don't over-lock in exit()

kernel/proc.c

@@ -358,14 +358,14 @@ exit(int status)
   p->cwd = 0;
 
   acquire(&proc_tree_lock);
-  
-  acquire(&p->lock);
 
   // Give any children to init.
   reparent(p);
 
   // Parent might be sleeping in wait().
   wakeup(p->parent);
+  
+  acquire(&p->lock);
 
   p->xstate = status;
   p->state = ZOMBIE;
@@ -393,7 +393,9 @@ wait(uint64 addr)
     havekids = 0;
     for(np = proc; np < &proc[NPROC]; np++){
       if(np->parent == p){
+        // make sure the child isn't still in exit() or swtch().
         acquire(&np->lock);
+
         havekids = 1;
         if(np->state == ZOMBIE){
           // Found one.