Messages in this thread | | | Date | Thu, 13 Nov 2008 03:31:35 +0100 | From | Andrea Arcangeli <> | Subject | Re: [PATCH 2/4] Add replace_page(), change the mapping of pte from one page into another |
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On Thu, Nov 13, 2008 at 03:00:59AM +0100, Andrea Arcangeli wrote: > CPU0 migrate.c CPU1 filemap.c > ------- ---------- > find_get_page > radix_tree_lookup_slot returns the oldpage > page_count still = expected_count > freeze_ref (oldpage->count = 0) > radix_tree_replace (too late, other side already got the oldpage) > unfreeze_ref (oldpage->count = 2) > page_cache_get_speculative(old_page) > set count to 3 and succeeds
After reading more of this lockless radix tree code, I realized this below check is the one that was intended to restart find_get_page and prevent it to return the oldpage:
if (unlikely(page != *pagep)) {
But there's no barrier there, atomic_add_unless would need to provide an atomic smp_mb() _after_ atomic_add_unless executed. In the old days the atomic_* routines had no implied memory barriers, you had to use smp_mb__after_atomic_add_unless if you wanted to avoid the race. I don't see much in the ppc implementation of atomic_add_unless that would provide an implicit smb_mb after the page_cache_get_speculative returns, so I can't see why the pagep can't be by find_get_page read before the other cpu executes radix_tree_replace in the above timeline.
I guess you intended to put an smp_mb() in between the page_cache_get_speculative and the *pagep to make the code safe on ppc too, but there isn't, and I think it must be fixed, either that or I don't understand ppc assembly right. The other side has a smp_wmb implicit inside radix_tree_replace_slot so it should be ok already to ensure we see the refcount going to 0 before we see the pagep changed (the fact the other side has a memory barrier, further confirms this side needs it too).
BTW, the radix_tree_deref_slot might miss a rcu_barrier_depends() after radix_tree_deref_slot returns but I'm not entirely sure and only alpha would be affected in the worst case.
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