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.

kernel/proc.h

@@ -88,12 +88,14 @@ struct proc {
 
   // p->lock must be held when using these:
   enum procstate state;        // Process state
-  struct proc *parent;         // Parent process
   void *chan;                  // If non-zero, sleeping on chan
   int killed;                  // If non-zero, have been killed
   int xstate;                  // Exit status to be returned to parent's wait
   int pid;                     // Process ID
 
+  // proc_tree_lock must be held when using this:
+  struct proc *parent;         // Parent process
+
   // these are private to the process, so p->lock need not be held.
   uint64 kstack;               // Virtual address of kernel stack
   uint64 sz;                   // Size of process memory (bytes)