Messages in this thread | | | Subject | Re: Possible memory ordering bug in page reclaim? | From | Benjamin Herrenschmidt <> | Date | Sat, 15 Oct 2005 17:43:16 +1000 |
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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 > 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).
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 ?
Ben.
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