[PATCH] Missing memory barrier in pte_chain_unlock

On a ppc64 machine running 2.5.28 we were hitting this BUG in
__free_pages_ok:

BUG_ON(page->pte.chain != NULL);

In pte_chain_lock we use test_and_set_bit which implies a memory
barrier. In pte_chain_unlock we use clear_bit which has no memory
barriers so we need to add one.

include/linux/page-flags.h

@@ -249,6 +249,7 @@ static inline void pte_chain_lock(struct page *page)
 
 static inline void pte_chain_unlock(struct page *page)
 {
+	smp_mb__before_clear_bit();
 	clear_bit(PG_chainlock, &page->flags);
 	preempt_enable();
 }