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    SubjectRe: [tip:sched/core] sched: Fix cgroup movement of forking process
    Hello, guys.

    On Wed, Dec 21, 2011 at 03:44:14AM -0800, tip-bot for Daisuke Nishimura wrote:
    > sched: Fix cgroup movement of forking process
    >
    > There is a small race between task_fork_fair() and sched_move_task(),
    > which is trying to move the parent.
    >
    > task_fork_fair() sched_move_task()
    > --------------------------------+---------------------------------
    > cfs_rq = task_cfs_rq(current)
    > -> cfs_rq is the "old" one.
    > curr = cfs_rq->curr
    > -> curr is set to the parent.
    > task_rq_lock()
    > dequeue_task()
    > ->parent.se.vruntime -= (old)cfs_rq->min_vruntime
    > enqueue_task()
    > ->parent.se.vruntime += (new)cfs_rq->min_vruntime
    > task_rq_unlock()
    > raw_spin_lock_irqsave(rq->lock)
    > se->vruntime = curr->vruntime
    > -> vruntime of the child is set to that of the parent
    > which has already been updated by sched_move_task().
    > se->vruntime -= (old)cfs_rq->min_vruntime.
    > raw_spin_unlock_irqrestore(rq->lock)
    >
    > As a result, vruntime of the child becomes far bigger than expected,
    > if (new)cfs_rq->min_vruntime >> (old)cfs_rq->min_vruntime.
    >
    > This patch fixes this problem by setting "cfs_rq" and "curr" after
    > holding the rq->lock.

    The race shouldn't happen with threadgroup locking scheduled to be
    merged for the coming merge window. sched_fork() and cgroup migration
    become exclusive and won't happen concurrently. Would still make
    sense for -stable tho.

    Thanks.

    --
    tejun


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