buddy: fix buddy_reqs_wait_prio_adjust()

This fixes two bugs:

The most relavant was that we did not increase buddy->wait_pri if the
new priority was higher, which could lead to all kinds of weird effects,
for example:

- starvation/lock up
- LRU removing all of the cache

because there were requests waiting, but buddy_wait_work() did not see
them.

The second bug is that we also called buddy_wait_work() if there
actually was no change (the critical region could find state ==
IW_SIGNALLED).

Also, for the new assertions, we need to set the proper wait_pri for the
case that the priority is lowered.

src/buddy.c

@@ -1812,8 +1812,10 @@ static inline void
 buddy_reqs_wait_prio_adjust(struct buddy_reqs *reqs)
 {
 	struct buddy_reqs_head *ohead, *nhead;
+	enum i_wait_state state;
 	struct i_wait *w;
 	buddy_t *buddy;
+	unsigned opri;
 
 	CHECK_OBJ_NOTNULL(reqs, BUDDY_REQS_MAGIC);
 	w = &reqs->i_wait;
@@ -1827,11 +1829,13 @@ buddy_reqs_wait_prio_adjust(struct buddy_reqs *reqs)
 	buddy = reqs->buddy;
 	ohead = &buddy->reqs_head[w->pri];
 	nhead = &buddy->reqs_head[reqs->pri];
+	opri = w->pri;
 
 	AZ(pthread_mutex_lock(&reqs->buddy->map_mtx));
 	AZ(pthread_mutex_lock(&w->wait_mtx));
 
-	if (w->state == IW_WAITING) {
+	state = w->state;
+	if (state == IW_WAITING) {
 		AN(buddy->waiting);
 		/* _INSERT_HEAD? _INSERT_TAIL?
 		 *
@@ -1844,8 +1848,15 @@ buddy_reqs_wait_prio_adjust(struct buddy_reqs *reqs)
 	}
 	AZ(pthread_mutex_unlock(&w->wait_mtx));
 
-	if (buddy->waiting && buddy->wait_pri == w->pri)
-		buddy_wait_work(buddy);
+	if (state == IW_WAITING) {
+		if (reqs->pri >= buddy->wait_pri) {
+			buddy->wait_pri = reqs->pri;
+			buddy_wait_work(buddy);
+		}
+		else
+			buddy_new_wait_pri(buddy, opri);
+	}
+
 	AZ(pthread_mutex_unlock(&reqs->buddy->map_mtx));
 }