* fix possible race in thread_set_priority

* fill in same parts of proj1.txt
* use thread_get_priority() whenever possible

1 files changed, 20 insertions, 1 deletions

diff --git a/threads/thread.c b/threads/thread.c
index 61ab5d9..cf404b6 100644
--- a/threads/thread.c
+++ b/threads/thread.c
@@ -379,12 +379,26 @@ thread_set_priority (int new_priority)
 void
 thread_other_set_priority (struct thread *t, int new_priority)
 {
+  enum intr_level old_level;
+  bool yield = false;
+
   ASSERT (is_thread (t));
   ASSERT (new_priority >= PRI_MIN && new_priority <= PRI_MAX);
 
   if (t->priority == new_priority)
     return;
 
+  /* we need to disable interrupts here because after changing the thread's
+     priority and a possible timer interrupt which triggers thread_yield() we
+     could run into a couple of races. e.g. no proper sorted ready list,
+     corrupt ready list (wrong internal pointers) or the timer interrupt fires
+     after calling list_empty() but before list_front() while having only two
+     threads (one active, the other one ready). the scheduler then schedules the
+     other thread which runs and terminates, thus removing itself from all lists.
+     after that our thread gets scheduled again and reads from an empty list,
+     although it wasn't empty before. */
+  old_level = intr_disable ();
+
   t->priority = new_priority;
 
   if (t->status == THREAD_READY)
@@ -398,8 +412,13 @@ thread_other_set_priority (struct thread *t, int new_priority)
       /* compare priority with the highest priority in the list */
       struct thread *t2 = list_entry (list_front (&ready_list), struct thread, elem);
       if (t2->priority > t->priority)
-        thread_yield ();
+        yield = true;
     }
+
+  intr_set_level (old_level);
+
+  if (yield)
+    thread_yield ();
 }
 
 void