[PATCH 2/5] ipc/mqueue.c: Update/document memory barriers

[Manfred Spraul <manfred@colorfullife.com>]

Update and document memory barriers for mqueue.c:
- ewp->state is read without any locks, thus READ_ONCE is required.

- add smp_aquire__after_ctrl_dep() after the RAED_ONCE, we need
  acquire semantics if the value is STATE_READY.

- document that the code relies on the barrier inside wake_q_add()

- document why __set_current_state() may be used:
  Reading task->state cannot happen before the wake_q_add() call,
  which happens while holding info->lock.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
---
 ipc/mqueue.c | 32 +++++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 3d920ff15c80..902167407737 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -646,17 +646,25 @@ static int wq_sleep(struct mqueue_inode_info *info, int sr,
 	wq_add(info, sr, ewp);
 
 	for (;;) {
+		/* memory barrier not required, we hold info->lock */
 		__set_current_state(TASK_INTERRUPTIBLE);
 
 		spin_unlock(&info->lock);
 		time = schedule_hrtimeout_range_clock(timeout, 0,
 			HRTIMER_MODE_ABS, CLOCK_REALTIME);
 
-		if (ewp->state == STATE_READY) {
+		if (READ_ONCE(ewp->state) == STATE_READY) {
+			/*
+			 * Pairs, together with READ_ONCE(), with
+			 * the barrier in wake_q_add().
+			 */
+			smp_acquire__after_ctrl_dep();
 			retval = 0;
 			goto out;
 		}
 		spin_lock(&info->lock);
+
+		/* we hold info->lock, so no memory barrier required */
 		if (ewp->state == STATE_READY) {
 			retval = 0;
 			goto out_unlock;
@@ -928,16 +936,11 @@ static inline void pipelined_send(struct wake_q_head *wake_q,
 {
 	receiver->msg = message;
 	list_del(&receiver->list);
+
 	wake_q_add(wake_q, receiver->task);
-	/*
-	 * Rely on the implicit cmpxchg barrier from wake_q_add such
-	 * that we can ensure that updating receiver->state is the last
-	 * write operation: As once set, the receiver can continue,
-	 * and if we don't have the reference count from the wake_q,
-	 * yet, at that point we can later have a use-after-free
-	 * condition and bogus wakeup.
-	 */
-	receiver->state = STATE_READY;
+
+	/* The memory barrier is provided by wake_q_add(). */
+	WRITE_ONCE(receiver->state, STATE_READY);
 }
 
 /* pipelined_receive() - if there is task waiting in sys_mq_timedsend()
@@ -956,8 +959,11 @@ static inline void pipelined_receive(struct wake_q_head *wake_q,
 		return;
 
 	list_del(&sender->list);
+
 	wake_q_add(wake_q, sender->task);
-	sender->state = STATE_READY;
+
+	/* The memory barrier is provided by wake_q_add(). */
+	WRITE_ONCE(sender->state, STATE_READY);
 }
 
 static int do_mq_timedsend(mqd_t mqdes, const char __user *u_msg_ptr,
@@ -1044,6 +1050,8 @@ static int do_mq_timedsend(mqd_t mqdes, const char __user *u_msg_ptr,
 		} else {
 			wait.task = current;
 			wait.msg = (void *) msg_ptr;
+
+			/* memory barrier not required, we hold info->lock */
 			wait.state = STATE_NONE;
 			ret = wq_sleep(info, SEND, timeout, &wait);
 			/*
@@ -1147,6 +1155,8 @@ static int do_mq_timedreceive(mqd_t mqdes, char __user *u_msg_ptr,
 			ret = -EAGAIN;
 		} else {
 			wait.task = current;
+
+			/* memory barrier not required, we hold info->lock */
 			wait.state = STATE_NONE;
 			ret = wq_sleep(info, RECV, timeout, &wait);
 			msg_ptr = wait.msg;
-- 
2.21.0