Merge branch 'stack-init-optimize' into 'master'

Optimize stack initialization by not using malloc, and remove use of 'goto' (for code review safety)

See merge request lab2425_autumn/pintos_22!41

src/userprog/process.c

@@ -49,7 +49,7 @@ struct process_start_data
                                       be started. */
     bool success; /* Indicates whether the process was successfully loaded. */
     struct semaphore loaded; /* Semaphore used to signal that the process has
-                                been loaded. */
+                                finished attempting to load. */
   };
 
 static thread_func start_process NO_RETURN;
@@ -64,14 +64,7 @@ process_execute (const char *cmd)
 {
   char *cmd_copy;
   tid_t tid;
-  
-  struct process_start_data *data = malloc (sizeof (struct process_start_data));
-  if (data == NULL)
-    {
-      return TID_ERROR;
-    }
-  sema_init (&data->loaded, 0);
-  data->success = false;
+  struct process_start_data data;
 
   /* Make a copy of command.
      Otherwise there's a race between the caller and load(). */
@@ -85,13 +78,10 @@ process_execute (const char *cmd)
 
   /* Retrieve first argument of command, which is the file name
      of the process. */
-  char *file_name = strtok_r (cmd_copy, " ", &data->cmd_saveptr);
+  char *file_name = strtok_r (cmd_copy, " ", &data.cmd_saveptr);
 
-  /* NOTE: Currently, the file being executed is closed in load () and then
-          reopened here. Because load is an exported public function, this
-          might be necessary. */
+  /* Validates that the current file to be executed can be opened/exists. */
   lock_acquire (&filesys_lock);
-  /* Validates that the current file to be executed is a valid file */
   bool valid_file = filesys_open (file_name) != NULL;
   lock_release (&filesys_lock);
   if (!valid_file)
@@ -100,19 +90,26 @@ process_execute (const char *cmd)
   /* Create a new thread to execute the command, by initializing
      it running the function 'start_process' with the appropriate
      arguments. For details of arguments, see 'start_process'. */
-  data->cmd = cmd_copy;
-  strlcpy (data->file_name, file_name, FNAME_MAX_LEN + 1);
+  data.cmd = cmd_copy;
+  strlcpy (data.file_name, file_name, FNAME_MAX_LEN + 1);
+  sema_init (&data.loaded, 0);
+  data.success = false;
 
-  tid = thread_create (file_name, PRI_DEFAULT, start_process, data);
+  tid = thread_create (file_name, PRI_DEFAULT, start_process, &data);
+
+  /* Return TID_ERROR and free resources if starting execution went wrong. */
   if (tid == TID_ERROR)
     palloc_free_page (cmd_copy);
   else
     {
-      sema_down (&data->loaded);
-      if (!data->success)
+      
+      /* Wait until process file has finished attempting to load via the child
+         thread before reporting success of starting execution. */
+      sema_down (&data.loaded);
+      if (!data.success)
         tid = TID_ERROR;
     }
-  free (data);
+
   return tid;
 }
 
@@ -124,13 +121,15 @@ static void *push_to_stack (void **esp, void *data, size_t data_size);
 /* Make the current thread execute 'cmd', passing in a copy of the
    command string used for processing, the saveptr used by strtok_r
    (in order to further tokenize the same command and retrieve its
-   arguments), as well as the name of the file being executed. This
-   involves loading the specified file and starting it running. */
+   arguments), the name of the file being executed, and a semaphore that
+   calls sema_up to indicate that the 'success' variable passed to it
+   has been updated to indicate whether the process file loading succeeded. 
+   This involves loading the specified file and calling its main () function
+   with the specified command arguments. */
 static void
 start_process (void *proc_start_data)
 {
   struct intr_frame if_;
-  bool success;
 
   struct process_start_data *data = proc_start_data;
 
@@ -147,34 +146,36 @@ start_process (void *proc_start_data)
   if (exec_file == NULL)
     {
       lock_release (&filesys_lock);
-      goto fail;
+      sema_up (&data->loaded);
+      thread_exit ();
     }
-  thread_current ()->exec_file = exec_file;
   file_deny_write (exec_file);
   lock_release (&filesys_lock);
+  thread_current ()->exec_file = exec_file;
 
-  success = load (data->file_name, &if_.eip, &if_.esp);
+  data->success = load (data->file_name, &if_.eip, &if_.esp);
 
   /* If load failed, quit. */
-  if (!success) 
+  if (!data->success) 
     {
       palloc_free_page (data->cmd);
-      goto fail;
+      sema_up (&data->loaded);
+      thread_exit ();
     }
 
   /* Initialize user process stack and free page used to store the
      command that executed the process. */
-  success = process_init_stack (data->cmd_saveptr, &if_.esp, data->file_name);
+  data->success = process_init_stack (data->cmd_saveptr, &if_.esp, data->file_name);
+  bool success = data->success;
   palloc_free_page (data->cmd);
+  sema_up (&data->loaded);
   
   /* If stack initialization failed, free resources and quit. */
   if (!success)
     {
-      goto fail;
+      thread_exit ();
     }
 
-  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
@@ -183,12 +184,6 @@ start_process (void *proc_start_data)
      and jump to it. */
   asm volatile ("movl %0, %%esp; jmp intr_exit" : : "g" (&if_) : "memory");
   NOT_REACHED ();
-
-  /* If starting the process failed, exit. */
-fail:
-  data->success = false;
-  sema_up (&data->loaded);
-  thread_exit ();
 }
 
 /* Helper function that initializes the stack of a newly created