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.

Signed-off-by: Daisuke Nishimura <[email protected]>
Acked-by: Paul Turner <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Tejun Heo <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions