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    SubjectRe: [tip:sched/urgent] sched: Fix rq->nr_uninterruptible update race
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    On Fri, Jan 27, 2012 at 2:25 AM, tip-bot for Peter Zijlstra
    <a.p.zijlstra@chello.nl> wrote:
    > Commit-ID:  4ca9b72b71f10147bd21969c1805f5b2c4ca7b7b
    > Gitweb:     http://git.kernel.org/tip/4ca9b72b71f10147bd21969c1805f5b2c4ca7b7b
    > Author:     Peter Zijlstra <a.p.zijlstra@chello.nl>
    > AuthorDate: Wed, 25 Jan 2012 11:50:51 +0100
    > Committer:  Ingo Molnar <mingo@elte.hu>
    > CommitDate: Thu, 26 Jan 2012 19:38:09 +0100
    >
    > sched: Fix rq->nr_uninterruptible update race
    >
    > KOSAKI Motohiro noticed the following race:
    >
    >  > CPU0                    CPU1
    >  > --------------------------------------------------------
    >  > deactivate_task()
    >  >                         task->state = TASK_UNINTERRUPTIBLE;
    >  > activate_task()
    >  >    rq->nr_uninterruptible--;
    >  >
    >  >                         schedule()
    >  >                           deactivate_task()
    >  >                             rq->nr_uninterruptible++;
    >  >
    >
    > Kosaki-San's scenario is possible when CPU0 runs
    > __sched_setscheduler() against CPU1's current @task.
    >
    > __sched_setscheduler() does a dequeue/enqueue in order to move
    > the task to its new queue (position) to reflect the newly provided
    > scheduling parameters. However it should be completely invariant to
    > nr_uninterruptible accounting, sched_setscheduler() doesn't affect
    > readyness to run, merely policy on when to run.
    >
    > So convert the inappropriate activate/deactivate_task usage to
    > enqueue/dequeue_task, which avoids the nr_uninterruptible accounting.
    >
    Why would we want to avoid nr_uninterruptible accounting?
    nr_uninterruptible has impact on load calculation, we might not get
    the proper load weight if we don't account it. isn't it?

    Thanks,
    Rakib
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