Re: [tip:sched/core] sched: Fix cgroup movement of forking process

[Tejun Heo]

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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