diff --git a/kernel/proc.c b/kernel/proc.c
index 87a81fb..d88e453 100644
--- a/kernel/proc.c
+++ b/kernel/proc.c
@@ -16,11 +16,14 @@ int nextpid = 1;
 struct spinlock pid_lock;
 
 extern void forkret(void);
-static void wakeup1(struct proc *chan);
 static void freeproc(struct proc *p);
 
 extern char trampoline[]; // trampoline.S
 
+// protects parent/child relationships.
+// must be held when using p->parent.
+// must be acquired before any p->lock.
+struct spinlock proc_tree_lock;
 
 // Allocate a page for each process's kernel stack.
 // Map it high in memory, followed by an invalid
@@ -45,6 +48,7 @@ procinit(void)
   struct proc *p;
   
   initlock(&pid_lock, "nextpid");
+  initlock(&proc_tree_lock, "proc_tree");
   for(p = proc; p < &proc[NPROC]; p++) {
       initlock(&p->lock, "proc");
       p->kstack = KSTACK((int) (p - proc));
@@ -113,6 +117,7 @@ allocproc(void)
 
 found:
   p->pid = allocpid();
+  p->state = USED;
 
   // Allocate a trapframe page.
   if((p->trapframe = (struct trapframe *)kalloc()) == 0){
@@ -283,8 +288,6 @@ fork(void)
   }
   np->sz = p->sz;
 
-  np->parent = p;
-
   // copy saved user registers.
   *(np->trapframe) = *(p->trapframe);
 
@@ -301,35 +304,30 @@ fork(void)
 
   pid = np->pid;
 
-  np->state = RUNNABLE;
+  release(&np->lock);
 
+  acquire(&proc_tree_lock);
+  np->parent = p;
+  release(&proc_tree_lock);
+
+  acquire(&np->lock);
+  np->state = RUNNABLE;
   release(&np->lock);
 
   return pid;
 }
 
 // Pass p's abandoned children to init.
-// Caller must hold p->lock.
+// Caller must hold proc_tree_lock.
 void
 reparent(struct proc *p)
 {
   struct proc *pp;
 
   for(pp = proc; pp < &proc[NPROC]; pp++){
-    // this code uses pp->parent without holding pp->lock.
-    // acquiring the lock first could cause a deadlock
-    // if pp or a child of pp were also in exit()
-    // and about to try to lock p.
     if(pp->parent == p){
-      // pp->parent can't change between the check and the acquire()
-      // because only the parent changes it, and we're the parent.
-      acquire(&pp->lock);
       pp->parent = initproc;
-      // we should wake up init here, but that would require
-      // initproc->lock, which would be a deadlock, since we hold
-      // the lock on one of init's children (pp). this is why
-      // exit() always wakes init (before acquiring any locks).
-      release(&pp->lock);
+      wakeup(initproc);
     }
   }
 }
@@ -359,41 +357,20 @@ exit(int status)
   end_op();
   p->cwd = 0;
 
-  // we might re-parent a child to init. we can't be precise about
-  // waking up init, since we can't acquire its lock once we've
-  // acquired any other proc lock. so wake up init whether that's
-  // necessary or not. init may miss this wakeup, but that seems
-  // harmless.
-  acquire(&initproc->lock);
-  wakeup1(initproc);
-  release(&initproc->lock);
-
-  // grab a copy of p->parent, to ensure that we unlock the same
-  // parent we locked. in case our parent gives us away to init while
-  // we're waiting for the parent lock. we may then race with an
-  // exiting parent, but the result will be a harmless spurious wakeup
-  // to a dead or wrong process; proc structs are never re-allocated
-  // as anything else.
-  acquire(&p->lock);
-  struct proc *original_parent = p->parent;
-  release(&p->lock);
+  acquire(&proc_tree_lock);
   
-  // we need the parent's lock in order to wake it up from wait().
-  // the parent-then-child rule says we have to lock it first.
-  acquire(&original_parent->lock);
-
   acquire(&p->lock);
 
   // Give any children to init.
   reparent(p);
 
   // Parent might be sleeping in wait().
-  wakeup1(original_parent);
+  wakeup(p->parent);
 
   p->xstate = status;
   p->state = ZOMBIE;
 
-  release(&original_parent->lock);
+  release(&proc_tree_lock);
 
   // Jump into the scheduler, never to return.
   sched();
@@ -409,20 +386,13 @@ wait(uint64 addr)
   int havekids, pid;
   struct proc *p = myproc();
 
-  // hold p->lock for the whole time to avoid lost
-  // wakeups from a child's exit().
-  acquire(&p->lock);
+  acquire(&proc_tree_lock);
 
   for(;;){
     // Scan through table looking for exited children.
     havekids = 0;
     for(np = proc; np < &proc[NPROC]; np++){
-      // this code uses np->parent without holding np->lock.
-      // acquiring the lock first would cause a deadlock,
-      // since np might be an ancestor, and we already hold p->lock.
       if(np->parent == p){
-        // np->parent can't change between the check and the acquire()
-        // because only the parent changes it, and we're the parent.
         acquire(&np->lock);
         havekids = 1;
         if(np->state == ZOMBIE){
@@ -436,7 +406,7 @@ wait(uint64 addr)
           }
           freeproc(np);
           release(&np->lock);
-          release(&p->lock);
+          release(&proc_tree_lock);
           return pid;
         }
         release(&np->lock);
@@ -445,12 +415,12 @@ wait(uint64 addr)
 
     // No point waiting if we don't have any children.
     if(!havekids || p->killed){
-      release(&p->lock);
+      release(&proc_tree_lock);
       return -1;
     }
     
     // Wait for a child to exit.
-    sleep(p, &p->lock);  //DOC: wait-sleep
+    sleep(p, &proc_tree_lock);  //DOC: wait-sleep
   }
 }
 
@@ -563,10 +533,9 @@ sleep(void *chan, struct spinlock *lk)
   // guaranteed that we won't miss any wakeup
   // (wakeup locks p->lock),
   // so it's okay to release lk.
-  if(lk != &p->lock){  //DOC: sleeplock0
-    acquire(&p->lock);  //DOC: sleeplock1
-    release(lk);
-  }
+
+  acquire(&p->lock);  //DOC: sleeplock1
+  release(lk);
 
   // Go to sleep.
   p->chan = chan;
@@ -578,10 +547,8 @@ sleep(void *chan, struct spinlock *lk)
   p->chan = 0;
 
   // Reacquire original lock.
-  if(lk != &p->lock){
-    release(&p->lock);
-    acquire(lk);
-  }
+  release(&p->lock);
+  acquire(lk);
 }
 
 // Wake up all processes sleeping on chan.
@@ -592,23 +559,13 @@ wakeup(void *chan)
   struct proc *p;
 
   for(p = proc; p < &proc[NPROC]; p++) {
-    acquire(&p->lock);
-    if(p->state == SLEEPING && p->chan == chan) {
-      p->state = RUNNABLE;
+    if(p != myproc()){
+      acquire(&p->lock);
+      if(p->state == SLEEPING && p->chan == chan) {
+        p->state = RUNNABLE;
+      }
+      release(&p->lock);
     }
-    release(&p->lock);
-  }
-}
-
-// Wake up p if it is sleeping in wait(); used by exit().
-// Caller must hold p->lock.
-static void
-wakeup1(struct proc *p)
-{
-  if(!holding(&p->lock))
-    panic("wakeup1");
-  if(p->chan == p && p->state == SLEEPING) {
-    p->state = RUNNABLE;
   }
 }
 
diff --git a/kernel/proc.h b/kernel/proc.h
index 9c16ea7..e86bde1 100644
--- a/kernel/proc.h
+++ b/kernel/proc.h
@@ -80,7 +80,7 @@ struct trapframe {
   /* 280 */ uint64 t6;
 };
 
-enum procstate { UNUSED, SLEEPING, RUNNABLE, RUNNING, ZOMBIE };
+enum procstate { UNUSED, USED, SLEEPING, RUNNABLE, RUNNING, ZOMBIE };
 
 // Per-process state
 struct proc {