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    SubjectRe: [RFC][PATCH 04/16] writeback: fix possible race when shutting down bdi
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    On Sun, 2010-07-18 at 02:47 -0400, Christoph Hellwig wrote:
    > On Fri, Jul 16, 2010 at 03:45:00PM +0300, Artem Bityutskiy wrote:
    > > From: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
    > >
    > > Current bdi code has the following race between 'bdi_wb_shutdown()'
    > > and 'bdi_forker_thread()'.
    > >
    > > Initial condition: BDI_pending is cleaned, bdi has no writeback thread,
    > > because it was inactive and exited, 'bdi_wb_shutdown()' and
    > > 'bdi_forker_thread()' are executed concurrently.
    >
    > Wouldn't it be better to have a per-bdi mutex to serialize thread
    > creation and shutdown?

    There are several parties which want to have some serialization with bdi
    trheads creation and shutdown:

    1. 'bdi_queue_work()' - this should not take any mutex and should be
    fast. It uses spinlock and this is should stay this way

    2. I'm going to modify '__mark_inode_dirty()' to wake-up bdi thread -
    this is similar to 'bdi_queue_work()'

    3. 'bdi_wb_shutdown()' - this uses the 'BDI_pending' for serialization
    now, but can use a mutex instead.

    I guess you mean that for 1 and 2 things stay the same, but for 3 we can
    use a mutex. Then the forker thread should also take this mutex. Right?

    If yes, this looks fine for me. I am going to try this approach. Then
    -->

    > And please also kill the bit wait in favour
    > of a proper wait queue - the bit wait interface really is just a hack
    > for structures that are very size sensitive, which the backing device
    > is not.

    --> the bit should go away and so no wait queue will be needed as well.

    --
    Best Regards,
    Artem Bityutskiy (Артём Битюцкий)

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