fix: synchronise pagedir which now may be accessed by other threads, with a lock

src/userprog/process.c

@@ -396,6 +396,7 @@ process_exit (void)
 
   /* Destroy the current process's page directory and switch back
      to the kernel-only page directory. */
+  lock_acquire (&cur->pagedir_lock);
   pd = cur->pagedir;
   if (pd != NULL) 
     {
@@ -410,6 +411,7 @@ process_exit (void)
       pagedir_activate (NULL);
       pagedir_destroy (pd);
     }
+  lock_release (&cur->pagedir_lock);
 }
 
 /* Destruct a process_result, with multi-thread awareness.
@@ -535,7 +537,9 @@ load (const char *file_name, void (**eip) (void), void **esp)
   lock_acquire (&filesys_lock);
 
   /* Allocate and activate page directory. */
+  lock_acquire (&t->pagedir_lock);
   t->pagedir = pagedir_create ();
+  lock_release (&t->pagedir_lock);
   if (t->pagedir == NULL) 
     goto done;
   process_activate ();
@@ -761,11 +765,16 @@ get_usr_kpage (enum palloc_flags flags, void *upage)
   void *page;
 #ifdef VM
   struct thread *t = thread_current ();
+  lock_acquire (&t->pagedir_lock);
   if (pagedir_get_page (t->pagedir, upage) != NULL)
-    return NULL;
+    {
+      lock_release (&t->pagedir_lock);
+      return NULL;
+    }
   else
     page = frame_alloc (flags, upage, t);
   pagedir_set_accessed (t->pagedir, upage, true);
+  lock_release (&t->pagedir_lock);
 #else
   page = palloc_get_page (flags | PAL_USER);
 #endif