From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Date: Sat, 6 Nov 2010 19:09:29 +0000 (-0400)
Subject: Add compiler barriers to read-side for mutex ordering
X-Git-Tag: v0.5.0~6
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=42a3f1f99827c1bd73270966d4fffd6157809a7c;p=userspace-rcu.git

Add compiler barriers to read-side for mutex ordering

Ensure that mutex acquisition/release is not reordered wrt rcu read lock/unlock.
No impact in practice because pthreads lock/unlock primitives act as memory
barriers. But we should provide the same guarantees we are expecting from other
locking primitives, hence this modification.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/urcu-bp-static.h b/urcu-bp-static.h
index 515be28..6d7e3c8 100644
--- a/urcu-bp-static.h
+++ b/urcu-bp-static.h
@@ -175,6 +175,7 @@ static inline void _rcu_read_lock(void)
 	if (unlikely(!rcu_reader))
 		rcu_bp_register();
 
+	barrier();	/* Ensure the compiler does not reorder us with mutex */
 	tmp = rcu_reader->ctr;
 	/*
 	 * rcu_gp_ctr is
@@ -199,6 +200,7 @@ static inline void _rcu_read_unlock(void)
 	 */
 	smp_mb();
 	_STORE_SHARED(rcu_reader->ctr, rcu_reader->ctr - RCU_GP_COUNT);
+	barrier();	/* Ensure the compiler does not reorder us with mutex */
 }
 
 #ifdef __cplusplus 
diff --git a/urcu-qsbr-static.h b/urcu-qsbr-static.h
index 0413617..147eb18 100644
--- a/urcu-qsbr-static.h
+++ b/urcu-qsbr-static.h
@@ -187,10 +187,12 @@ static inline void _rcu_thread_offline(void)
 	STORE_SHARED(rcu_reader.ctr, 0);
 	smp_mb();	/* write rcu_reader.ctr before read futex */
 	wake_up_gp();
+	barrier();	/* Ensure the compiler does not reorder us with mutex */
 }
 
 static inline void _rcu_thread_online(void)
 {
+	barrier();	/* Ensure the compiler does not reorder us with mutex */
 	_STORE_SHARED(rcu_reader.ctr, LOAD_SHARED(rcu_gp_ctr));
 	smp_mb();
 }
diff --git a/urcu-static.h b/urcu-static.h
index dce15b7..eea39bb 100644
--- a/urcu-static.h
+++ b/urcu-static.h
@@ -259,6 +259,7 @@ static inline void _rcu_read_lock(void)
 {
 	unsigned long tmp;
 
+	barrier();	/* Ensure the compiler does not reorder us with mutex */
 	tmp = rcu_reader.ctr;
 	/*
 	 * rcu_gp_ctr is
@@ -294,6 +295,7 @@ static inline void _rcu_read_unlock(void)
 	} else {
 		_STORE_SHARED(rcu_reader.ctr, rcu_reader.ctr - RCU_GP_COUNT);
 	}
+	barrier();	/* Ensure the compiler does not reorder us with mutex */
 }
 
 #ifdef __cplusplus