Re: [tip:sched/core] sched: Fix cgroup movement of forking process
From: Tejun Heo
Date: Wed Dec 21 2011 - 12:37:40 EST
(cc'ing Frederic)
On Wed, Dec 21, 2011 at 09:26:32AM -0800, Tejun Heo wrote:
> 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.
I retract that. sched_move_task() can also be called from
cgroup_exit() which is outside of threadgroup locking.
Frederic, so, it seems we actually have race conditions here. I
really wish cgroup made sure that things like this can't happen even
if we pay a bit of overhead in relatively cold paths. I could be
being unrealistic tho. Any ideas?
Thanks.
--
tejun
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