printk: add missing memory barrier to wake_up_klogd()

It is important that any new records are visible to preparing
waiters before the waker checks if the wait queue is empty.
Otherwise it is possible that:

- there are new records available
- the waker sees an empty wait queue and does not wake
- the preparing waiter sees no new records and begins to wait

This is exactly the problem that the function description of
waitqueue_active() warns about.

Use wq_has_sleeper() instead of waitqueue_active() because it
includes the necessary full memory barrier.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220421212250.565456-4-john.ogness@linutronix.de

kernel/printk/printk.c

@@ -746,8 +746,19 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
 			goto out;
 		}
 
+		/*
+		 * Guarantee this task is visible on the waitqueue before
+		 * checking the wake condition.
+		 *
+		 * The full memory barrier within set_current_state() of
+		 * prepare_to_wait_event() pairs with the full memory barrier
+		 * within wq_has_sleeper().
+		 *
+		 * This pairs with wake_up_klogd:A.
+		 */
 		ret = wait_event_interruptible(log_wait,
-				prb_read_valid(prb, atomic64_read(&user->seq), r));
+				prb_read_valid(prb,
+					atomic64_read(&user->seq), r)); /* LMM(devkmsg_read:A) */
 		if (ret)
 			goto out;
 	}
@@ -1513,7 +1524,18 @@ static int syslog_print(char __user *buf, int size)
 		seq = syslog_seq;
 
 		mutex_unlock(&syslog_lock);
-		len = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL));
+		/*
+		 * Guarantee this task is visible on the waitqueue before
+		 * checking the wake condition.
+		 *
+		 * The full memory barrier within set_current_state() of
+		 * prepare_to_wait_event() pairs with the full memory barrier
+		 * within wq_has_sleeper().
+		 *
+		 * This pairs with wake_up_klogd:A.
+		 */
+		len = wait_event_interruptible(log_wait,
+				prb_read_valid(prb, seq, NULL)); /* LMM(syslog_print:A) */
 		mutex_lock(&syslog_lock);
 
 		if (len)
@@ -3316,7 +3338,18 @@ void wake_up_klogd(void)
 		return;
 
 	preempt_disable();
-	if (waitqueue_active(&log_wait)) {
+	/*
+	 * Guarantee any new records can be seen by tasks preparing to wait
+	 * before this context checks if the wait queue is empty.
+	 *
+	 * The full memory barrier within wq_has_sleeper() pairs with the full
+	 * memory barrier within set_current_state() of
+	 * prepare_to_wait_event(), which is called after ___wait_event() adds
+	 * the waiter but before it has checked the wait condition.
+	 *
+	 * This pairs with devkmsg_read:A and syslog_print:A.
+	 */
+	if (wq_has_sleeper(&log_wait)) { /* LMM(wake_up_klogd:A) */
 		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
 		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
 	}