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    SubjectRe: preempt-related hangs
    On Sun, 2002-03-24 at 21:11, Andrew Morton wrote:

    > OK, this patch fixed it. I don't know why.

    Eh, odd. That effectively disables kernel preemption around
    set_cpus_allowed, but preemption is already disabled by the task_rq_lock
    call. Note, however, preemption is enabled by the task_rq_unlock and
    thus wake_up_process is called with preemption enabled.

    With your patch, preemption is now disabled across the call, and
    subsequently the task_rq_unlock in try_to_wake_up will never call
    preempt_schedule and your lock does not happen.

    The actual problem may be elsewhere, and this just hides it. This is
    pretty clear, since we would get a similar effect just wrapping
    wake_up_process in preempt_disable. But, oh, try_to_wake_up disables
    preempt, too ... hrm.

    Hm, what if try_to_wake_up wakes up a process and then preemptively
    schedules into it and it wants to acquire the req.sem semaphore, but
    cannot, as it is still taken by set_cpus_allowed? The semaphore seems
    to just be used in the migration code.

    So we have init spinning on softirq threads to come up and then we have
    a deadlock on req.sem from set_cpus_allowed and into the migration

    Bleh ... Ingo?

    Robert Love

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