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    SubjectRe: [BUG] cpu-hotplug: Can't offline the CPU with naughty realtime processes
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    On Mon, 2007-05-07 at 19:10 +0900, Satoru Takeuchi wrote:
    > Hi,
    >
    > I found a bug on 2.6.21 cpu-hotplug code.
    >
    > When process A on CPU0 try to offline the CPU1 on which the process B,
    > realtime process (its task->policy == SCHED_FIFO or SCHED_RR) running
    > without sleep or yield, both CPU0 and CPU1 get hang. It's because of
    > the following code on __stop_machine_run().
    >
    > struct task_struct *__stop_machine_run(int (*fn)(void *), void *data,
    > unsigned int cpu)
    > {
    > ...
    > p = kthread_create(do_stop, &smdata, "kstopmachine");
    > if (!IS_ERR(p)) {
    > kthread_bind(p, cpu);
    > wake_up_process(p);
    > wait_for_completion(&smdata.done);
    > }
    > ...
    > }
    >
    > kstopmachine is created, bound to the CPU1, and woken up here, but
    > this process can't start to run because reschedule doesn't occur on
    > CPU1. Hence CPU0 also be able to run because it's waiting completion
    > of CPU1's offline work.

    Yes, we should probably move the set_scheduler call in stop_machine
    (where the thread up-prioritizes itself) to before wake_up_process(p),
    to avoid this happening.

    Others have suggested we use the freezer; I've always distrusted that
    code. It's much trickier than stop_machine().

    I look forward to your patch!
    Rusty.

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