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Subject[PATCH] mm: fix hang on anon_vma->root->lock
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.

Signed-off-by: Hugh Dickins <hughd@google.com>
---

mm/rmap.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)

--- 2.6.36-rc2/mm/rmap.c 2010-08-16 00:18:01.000000000 -0700
+++ linux/mm/rmap.c 2010-08-24 03:22:17.000000000 -0700
@@ -316,7 +316,7 @@ void __init anon_vma_init(void)
*/
struct anon_vma *page_lock_anon_vma(struct page *page)
{
- struct anon_vma *anon_vma;
+ struct anon_vma *anon_vma, *root_anon_vma;
unsigned long anon_mapping;

rcu_read_lock();
@@ -327,8 +327,21 @@ struct anon_vma *page_lock_anon_vma(stru
goto out;

anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
- anon_vma_lock(anon_vma);
- return anon_vma;
+ root_anon_vma = ACCESS_ONCE(anon_vma->root);
+ spin_lock(&root_anon_vma->lock);
+
+ /*
+ * If this page is still mapped, then its anon_vma cannot have been
+ * freed. But if it has been unmapped, we have no security against
+ * the anon_vma structure being freed and reused (for another anon_vma:
+ * SLAB_DESTROY_BY_RCU guarantees that - so the spin_lock above cannot
+ * corrupt): with anon_vma_prepare() or anon_vma_fork() redirecting
+ * anon_vma->root before page_unlock_anon_vma() is called to unlock.
+ */
+ if (page_mapped(page))
+ return anon_vma;
+
+ spin_unlock(&root_anon_vma->lock);
out:
rcu_read_unlock();
return NULL;

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