fix: synchronise destruction of page directories with frame table

src/userprog/pagedir.c

@@ -6,6 +6,7 @@
 #include "threads/init.h"
 #include "threads/pte.h"
 #include "threads/palloc.h"
+#include "threads/synch.h"
 #include "vm/frame.h"
 #include "vm/page.h"
 
@@ -35,6 +36,7 @@ pagedir_destroy (uint32_t *pd)
     return;
 
   ASSERT (pd != init_page_dir);
+  lock_acquire (&lru_lock);
   for (pde = pd; pde < pd + pd_no (PHYS_BASE); pde++)
     if (*pde & PTE_P) 
       {
@@ -53,6 +55,7 @@ pagedir_destroy (uint32_t *pd)
         palloc_free_page (pt);
       }
   palloc_free_page (pd);
+  lock_release (&lru_lock);
 }
 
 /* Returns the address of the page table entry for virtual

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
@@ -206,8 +209,13 @@ frame_free (void *frame)
              "but this address is not allocated!\n",
              frame);
 
+    bool lock_held = lock_held_by_current_thread (&lru_lock);
+
     free_owners (&frame_metadata->owners);
+
+    if (!lock_held)
       lock_acquire (&lru_lock);
+
     hash_delete (&frame_table, &frame_metadata->hash_elem);
     list_remove (&frame_metadata->list_elem);
 
@@ -221,6 +229,8 @@ frame_free (void *frame)
         else
           next_victim = lru_next (next_victim);
       }
+
+    if (!lock_held)
       lock_release (&lru_lock);
 
     free (frame_metadata);
@@ -372,7 +382,6 @@ 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);