On Wed, Nov 09, 2016 at 05:59:33PM +0100, Oleg Nesterov wrote: > We need to ensure that autogroup/tg returned by autogroup_task_group() > can't go away if we race with autogroup_move_group(), and unless the > caller holds ->siglock we rely on fact that autogroup_move_group() > will a) see this task and b) do sched_move_task() which needs the same > same rq->lock. > > However. autogroup_move_group() skips for_each_thread/sched_move_task > if sysctl_sched_autogroup_enabled == 0. > > So. Doesn't this mean that cgroup migration to the root cgroup can race > with autogroup_move_group() and use the soon-to-be-freed autogroup->tg?
Argh, its too late for this, also jet-lag. But maybe, I can sort of feel a hole here but cannot for the life of me still think. > although this is a bit off-topic. Another question is that I fail to > understand why sched_autogroup_create_attach() does autogroup_create() > and changes signal->autogroup even if !sysctl_sched_autogroup_enabled. I really cannot remember back that far, but it could be to allow flipping it back on. Then again, I don't think the fork path puts new tasks in, even if the autogroup exists.
