Fix process_result locking by acquiring in process_wait as well to prevent freeing memory too early

src/threads/thread.c

@@ -647,6 +647,7 @@ 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

@@ -34,9 +34,10 @@ typedef int tid_t;
 /* A process result, synchronised between parent and child. */
 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. */
+  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 the exit_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

@@ -224,11 +224,16 @@ process_wait (tid_t child_tid UNUSED)
     return -1;
   /* Wait for child to die. */
   sema_down (&child_result->sema);
+  /* We need lock release in process_exit, so we need to acquire (and possibly
+     wait) for it here to ensure we don't free the lock memory before it is
+     released in process_exit. */
+  lock_acquire (&child_result->lock);
   /* To prevent waiting for child twice, remove it from the list.
      No need to use lock since this is the only thread with access to
      the struct process_result now. */
   list_remove (&child_result->elem);
   int exit_status = child_result->exit_status;
+  lock_release (&child_result->lock);
   free (child_result);
   return exit_status;
 }
@@ -245,14 +250,21 @@ 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))
-        free (cur->result);
+        {
+          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);
+        {
+          sema_up (&cur->result->sema);
+          lock_release (&cur->result->lock);
+        }
     }
 
   struct list_elem *e;
@@ -261,12 +273,19 @@ 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))
-        free (result);
+        {
+          lock_release (&result->lock);
+          free (result);
+        }
       /* Child is still alive, signal via sema that parent has died. */
       else
-        sema_up (&result->sema);
+        {
+          sema_up (&result->sema);
+          lock_release (&result->lock);
+        }
     }
 
   /* Destroy the current process's page directory and switch back