Re: Possible memory ordering bug in page reclaim?

[Nick Piggin <nickpiggin@yahoo.com.au>]

Benjamin Herrenschmidt wrote:
> On Sat, 2005-10-15 at 07:17 +0100, Hugh Dickins wrote:
>
> > On Sat, 15 Oct 2005, Nick Piggin wrote:

> > > 1                                2
> > > find_get_page();
> > > write to page                    write_lock(tree_lock);
> > > SetPageDirty();                  if (page_count != 2
> > > put_page();                          || PageDirty())
> > >
> > > Now I'm worried that 2 might see PageDirty *before* SetPageDirty in
> >
> >                                  page->flags
> >
> > > 1, and page_count *after* put_page in 1.
> >
> > I think you're right.  But I'm the last person to ask
> > barrier/ordering questions of.  CC'ed Ben and Andrea.
>
>
> yup, now the question is wether PG_Dirty will be visible to CPU 2 before
> the page count is decremented right ? That depends on put_page, I
> suppose. If it's doing a simple atomic, there is an issue. But atomics
> with return has been so often abused as locks that they may have been
> implemented with a barrier... (On ppc64, it will do an eieio, thus I
> think it should be ok).

Well yes, that's on the store side (1, above). However can't a CPU
still speculatively (eg. guess the branch) load the page->flags
cacheline which might be satisfied from memory before the page->count
cacheline loads? Ie. you can still have the correct write ordering
but have incorrect read ordering?

Because neither PageDirty nor page_count is a barrier, and there is
no read barrier between them.

Nick

--
SUSE Labs, Novell Inc.

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