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    SubjectRe: [RFC PATCH] cifs: Fix possible deadlock with cifs and work queues
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    2014-03-21 6:23 GMT+04:00 Steven Rostedt <rostedt@goodmis.org>:
    > On Thu, 20 Mar 2014 17:02:39 -0400
    > Jeff Layton <jlayton@redhat.com> wrote:
    >
    >> Eventually the server should just allow the read to complete even if
    >> the client doesn't respond to the oplock break. It has to since clients
    >> can suddenly drop off the net while holding an oplock. That should
    >> allow everything to unwedge eventually (though it may take a while).
    >>
    >> If that's not happening then I'd be curious as to why...
    >
    > The problem is that the data is being filled in the page and the reader
    > is waiting for the page lock to be released. The kworker for the reader
    > will issue the complete() and unlock the page to wake up the reader.
    >
    > But because the other workqueue callback calls down_read(), and there
    > can be a down_write() waiting for the reader to finish, this
    > down_read() will block on the lock as well (rwsems are fair locks).
    > This blocks the other workqueue callback from issuing the complete and
    > page_unlock() that will wake up the reader that is holding the rwsem
    > with down_read().
    >
    > DEADLOCK.

    Thank you for reporting and clarifying the issue!

    Read and write codepaths both obtain lock_sem for read and then wait
    for cifsiod_wq to complete and release lock_sem. They don't do any
    lock_sem operations inside their work task queued to cifsiod_wq. But
    oplock code can obtain/release lock_sem in its work task. So, that's
    why I agree with Jeff and suggest to move the oplock code to a
    different work queue (cifsioopd_wq?) but leave read and write
    codepaths use cifsiod_wq.

    --
    Best regards,
    Pavel Shilovsky.


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