Update user proc stack initialization comments to be more helpful

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;
@@ -64,7 +64,6 @@ process_execute (const char *cmd)
 {
   char *cmd_copy;
   tid_t tid;
-  
   struct process_start_data data;
 
   /* Make a copy of command.
@@ -81,11 +80,8 @@ process_execute (const char *cmd)
      of the process. */
   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)
@@ -100,14 +96,20 @@ process_execute (const char *cmd)
   data.success = false;
 
   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
     {
+      
+      /* 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;
     }
+
   return tid;
 }
 
@@ -119,8 +121,11 @@ 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)
 {