Move the process_fork event out from under Giant. This one is easy,

since there are no consumers in the tree.  Document this.

sys/kern/kern_fork.c

@@ -689,14 +689,16 @@ again:
 		cnt.v_rforkpages += p2->p_vmspace->vm_dsize +
 		    p2->p_vmspace->vm_ssize;
 	}
+	mtx_unlock(&Giant);
 
 	/*
 	 * Both processes are set up, now check if any loadable modules want
 	 * to adjust anything.
 	 *   What if they have an error? XXX
+	 *
+	 * Handlers must be MPSAFE, or aquire Giant themselves if not.
 	 */
 	EVENTHANDLER_INVOKE(process_fork, p1, p2, flags);
-	mtx_unlock(&Giant);
 
 	/*
 	 * Set the child start time and mark the process as being complete.

sys/sys/eventhandler.h

@@ -156,7 +156,11 @@ typedef void (*vm_lowmem_handler_t)(void *, int);
 #define	LOWMEM_PRI_DEFAULT	EVENTHANDLER_PRI_FIRST
 EVENTHANDLER_DECLARE(vm_lowmem, vm_lowmem_handler_t);
 
-/* Process events */
+/*
+ * Process events
+ * process_fork handlers are called without Giant.
+ * exit/exec handlers are called with Giant.
+ */
 struct proc;
 
 typedef void (*exitlist_fn)(void *, struct proc *);