lkml.org 
[lkml]   [2007]   [Feb]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: PREEMPT_RCU breaks anon_vma locking ?
On 02/24, Hugh Dickins wrote:
>
> On Sat, 24 Feb 2007, Oleg Nesterov wrote:
>
> > So page_lock_anon_vma() works correctly due to SLAB_DESTROY_BY_RCU even if
> > anon_vma_unlink() has already freed anon_vma. In that case we should see
> > list_empty(&anon_vma->head), we are safe.
>
> (It doesn't affect your argument, but we won't necessarily see list_empty
> there: the anon_vma slot may already have got reused for a different
> bundle of vmas completely; but its lock remains a lock and its list
> remains a list of vmas, and the worst that happens is that
> page_referenced_anon or try_to_unmap_anon wanders through an irrelevant
> bundle of vmas, looking for a page that cannot be found there.)

Yes, but in that case we are safe, right? We hold the lock, anon_vma can't be
freed. But thanks for clarification! Somehow I missed that not only unlock()
is unsafe (in theory). If anon_vma's memory was re-used for something else, we
can't assume that we will see list_empty(&anon_vma->head).

> > static inline void page_lock_anon_vma(struct anon_vma *anon_vma)
>
> It might be wiser to call that one page_unlock_anon_vma ;)

Congratulations, you passed the test! Paul didn't :)

> (I'm slightly disgruntled that page_lock_anon_vma takes a struct page *,
> but page_unlock_anon_vma no struct page *. But it would be silly to do
> it differently, or mess with the naming: besides, it's a static function
> and the prototype guards against error anyway.)

OK. I thought about "unlock_anon_vma", but symmetry is good indeed.

Oleg.

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-02-25 21:09    [W:0.075 / U:0.256 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site