[PATCH 4/5] ipc,msg: document volatile r_msg

[Davidlohr Bueso <davidlohr@hp.com>]

The need for volatile is not obvious, document it.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
---
 ipc/msg.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/ipc/msg.c b/ipc/msg.c
index c2cdb5b..956cd65 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -42,9 +42,7 @@
 #include <linux/uaccess.h>
 #include "util.h"
 
-/*
- * one msg_receiver structure for each sleeping receiver:
- */
+/* one msg_receiver structure for each sleeping receiver */
 struct msg_receiver {
 	struct list_head	r_list;
 	struct task_struct	*r_tsk;
@@ -54,6 +52,12 @@ struct msg_receiver {
 	long			r_msgtype;
 	long			r_maxsize;
 
+	/*
+	 * Mark r_msg volatile so that the compiler
+	 * does not try to get smart and optimize
+	 * it. We rely on this for the lockless
+	 * receive algorithm.
+	 */
 	struct msg_msg		*volatile r_msg;
 };
 
-- 
1.8.1.4