-qsbr-static.h
index 86ae7d4..b18affe 100644
--- a/urcu-qsbr-static.h
+++ b/urcu-qsbr-static.h
@@ -171,10 +171,12 @@ extern long __thread rcu_reader_qs_gp;
 
 static inline int rcu_gp_ongoing(long *value)
 {
+	long reader_gp;
+
 	if (value == NULL)
 		return 0;
-
-	return LOAD_SHARED(*value) & 1;
+	reader_gp = LOAD_SHARED(*value);
+	return (reader_gp & 1) && (reader_gp - urcu_gp_ctr < 0);
 }
 
 static inline void _rcu_read_lock(void)
@@ -189,19 +191,19 @@ static inline void _rcu_read_unlock(void)
 static inline void _rcu_quiescent_state(void)
 {
 	smp_mb();	
-	rcu_reader_qs_gp = ACCESS_ONCE(urcu_gp_ctr) + 1;
+	_STORE_SHARED(rcu_reader_qs_gp, _LOAD_SHARED(urcu_gp_ctr) + 1);
 	smp_mb();
 }
 
 static inline void _rcu_thread_offline(void)
 {
 	smp_mb();
-	rcu_reader_qs_gp = 0;
+	STORE_SHARED(rcu_reader_qs_gp, 0);
 }
 
 static inline void _rcu_thread_online(void)
 {
-	rcu_reader_qs_gp = ACCESS_ONCE(urcu_gp_ctr) + 1;
+	_STORE_SHARED(rcu_reader_qs_gp, LOAD_SHARED(urcu_gp_ctr) + 1);
 	smp_mb();
 }
 
diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index ccecab4..e077eac 100644
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -122,8 +122,7 @@ static void wait_for_quiescent_state(void)
 	 */
 	for (index = registry; index < registry + num_readers; index++) {
 #ifndef HAS_INCOHERENT_CACHES
-		while (rcu_gp_ongoing(index->rcu_reader_qs_gp) &&
-		       (*index->rcu_reader_qs_gp - urcu_gp_ctr < 0))
+		while (rcu_gp_ongoing(index->rcu_reader_qs_gp))
 			cpu_relax();
 #else /* #ifndef HAS_INCOHERENT_CACHES */
 		int wait_loops = 0;
@@ -131,8 +130,7 @@ static void wait_for_quiescent_state(void)
 		 * BUSY-LOOP. Force the reader thread to commit its
 		 * rcu_reader_qs_gp update to memory if we wait for too long.
 		 */
-		while (rcu_gp_ongoing(index->rcu_reader_qs_gp) &&
-		       (*index->rcu_reader_qs_gp - urcu_gp_ctr < 0)) {
+		while (rcu_gp_ongoing(index->rcu_reader_qs_gp)) {
 			if (wait_loops++ == KICK_READER_LOOPS) {
 				force_mb_single_thread(index);
 				wait_loops = 0;
@@ -160,7 +158,7 @@ void synchronize_rcu(void)
 
 	internal_urcu_lock();
 	force_mb_all_threads();
-	urcu_gp_ctr += 2;
+	STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr + 2);
 	wait_for_quiescent_state();
 	force_mb_all_threads();
 	internal_urcu_unlock();