On Wed, 12 May 2010, Rik van Riel wrote: > > I suspect the atomic_dec_and_lock in the KVM code is being used > to prevent the following race: > > 1) KSM code reduces the refcount to 0 > > 2) munmap on other CPU frees the anon_vma > > 3) KSM code takes the anon_vma lock, > which now lives in freed memory Hmm. Well, if it were just about the lock, then that would be fine. That's why we do the whole anon_vma RCU freeing dance, after all. But I guess you're right - although not because of the lock. You're right because it would be a double-free - both parties would decide that they can free the damn thing, because it's not a pure atomic refcount, it's a "refcount or list_empty()" thing. If _everybody_ was using the refcount, we could just do the atomic_dec_and_test(). But they aren't. So yeah, I guess we do want that nasty dec-and-lock version. Linus -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>