2.1: wrote pick_highest_priority_runnable_proc(), it passes all tests

kernel/proc.c

@@ -509,6 +509,49 @@ wait(uint64 addr)
   }
 }
 
+// Finds highest priority, runnable, process and 
+// returns it, returns 0 if no runnable process is found.
+struct proc*
+pick_highest_priority_runnable_proc() {
+  for (int i = 0; i < NPRIO; i++) {
+    struct list_proc* list = prio[i];
+    acquire(&prio_lock);
+    struct list_proc* observed = list;
+
+    while(observed) {
+      struct proc* p = observed->p;
+      acquire(&p->lock);
+      if (p->state == RUNNABLE) {
+        return p;
+      }
+      release(&p->lock);
+      observed = observed->next;
+    }
+
+    release(&prio_lock);
+  }
+  return 0;
+
+  // for (struct list_proc* lp = prio[0]; lp < prio[NPRIO]; lp++) { // go through all priority lists
+  //   acquire(&prio_lock);
+  //   struct list_proc* observed = lp;
+
+  //   while(observed) {
+  //     struct proc* p = observed->p;
+  //     acquire(&p->lock);
+  //     if (p->state == RUNNABLE) {
+  //       return 0;
+  //     }
+  //     release(&p->lock);
+  //     observed = observed->next;
+  //   }
+  
+  //   release(&prio_lock);
+  // }
+  // return 0; // if no runnable process is found, return 0, holding no lock
+}
+
+
 // Per-CPU process scheduler.
 // Each CPU calls scheduler() after setting itself up.
 // Scheduler never returns.  It loops, doing: