From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Date: Sun, 14 Mar 2010 14:27:23 +0000 (-0400)
Subject: urcu-qsbr: document compiler barrier
X-Git-Tag: v0.4.4~8
X-Git-Url: https://git.lttng.org/?p=urcu.git;a=commitdiff_plain;h=7a5a38f52b8814d0188b74a91cb635bc205df96c

urcu-qsbr: document compiler barrier

This compiler barrier is, at the implementation level, already provided by the
volatile accesses for STORE/LOAD_SHARED. However, given that these primitives
does not provide this guarantee at the high-level (and might use lighter gcc
primitives eventually), we need to add an explicit barrier().

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

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index 84789f2..25074d0 100644
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -120,6 +120,13 @@ static void update_counter_and_wait(void)
 	STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
 #endif	/* !(BITS_PER_LONG < 64) */
 
+	/*
+	 * Enforce compiler-order of store to rcu_gp_ctr before before
+	 * load rcu_reader ctr.
+	 * This ensures synchronize_rcu() cannot be starved by readers.
+	 */
+	barrier();
+
 	/*
 	 * Wait for each thread rcu_reader_qs_gp count to become 0.
 	 */