Re: [PATCH] sched: Fix race between task_group and sched_task_group

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 29, 2014 at 06:20:48AM +0300, Kirill Tkhai wrote:
> > And cgroup_task_migrate() can free ->cgroups via call_rcu(). Of course,
> > in practice raw_spin_lock_irq() should also act as rcu_read_lock(), but
> > we should not rely on implementation details.
> 
> Do you mean cgroup_task_migrate()->put_css_set_locked()? It's not
> possible there, because old_cset->refcount is lager than 1. We increment
> it in cgroup_migrate_add_src() and real freeing happens in
> cgroup_migrate_finish(). These functions are around task_migrate(), they
> are pair brackets.
> 
> > task_group = tsk->cgroups[cpu_cgrp_id] can't go away because yes, if we
> > race with migrate then ->attach() was not called. But it seems that in
> > theory it is not safe to dereference tsk->cgroups.
> 
> old_cset can't be freed in cgroup_task_migrate(), so we can safely
> dereference it. If we've got old_cset in
> cgroup_post_fork()->sched_move_task(), the right sched_task_group will
> be installed by attach->sched_move_task().


Would it be fair to summarise your argument thusly:

 "Because sched_move_task() is only called from cgroup_subsys methods
  the cgroup infrastructure itself holds reference on the relevant
  css sets, and therefore their existence is guaranteed."

?

The question then would be how do we guarantee/assert the assumption
that sched_move_task() is indeed only ever called from such a method.