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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>
This commit is contained in:
Hugh Dickins 2010-08-25 23:12:54 -07:00 committed by Linus Torvalds
parent d4348c6789
commit f18194275c
1 changed files with 16 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
mm/rmap.c
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@ -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(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);
out:
rcu_read_unlock();
return NULL;