Synchronise process_execute return with child process load

src/userprog/process.c

@@ -47,6 +47,9 @@ struct process_start_data
                           tokens while maintaining state. */
     char file_name[FNAME_MAX_LEN + 1]; /* Name of the file of the process to
                                       be started. */
+    bool success; /* Indicates whether the process was successfully loaded. */
+    struct semaphore loaded; /* Semaphore used to signal that the process has
+                                been loaded. */
   };
 
 static thread_func start_process NO_RETURN;
@@ -67,6 +70,8 @@ process_execute (const char *cmd)
     {
       return TID_ERROR;
     }
+  sema_init (&data->loaded, 0);
+  data->success = false;
 
   /* Make a copy of command.
      Otherwise there's a race between the caller and load(). */
@@ -101,6 +106,13 @@ process_execute (const char *cmd)
   tid = thread_create (file_name, PRI_DEFAULT, start_process, data);
   if (tid == TID_ERROR)
     palloc_free_page (cmd_copy);
+  else
+    {
+      sema_down (&data->loaded);
+      if (!data->success)
+        tid = TID_ERROR;
+    }
+  free (data);
   return tid;
 }
 
@@ -132,6 +144,11 @@ start_process (void *proc_start_data)
   
   /* Prevent writing to the file being executed. */
   struct file *exec_file = filesys_open (data->file_name);
+  if (exec_file == NULL)
+    {
+      lock_release (&filesys_lock);
+      goto fail;
+    }
   thread_current ()->exec_file = exec_file;
   file_deny_write (exec_file);
   lock_release (&filesys_lock);
@@ -157,6 +174,8 @@ start_process (void *proc_start_data)
       goto fail;
     }
 
+  data->success = true;
+  sema_up (&data->loaded);
   /* Start the user process by simulating a return from an
      interrupt, implemented by intr_exit (in
      threads/intr-stubs.S).  Because intr_exit takes all of its
@@ -166,9 +185,10 @@ start_process (void *proc_start_data)
   asm volatile ("movl %0, %%esp; jmp intr_exit" : : "g" (&if_) : "memory");
   NOT_REACHED ();
 
-/* If starting the process failed, free its common resources and exit. */
- fail:
-  free (data);
+  /* If starting the process failed, exit. */
+fail:
+  data->success = false;
+  sema_up (&data->loaded);
   thread_exit ();
 }
 
@@ -328,7 +348,6 @@ process_exit (void)
   if (cur->exec_file != NULL)
     {
       lock_acquire (&filesys_lock);
-      file_allow_write (cur->exec_file);
       file_close (cur->exec_file);
       lock_release (&filesys_lock);
     }