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    Subject[PATCH 1/2] stop_machine: check work->done while handling enqueued works
    From
    In cpu_stopper_thread(), @work->done may be NULL if the cpu stop work
    is queued from stop_one_cpu_nowait(); however, cpu_stopper_thread()
    updates @done->ret without checking whether @done exists or not when
    the work function fails.

    While this can lead to oops, the only current user of
    stop_one_cpu_nowait() - active_load_balance_cpu_stop() - always
    returns 0 and thus there's no in-kernel user which triggers this bug.

    Fix it by checking whether @done exists before updating @done->ret.

    Thanks Tejun for sharing commit message.

    Signed-off-by: Hillf Danton <dhillf@gmail.com>
    Reviewed-by: Namhyung Kim <namhyung@kernel.org>
    ---

    --- a/kernel/stop_machine.c Sun Feb 10 12:51:46 2013
    +++ b/kernel/stop_machine.c Sun Feb 10 12:58:58 2013
    @@ -279,7 +279,7 @@ repeat:
    preempt_disable();

    ret = fn(arg);
    - if (ret)
    + if (ret && done)
    done->ret = ret;

    /* restore preemption and check it's still balanced */
    --

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