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    SubjectBlock driver freezes when using CFQ
    Hi all,

    I have written a block driver that registers a virtual device and
    routes requests to appropriate real devices after some re-mapping of
    the requests. I am testing the driver by creating a filesystem on the
    virtual device and copying a large number of files on to it. The test
    causes the device to become unresponsive after some time. After some
    debugging, I noticed that this happens only if the I/O scheduler being
    used is CFQ. I have not had any trouble if the scheduler is noop,
    anticipatory or deadline. The problem occurs on all the kernels I have
    tested - 2.6.18-rc2, 2.6.18-rc4, 2.6.19-rc3.

    Below are some details about the driver and what I have observed during

    The request function registered by my driver is a simple loop -

    while ((req = elv_next_request(q))) {

    Add request to an internal queue for further processing
    Wake up thread to start processing the queue
    Update some variables for book-keeping

    Completed requests are handled in a different thread -
    while (work to be done) {
    Dequeue completed requests from internal queue
    Call end_that_request_first() and end_that_request_last()
    Update some variables for book-keeping

    Several times during the test run, the while() loop in the request
    function comes out without dequeuing any request even though the
    elevator queue is not empty. (Confirmed by printing the return value of
    elv_queue_empty(), and the values of q->rq.count[] outside the loop).
    After one such occurrence, the request function is not called at all
    and the device becomes unresponsive.
    I added some code that lets me trigger the request function from userspace.
    If I nudge the driver this way, I/Os continue for a short while and stop

    Since CFQ is the default I/O scheduler in current kernels, it has been
    widely used and tested. So I suspect I am not doing something right in my
    driver. Since the driver works well with the other schedulers, is there
    something CFQ-specific that I should take care of?

    Please Cc me on the responses since I am not subscribed to lkml.

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      Last update: 2006-10-28 08:59    [W:0.021 / U:12.844 seconds]
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