fix: acquire lru_lock before pinning frames to avoid race condition with eviction

src/vm/frame.c

@@ -28,9 +28,6 @@ struct list lru_list;
    victim. Otherwise, the next element in the queue is similarly considered. */
 struct list_elem *next_victim = NULL;
 
-/* Synchronisation variables. */
-/* Protects access to 'lru_list'. */
-struct lock lru_lock;
 
 struct frame_metadata
   {
@@ -175,23 +172,29 @@ frame_alloc (enum palloc_flags flags, void *upage, struct thread *owner)
 void
 frame_pin (void *frame)
 {
+  ASSERT (frame != NULL);
+  lock_acquire (&lru_lock);
   struct frame_metadata *frame_metadata = frame_metadata_get (frame);
   if (frame_metadata == NULL)
     PANIC ("Attempted to pin a frame at an unallocated kernel address '%p'\n",
            frame);
 
   frame_metadata->pinned = true;
+  lock_release (&lru_lock);
 }
   
 void
 frame_unpin (void *frame)
 {
+  ASSERT (frame != NULL);
+  lock_acquire (&lru_lock);
   struct frame_metadata *frame_metadata = frame_metadata_get (frame);
   if (frame_metadata == NULL)
     PANIC ("Attempted to unpin a frame at an unallocated kernel address '%p'\n",
            frame);
 
   frame_metadata->pinned = false;
+  lock_release (&lru_lock);
 }
 
 /* Attempt to deallocate a frame for a user process by removing it from the
@@ -372,7 +375,6 @@ frame_metadata_get (void *frame)
 
   struct hash_elem *e = hash_find (&frame_table, &key_metadata.hash_elem);
   if (e == NULL) return NULL;
-
   return hash_entry (e, struct frame_metadata, hash_elem);
 }
 

src/vm/frame.h

@@ -10,7 +10,11 @@ struct frame_owner
   struct list_elem elem; /* List element for the list of owners. */
 };
 
+/* Synchronisation variables. */
+/* Protects access to 'lru_list'. */
+struct lock lru_lock;
 void frame_init (void);
+
 void *frame_alloc (enum palloc_flags, void *, struct thread *);
 void frame_pin (void *frame);
 void frame_unpin (void *frame);