Missing memory barrier in pte_chain_unlock

Hi,

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.

Anton

===== include/linux/page-flags.h 1.12 vs edited =====

@@ -249,6 +248,7 @@
 
 static inline void pte_chain_unlock(struct page *page)
 {
+	smp_mb__before_clear_bit();
 	clear_bit(PG_chainlock, &page->flags);
 	preempt_enable();
 }

Bah. The problem with this smp_mb thing is that nobody knows
what it does, nobody remembers to do it and it's as ugly as sin.

I bet there are plenty of identical bugs around the place which haven't been discovered yet.

Is there some clean, centralised way of fixing this problem permanently?

Correctness comes first. Why not move the barrier into clear_bit() and then have a clear_bit_no_mb() operation for those performance-sensitive places where the barrier is not needed?

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