From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Wed, 6 Sep 2017 20:35:59 +0000 (-0400)
Subject: Fix: don't use membarrier SHARED syscall command in liburcu-bp
X-Git-Tag: v0.9.5~2
X-Git-Url: http://git.liburcu.org/?p=userspace-rcu.git;a=commitdiff_plain;h=d4dae7fc4a6fcb4c88c65a2445ee1ddc7a8ac500

Fix: don't use membarrier SHARED syscall command in liburcu-bp

One main user of liburcu-bp (lttng-ust) invokes synchronize_rcu()
repeatedly, without batching (does not use call_rcu).

Those delays introduced by sys_membarrier SHARED command significantly
impacts application startup time. Therefore, revert to not using the
membarrier SHARED command.

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

diff --git a/urcu-bp.c b/urcu-bp.c
index d0bfb0d..9c7da3d 100644
--- a/urcu-bp.c
+++ b/urcu-bp.c
@@ -581,6 +581,17 @@ void urcu_bp_thread_exit_notifier(void *rcu_key)
 	rcu_bp_unregister(rcu_key);
 }
 
+static
+void rcu_sys_membarrier_status(int available)
+{
+	/*
+	 * membarrier has blocking behavior, which changes the
+	 * application behavior too much compared to using barriers when
+	 * synchronize_rcu is used repeatedly (without using call_rcu).
+	 * Don't use membarrier for now.
+	 */
+}
+
 static
 void rcu_bp_init(void)
 {
@@ -593,9 +604,8 @@ void rcu_bp_init(void)
 		if (ret)
 			abort();
 		ret = membarrier(MEMBARRIER_CMD_QUERY, 0);
-		if (ret >= 0 && (ret & MEMBARRIER_CMD_SHARED)) {
-			urcu_bp_has_sys_membarrier = 1;
-		}
+		rcu_sys_membarrier_status(ret >= 0
+				&& (ret & MEMBARRIER_CMD_SHARED));
 		initialized = 1;
 	}
 	mutex_unlock(&init_lock);