Messages in this thread | | | Date | Thu, 10 Nov 2016 14:09:13 +0100 | From | Oleg Nesterov <> | Subject | Re: sched/autogroup: race if !sysctl_sched_autogroup_enabled ? |
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On 11/09, Peter Zijlstra wrote: > > 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.
And the 3rd case which I didn't think about yesterday. And now I really hope it can explain the vmcore we have.
If sysctl_sched_autogroup_enabled was enabled and then disabled, it is possible that the "autogrouped" process runs with ag->kref.refcount == 1, and if it does setsid() it frees its active task_group.
> > 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.
Yes, I thought about this too, but I think it is hardly possible to explain what do we actually want when sysctl_sched_autogroup_enabled changes from 0 to 1.
So I am going to send the patch which simply moves the sysctl check from autogroup_move_group() to sched_autogroup_create_attach(), but perhaps I should split this change?
I mean, the first patch for -stable could just remove the current check, the 2nd one will add it into sched_autogroup_create_attach().
Oleg.
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