Hi Hugh, On Wed, Aug 25, 2010 at 11:12:54PM -0700, Hugh Dickins wrote: > After several hours, kbuild tests hang with anon_vma_prepare() spinning on > a newly allocated anon_vma's lock - on a box with CONFIG_TREE_PREEMPT_RCU=y > (which makes this very much more likely, but it could happen without). > > The ever-subtle page_lock_anon_vma() now needs a further twist: since > anon_vma_prepare() and anon_vma_fork() are liable to change the ->root > of a reused anon_vma structure at any moment, page_lock_anon_vma() > needs to check page_mapped() again before succeeding, otherwise > page_unlock_anon_vma() might address a different root->lock. I don't get it, the anon_vma can be freed and reused only after we run rcu_read_unlock(). And the anon_vma->root can't change unless the anon_vma is freed and reused. Last but not the least by the time page->mapping points to "anon_vma" the "anon_vma->root" is already initialized and stable. The page_mapped test is only relevant against the rcu_read_lock, not the spin_lock, so how it can make a difference to run it twice inside the same rcu_read_lock protected critical section? The first one still is valid also after the anon_vma_lock() returns, it's not like that anon_vma_lock drops the rcu_read_lock internally. Furthermore no need of ACCESS_ONCE on the anon_vma->root because it can't change from under us as the anon_vma can't be freed from under us until rcu_read_unlock returns (after we verified the first time that page_mapped is true under the rcu_read_lock, which we already do before trying to take the anon_vma_lock). -- 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>