Update start_process to acquire filesys lock when loading user process file

src/userprog/process.c

@@ -8,6 +8,7 @@
 #include <string.h>
 #include "userprog/gdt.h"
 #include "userprog/pagedir.h"
+#include "userprog/syscall.h"
 #include "userprog/tss.h"
 #include "filesys/directory.h"
 #include "filesys/file.h"
@@ -81,6 +82,12 @@ 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. */
+struct file *exec_file = filesys_open (file_name);
+file_deny_write (exec_file);
+
 /* Validates that the current file to be executed is a valid file */
 if (filesys_open (file_name) == NULL)
     return TID_ERROR;
@@ -120,7 +127,10 @@ start_process (void *proc_start_data)
   if_.gs = if_.fs = if_.es = if_.ds = if_.ss = SEL_UDSEG;
   if_.cs = SEL_UCSEG;
   if_.eflags = FLAG_IF | FLAG_MBS;
+
+  lock_acquire (&filesys_lock);
   success = load (data->file_name, &if_.eip, &if_.esp);
+  lock_release (&filesys_lock);
 
   /* If load failed, quit. */
   if (!success) 
@@ -141,13 +151,6 @@ start_process (void *proc_start_data)
       goto fail;
     }
 
-  /* 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. */
-  struct file *exec_file = filesys_open (data->file_name);
-  thread_current ()->exec_file = exec_file;
-  file_deny_write (exec_file);
-
   /* 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