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    SubjectRe: SCSI access creating lost time
    On Sun, 7 Mar 1999, Doug Ledford wrote:

    >The races include do_scsi_cmnd() vs. scsi_done() vs. scsi_times_out().

    The harm is not the locking overkill, but instead the fact that during the
    locked-atomic region we have the cpu __cli()ed.

    And the _only_ reason we need the cpu clied in order to do the locking
    safe, is that all places are currently looping infinitely on their lock
    (in this case a -spin-lock) and not always-taking a fail-safe path.

    I think instead that all places that could run in a irq handler should do:

    struct semaphore scsi_sem = MUTEX;

    scsi_done()
    {
    if (down_trylock(&scsi_sem))
    {
    queue_task();
    return;
    }
    /* lock held */
    up(&scsi_sem);
    }

    and places that can be recalled only from normal kernel context (I
    think none in the scsi code though ;) should do:

    down()
    /* Critical section */
    up();

    That way we would _never_ need the cpu clied.

    Comments?

    Andrea Arcangeli


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