ipc,msg: document barriers

Both expunge_all() and pipeline_send() rely on both a nil msg value and
a full barrier to guarantee the correct ordering when waking up a task.

While its counterpart at the receiving end is well documented for the
lockless recv algorithm, we still need to document these specific
smp_mb() calls.

[akpm@linux-foundation.org: fix typo, per Mike]
[akpm@linux-foundation.org: mroe tpyos]
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/msg.c

@@ -253,8 +253,14 @@ static void expunge_all(struct msg_queue *msq, int res)
 	struct msg_receiver *msr, *t;
 
 	list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {
-		msr->r_msg = NULL;
+		msr->r_msg = NULL; /* initialize expunge ordering */
 		wake_up_process(msr->r_tsk);
+		/*
+		 * Ensure that the wakeup is visible before setting r_msg as
+		 * the receiving end depends on it: either spinning on a nil,
+		 * or dealing with -EAGAIN cases. See lockless receive part 1
+		 * and 2 in do_msgrcv().
+		 */
 		smp_mb();
 		msr->r_msg = ERR_PTR(res);
 	}
@@ -638,15 +644,22 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
 
 			list_del(&msr->r_list);
 			if (msr->r_maxsize < msg->m_ts) {
+				/* initialize pipelined send ordering */
 				msr->r_msg = NULL;
 				wake_up_process(msr->r_tsk);
-				smp_mb();
+				smp_mb(); /* see barrier comment below */
 				msr->r_msg = ERR_PTR(-E2BIG);
 			} else {
 				msr->r_msg = NULL;
 				msq->q_lrpid = task_pid_vnr(msr->r_tsk);
 				msq->q_rtime = get_seconds();
 				wake_up_process(msr->r_tsk);
+				/*
+				 * Ensure that the wakeup is visible before
+				 * setting r_msg, as the receiving end depends
+				 * on it. See lockless receive part 1 and 2 in
+				 * do_msgrcv().
+				 */
 				smp_mb();
 				msr->r_msg = msg;
 
@@ -654,6 +667,7 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
 			}
 		}
 	}
+
 	return 0;
 }
 
@@ -716,6 +730,7 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
 			goto out_unlock0;
 		}
 
+		/* enqueue the sender and prepare to block */
 		ss_add(msq, &s);
 
 		if (!ipc_rcu_getref(msq)) {