genwait: Keep queues in order by thread priority.

Currently, new waiting threads are always added to the end
of their genwait tailq and the first thread in the tailq is
the first woken. This means that unless a thread is being woken
and readded (like can happen with a `genwait_wake_all`) they will
always be awoken based on how long they have been waiting.

The downside to this method is that it ignores thread priorities,
which should be the controlling factor in which thread gets woken
first. By inserting into the queue by priority order, wakes will now
happen based on priority rather than time waiting.

The mild overhead of seeking through the waiting queues should be
outweighed greatly by allowing priority management via thread priorities.

Author: QuzarDC

kernel/thread/genwait.c

@@ -69,7 +69,7 @@ static kthread_t *tq_next(void) {
 }
 
 int genwait_wait(void *obj, const char *mesg, int timeout, void (*callback)(void *)) {
-    kthread_t   *me;
+    kthread_t   *me, *t;
 
     /* Twiddle interrupt state */
     if(irq_inside_int()) {
@@ -95,8 +95,17 @@ int genwait_wait(void *obj, const char *mesg, int timeout, void (*callback)(void
 
     me->wait_callback = callback;
 
-    /* Insert us on the appropriate wait queue */
-    TAILQ_INSERT_TAIL(&slpque[LOOKUP(obj)], me, thdq);
+    /* Go through and find where to insert */
+    TAILQ_FOREACH(t, &slpque[LOOKUP(obj)], thdq) {
+        if(me->prio < t->prio) {
+            TAILQ_INSERT_BEFORE(t, me, thdq);
+            break;
+        }
+    }
+
+    /* We got to the end of the list, so insert at end */
+    if(!t)
+        TAILQ_INSERT_TAIL(&slpque[LOOKUP(obj)], me, thdq);
 
     /* Block us until we're signaled */
     return thd_block_now(&me->context);