Possible memory ordering bug in page reclaim?

[Nick Piggin <nickpiggin@yahoo.com.au>]

Is there anything that prevents PageDirty from theoretically being
speculatively loaded before page_count here? (see patch)

It would result in pagecache corruption in the following situation:

1                                2
find_get_page();
write to page                    write_lock(tree_lock);
SetPageDirty();                  if (page_count != 2
put_page();                          || PageDirty())

Now I'm worried that 2 might see PageDirty *before* SetPageDirty in
1, and page_count *after* put_page in 1.

Or am I seeing things that aren't there?

Thanks,

--
SUSE Labs, Novell Inc.

Index: linux-2.6/mm/vmscan.c
===================================================================
--- linux-2.6.orig/mm/vmscan.c
+++ linux-2.6/mm/vmscan.c
@@ -511,7 +511,12 @@ static int shrink_list(struct list_head 
 		 * PageDirty _after_ making sure that the page is freeable and
 		 * not in use by anybody. 	(pagecache + us == 2)
 		 */
-		if (page_count(page) != 2 || PageDirty(page)) {
+		if (page_count(page) != 2) {
+			write_unlock_irq(&mapping->tree_lock);
+			goto keep_locked;
+		}
+		smp_rmb();
+		if (PageDirty(page)) {
 			write_unlock_irq(&mapping->tree_lock);
 			goto keep_locked;
 		}