[PATCH 3/6] ipc/mqueue.c: Update/document memory barriers

[Manfred Spraul]

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 READ_ONCE, we need
  acquire semantics if the value is STATE_READY.

- add an explicit memory barrier to __pipelined_op(), the
  refcount must have been increased before the updated state becomes
  visible

- 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. Thus the spin_unlock()
  is the RELEASE, and the spin_lock() is the ACQUIRE.

For completeness: there is also a 3 CPU szenario, if the to be woken
up task is already on another wake_q.
Then:
- CPU1: spin_unlock() of the task that goes to sleep is the RELEASE
- CPU2: the spin_lock() of the waker is the ACQUIRE
- CPU2: smp_mb__before_atomic inside wake_q_add() is the RELEASE
- CPU3: smp_mb__after_spinlock() inside try_to_wake_up() is the ACQUIRE

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

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index be48c0ba92f7..b80574822f0a 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -646,18 +646,26 @@ 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 __pipelined_op().
+			 */
+			smp_acquire__after_ctrl_dep();
 			retval = 0;
 			goto out;
 		}
 		spin_lock(&info->lock);
-		if (ewp->state == STATE_READY) {
+
+		/* we hold info->lock, so no memory barrier required */
+		if (READ_ONCE(ewp->state) == STATE_READY) {
 			retval = 0;
 			goto out_unlock;
 		}
@@ -925,14 +933,12 @@ static inline void __pipelined_op(struct wake_q_head *wake_q,
 	list_del(&this->list);
 	wake_q_add(wake_q, this->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.
+	 * The barrier is required to ensure that the refcount increase
+	 * inside wake_q_add() is completed before the state is updated.
+	 *
+	 * The barrier pairs with READ_ONCE()+smp_mb__after_ctrl_dep().
 	 */
-        this->state = STATE_READY;
+        smp_store_release(&this->state, STATE_READY);
 }
 
 /* pipelined_send() - send a message directly to the task waiting in
@@ -1049,7 +1055,9 @@ static int do_mq_timedsend(mqd_t mqdes, const char __user *u_msg_ptr,
 		} else {
 			wait.task = current;
 			wait.msg = (void *) msg_ptr;
-			wait.state = STATE_NONE;
+
+			/* memory barrier not required, we hold info->lock */
+			WRITE_ONCE(wait.state, STATE_NONE);
 			ret = wq_sleep(info, SEND, timeout, &wait);
 			/*
 			 * wq_sleep must be called with info->lock held, and
@@ -1152,7 +1160,9 @@ static int do_mq_timedreceive(mqd_t mqdes, char __user *u_msg_ptr,
 			ret = -EAGAIN;
 		} else {
 			wait.task = current;
-			wait.state = STATE_NONE;
+
+			/* memory barrier not required, we hold info->lock */
+			WRITE_ONCE(wait.state, STATE_NONE);
 			ret = wq_sleep(info, RECV, timeout, &wait);
 			msg_ptr = wait.msg;
 		}
-- 
2.21.0