diff options
authorHugh Dickins <hughd@google.com>2010-08-25 23:12:54 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-08-28 13:54:12 -0700
commitf18194275c39835cb84563500995e0d503a32d9a (patch)
parentd4348c678977c7093438bbbf2067c49396ae941b (diff)
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> Reviewed-by: Rik van Riel <riel@redhat.com> Cc: Christoph Lameter <cl@linux.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 files changed, 16 insertions, 3 deletions
diff --git a/mm/rmap.c b/mm/rmap.c
index 87b9e8ad4509..f6f0d2dda2ea 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -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;
@@ -327,8 +327,21 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
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);
return NULL;

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