Remove process_result lock since it is an invalid solution

TODO : synchronise process_result in another way

src/threads/thread.c

@@ -647,7 +647,6 @@ init_process_result (struct thread *t)
   struct process_result *result = malloc (sizeof (struct process_result));
   result->tid = t->tid;
   result->exit_status = t->exit_status;
-  lock_init (&result->lock);
   sema_init (&result->sema, 0);
   t->result = result;
 }

src/threads/thread.h

@@ -37,7 +37,6 @@ struct process_result
   tid_t tid;        /* The tid of the child process. */
   int exit_status;  /* The exit status of the child process. Initially set to
                        -1, then to exit_status when child dies. */
-  struct lock lock; /* Lock to synchronise access to the status and sema. */
   struct semaphore sema; /* Semaphore to signal the parent that the exit_status
                             has been set. */
   struct list_elem elem; /* List element for the parent's children list. */

src/userprog/process.c

@@ -245,21 +245,14 @@ process_exit (void)
   /* Update process result. */
   if (cur->result != NULL)
     {
-      lock_acquire (&cur->result->lock);
       cur->result->exit_status = cur->exit_status;
       /* Parent has died, child has to free the struct process_result * */
       if (sema_try_down (&cur->result->sema))
-        {
-          lock_release (&cur->result->lock);
         free (cur->result);
-        }
       /* Parent is still alive and will be the one to free the
          struct process_result *, and may be waiting so call sema_up */
       else
-        {
         sema_up (&cur->result->sema);
-          lock_release (&cur->result->lock);
-        }
     }
 
   struct list_elem *e;
@@ -268,19 +261,12 @@ process_exit (void)
     {
       struct process_result *result
           = list_entry (e, struct process_result, elem);
-      lock_acquire (&result->lock);
       /* Child has died (and was not waited for). Free the result. */
       if (sema_try_down (&result->sema))
-        {
-          lock_release (&result->lock);
         free (result);
-        }
       /* Child is still alive, signal via sema that parent has died. */
       else
-        {
         sema_up (&result->sema);
-          lock_release (&result->lock);
-        }
     }
 
   /* Destroy the current process's page directory and switch back