Add a comment explaining the race fixed by r310423.

Suggested and reviewed by: jhb
X-MFC With:	r310423

sys/kern/subr_sleepqueue.c

@@ -892,7 +892,12 @@ sleepq_broadcast(void *wchan, int flags, int pri, int queue)
 	KASSERT(sq->sq_type == (flags & SLEEPQ_TYPE),
 	    ("%s: mismatch between sleep/wakeup and cv_*", __func__));
 
-	/* Resume all blocked threads on the sleep queue. */
+	/*
+	 * Resume all blocked threads on the sleep queue.  The last thread will
+	 * be given ownership of sq and may re-enqueue itself before
+	 * sleepq_resume_thread() returns, so we must cache the "next" queue
+	 * item at the beginning of the final iteration.
+	 */
 	wakeup_swapper = 0;
 	TAILQ_FOREACH_SAFE(td, &sq->sq_blocked[queue], td_slpq, tdn) {
 		thread_lock(td);