> Peter Zijlstra <peterz@infradead.org> writes:
>
> > On Wed, 2008-06-18 at 14:09 -0400, Jeff Moyer wrote:
> >> Hi, Andrew,
> >>
> >> This is a follow-up to:
> >>
> >> commit bdb76ef5a4bc8676a81034a443f1eda450b4babb
> >> Author: Zach Brown <zach.brown@oracle.com>
> >> Date: Tue Oct 30 11:45:46 2007 -0700
> >>
> >> dio: fix cache invalidation after sync writes
> >>
> >> Commit commit 65b8291c4000e5f38fc94fb2ca0cb7e8683c8a1b ("dio: invalidate
> >> clean pages before dio write") introduced a bug which stopped dio from
> >> ever invalidating the page cache after writes. It still invalidated it
> >> before writes so most users were fine.
> >>
> >> Karl Schendel reported (
http://lkml.org/lkml/2007/10/26/481 ) hitting
> >> this bug when he had a buffered reader immediately reading file data
> >> after an O_DIRECT [writer] had written the data. The kernel issued
> >> read-ahead beyond the position of the reader which overlapped with the
> >> O_DIRECT writer. The failure to invalidate after writes caused the
> >> reader to see stale data from the read-ahead.
> >>
> >> The following patch is originally from Karl. The following commentary
> >> is his:
> >>
> >> The below 3rd try takes on your suggestion of just invalidating
> >> no matter what the retval from the direct_IO call. I ran it
> >> thru the test-case several times and it has worked every time.
> >> The post-invalidate is probably still too early for async-directio,
> >> but I don't have a testcase for that; just sync. And, this
> >> won't be any worse in the async case.
> >>
> >> I added a test to the aio-dio-regress repository which mimics Karl's IO
> >> pattern. It verifed the bad behaviour and that the patch fixed it. I
> >> agree with Karl, this still doesn't help the case where a buffered
> >> reader follows an AIO O_DIRECT writer. That will require a bit more
> >> work.
> >>
> >> This gives up on the idea of returning EIO to indicate to userspace that
> >> stale data remains if the invalidation failed.
> >>
> >> Note the second-to-last paragraph, where it mentions that this does not fix
> >> the AIO case. I updated the regression test to also perform asynchronous
> >> I/O and verified that the problem does exist.
> >>
> >> To fix the problem, we need to invalidate the pages that were under write
> >> I/O after the I/O completes. Because the I/O completion handler can be called
> >> in interrupt context (and invalidate_inode_pages2 cannot be called in interrupt
> >> context), this patch opts to defer the completion to a workqueue. That
> >> workqueue is responsible for invalidating the page cache pages and completing
> >> the I/O.
> >>
> >> I verified that the test case passes with the following patch applied.
> >
> > I'm utterly ignorant of all thing [AD]IO, but doesn't deferring the
> > invalidate open up/widen a race window?
>
> We weren't doing the invalidate at all before this patch. This patch
> introduces the invalidation, but we can't do it in interrupt context.
