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SubjectRe: [PATCH 00/21] mm: Preemptibility -v6
On Fri, Jan 21, 2011 at 04:33:54PM +0100, Peter Zijlstra wrote:
> On Thu, 2011-01-20 at 11:57 -0800, Hugh Dickins wrote:
> > > > 21/21 mm-optimize_page_lock_anon_vma_fast-path.patch
> > > > I certainly see the call for this patch, I want to eliminate those
> > > > doubled atomics too. This appears correct to me, and I've not dreamt
> > > > up an alternative; but I do dislike it, and I suspect you don't like
> > > > it much either. I'm ambivalent about it, would love a better patch.
> > >
> > > Like said, I fully agree with that sentiment, just haven't been able to
> > > come up with anything saner :/ Although I can optimize the
> > > __put_anon_vma() path a bit by doing something like:
> > >
> > > if (mutex_is_locked()) { anon_vma_lock(); anon_vma_unlock(); }
> > >
> > > But I bet that wants a barrier someplace and my head hurts..
> >
> > Without daring to hurt my head very much, yes, I'd say those kind
> > of "optimizations" have a habit of turning out to be racily wrong.
> >
> > But you put your finger on it: if you hadn't had to add that lock-
> > unlock pair into __put_anon_vma(), I wouldn't have minded the
> > contortions added to page_lock_anon_vma().
>
> I think there's just about enough implied barriers there that the
> 'simple' code just works ;-)
>
> But given that I'm trying to think with snot for brains thanks to some
> cold, I don't trust myself at all to have gotten this right.
>
> [ for Oleg and Paul: https://lkml.org/lkml/2010/11/26/213 contains the
> full patch this is against ]
>
> ---
> Index: linux-2.6/mm/rmap.c
> ===================================================================
> --- linux-2.6.orig/mm/rmap.c
> +++ linux-2.6/mm/rmap.c
> @@ -1559,9 +1559,20 @@ void __put_anon_vma(struct anon_vma *ano
> * Synchronize against page_lock_anon_vma() such that
> * we can safely hold the lock without the anon_vma getting
> * freed.
> + *
> + * Relies on the full mb implied by the atomic_dec_and_test() from
> + * put_anon_vma() against the full mb implied by mutex_trylock() from
> + * page_lock_anon_vma(). This orders:
> + *
> + * page_lock_anon_vma() VS put_anon_vma()
> + * mutex_trylock() atomic_dec_and_test()
> + * smp_mb() smp_mb()
> + * atomic_read() mutex_is_locked()
> */
> - anon_vma_lock(anon_vma);
> - anon_vma_unlock(anon_vma);
> + if (mutex_is_locked(&anon_vma->root->mutex)) {
> + anon_vma_lock(anon_vma);
> + anon_vma_unlock(anon_vma);
> + }
>
> if (anon_vma->root != anon_vma)
> put_anon_vma(anon_vma->root);
>

OK, so the anon_vma slab cache is SLAB_DESTROY_BY_RCU. Presumably
all callers of page_lock_anon_vma() check the identity of the page
that got locked, since it might be recycled at any time. But when
I look at 2.6.37, I only see checks for NULL. So I am assuming
that this code is supposed to prevent such recycling.

I am not sure that I am seeing a consistent snapshot of all of the
relevant code, in particular, I am guessing that the ->lock and ->mutex
are the result of changes rather than there really being both a spinlock
and a mutex in anon_vma. Mainline currently has a lock, FWIW. But from
what I do see, I am concerned about the following sequence of events:

o CPU 0 starts executing page_lock_anon_vma() as shown at
https://lkml.org/lkml/2010/11/26/213, fetches the pointer
to anon_vma->root->lock, but does not yet invoke
mutex_trylock().

o CPU 1 executes __put_anon_vma() above on the same VMA
that CPU 0 is attempting to use. It sees that the
anon_vma->root->mutex (presumably AKA ->lock) is not held,
so it calls anon_vma_free().

o CPU 2 reallocates the anon_vma freed by CPU 1, so that it
now has a non-zero reference count.

o CPU 0 continues execution, incorrectly acquiring a reference
to the now-recycled anon_vma.

Or am I misunderstanding what this code is trying to do?

Thanx, Paul


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