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: v0.11.0~52
X-Git-Url: http://git.lttng.org/?p=urcu.git;a=commitdiff_plain;h=64478021edcf7a5ac3bca3fa9e8b8108d2fbb9b6

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/src/urcu-bp.c b/src/urcu-bp.c
index c0fac83..ebe96c5 100644
--- a/src/urcu-bp.c
+++ b/src/urcu-bp.c
@@ -593,8 +593,13 @@ void rcu_sys_membarrier_status(int available)
 static
 void rcu_sys_membarrier_status(int available)
 {
-	if (available)
-		urcu_bp_has_sys_membarrier = 1;
+	/*
+	 * 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, unless its use has been
+	 * explicitly forced when building liburcu.
+	 */
 }
 #endif