From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Date: Fri, 15 May 2009 02:09:24 +0000 (-0400)
Subject: Move reader barrier within if statement for outermost nesting
X-Git-Tag: v0.1~224
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=67ef1a2cd793e5dde7db32ef5210fc002c3325cd;p=userspace-rcu.git

Move reader barrier within if statement for outermost nesting

Innermost nesting levels do not need this barrier. Slight runtime cost when
compiling with reader barriers in place. Does not change anything when using the
signal-based scheme.

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

diff --git a/urcu-static.h b/urcu-static.h
index f1aab2f..1b3527f 100644
--- a/urcu-static.h
+++ b/urcu-static.h
@@ -212,22 +212,16 @@ static inline void _rcu_read_lock(void)
 
 	tmp = urcu_active_readers;
 	/* urcu_gp_ctr = RCU_GP_COUNT | (~RCU_GP_CTR_BIT or RCU_GP_CTR_BIT) */
-	/*
-	 * The data dependency "read urcu_gp_ctr, write urcu_active_readers",
-	 * serializes those two memory operations. The memory barrier in the
-	 * signal handler ensures we receive the proper memory commit barriers
-	 * required by _STORE_SHARED and _LOAD_SHARED whenever communication
-	 * with the writer is needed.
-	 */
-	if (likely(!(tmp & RCU_GP_CTR_NEST_MASK)))
+	if (likely(!(tmp & RCU_GP_CTR_NEST_MASK))) {
 		_STORE_SHARED(urcu_active_readers, _LOAD_SHARED(urcu_gp_ctr));
-	else
+		/*
+		 * Set active readers count for outermost nesting level before
+		 * accessing the pointer. See force_mb_all_threads().
+		 */
+		reader_barrier();
+	} else {
 		_STORE_SHARED(urcu_active_readers, tmp + RCU_GP_COUNT);
-	/*
-	 * Increment active readers count before accessing the pointer.
-	 * See force_mb_all_threads().
-	 */
-	reader_barrier();
+	}
 }
 
 static inline void _rcu_read_unlock(void)
@@ -236,6 +230,9 @@ static inline void _rcu_read_unlock(void)
 	/*
 	 * Finish using rcu before decrementing the pointer.
 	 * See force_mb_all_threads().
+	 * Formally only needed for outermost nesting level, but leave barrier
+	 * in place for nested unlocks to remove a branch from the common case
+	 * (no nesting).
 	 */
 	_STORE_SHARED(urcu_active_readers, urcu_active_readers - RCU_GP_COUNT);
 }
@@ -263,7 +260,7 @@ static inline void _rcu_read_unlock(void)
 
 /**
  * _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous
- * pointer to the data structure, which can be safely freed after waitin for a
+ * pointer to the data structure, which can be safely freed after waiting for a
  * quiescent state using synchronize_rcu().
  */