Re: Possible memory ordering bug in page reclaim?
From: Benjamin Herrenschmidt
Date: Sat Oct 15 2005 - 02:46:05 EST
On Sat, 2005-10-15 at 07:17 +0100, Hugh Dickins wrote:
> On Sat, 15 Oct 2005, Nick Piggin wrote:
> > Is there anything that prevents PageDirty from theoretically being
> > speculatively loaded before page_count here? (see patch)
> > It would result in pagecache corruption in the following situation:
> > 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
> > 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).
There is also a problem the other way around. Write to page, then set
page dirty... those writes may be visible to CPU 2 (that is the page
content be dirty) before find_get_page even increased the page count,
unless there is a barrier in there too.
Paul, Anton ?
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