[PATCH 2/5] ipc,msg: provide barrier pairings for lockless receive

[Davidlohr Bueso <dave@stgolabs.net>]

We currently use a full barrier on the sender side to
to avoid receiver tasks disappearing on us while still
performing on the sender side wakeup. We lack however,
the proper CPU-CPU interactions pairing on the receiver
side which busy-waits for the message. Similarly, we do
not need a full smp_mb, and can relax the semantics for
the writer and reader sides of the message. This is safe
as we are only ordering loads and stores to r_msg. And in
both smp_wmb and smp_rmb, there are no stores after the
calls _anyway_.

This obviously applies for pipelined_send and expunge_all,
for EIRDM when destroying a queue.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 ipc/msg.c | 48 ++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 38 insertions(+), 10 deletions(-)

diff --git a/ipc/msg.c b/ipc/msg.c
index 2b6fdbb..a9c3c51 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -196,7 +196,7 @@ static void expunge_all(struct msg_queue *msq, int res)
 		 * or dealing with -EAGAIN cases. See lockless receive part 1
 		 * and 2 in do_msgrcv().
 		 */
-		smp_mb();
+		smp_wmb(); /* barrier (B) */
 		msr->r_msg = ERR_PTR(res);
 	}
 }
@@ -580,7 +580,8 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
 				/* initialize pipelined send ordering */
 				msr->r_msg = NULL;
 				wake_up_process(msr->r_tsk);
-				smp_mb(); /* see barrier comment below */
+				/* barrier (B) see barrier comment below */
+				smp_wmb();
 				msr->r_msg = ERR_PTR(-E2BIG);
 			} else {
 				msr->r_msg = NULL;
@@ -589,11 +590,12 @@ static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
 				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().
+				 * setting r_msg, as the receiving can otherwise
+				 * exit - once r_msg is set, the receiver can
+				 * continue. See lockless receive part 1 and 2
+				 * in do_msgrcv(). Barrier (B).
 				 */
-				smp_mb();
+				smp_wmb();
 				msr->r_msg = msg;
 
 				return 1;
@@ -932,12 +934,38 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgfl
 		/* Lockless receive, part 2:
 		 * Wait until pipelined_send or expunge_all are outside of
 		 * wake_up_process(). There is a race with exit(), see
-		 * ipc/mqueue.c for the details.
+		 * ipc/mqueue.c for the details. The correct serialization
+		 * ensures that a receiver cannot continue without the wakeup
+		 * being visibible _before_ setting r_msg:
+		 *
+		 * CPU 0                             CPU 1
+		 * <loop receiver>
+		 *   smp_rmb(); (A) <-- pair -.      <waker thread>
+		 *   <load ->r_msg>           |        msr->r_msg = NULL;
+		 *                            |        wake_up_process();
+		 * <continue>                 `------> smp_wmb(); (B)
+		 *                                     msr->r_msg = msg;
+		 *
+		 * Where (A) orders the message value read and where (B) orders
+		 * the write to the r_msg -- done in both pipelined_send and
+		 * expunge_all.
 		 */
-		msg = (struct msg_msg *)msr_d.r_msg;
-		while (msg == NULL) {
-			cpu_relax();
+		for (;;) {
+			/*
+			 * Pairs with writer barrier in pipelined_send
+			 * or expunge_all.
+			 */
+			smp_rmb(); /* barrier (A) */
 			msg = (struct msg_msg *)msr_d.r_msg;
+			if (msg)
+				break;
+
+			/*
+			 * The cpu_relax() call is a compiler barrier
+			 * which forces everything in this loop to be
+			 * re-loaded.
+			 */
+			cpu_relax();
 		}
 
 		/* Lockless receive, part 3:
-- 
2.1.4

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