[Bug] race condition at rebind_subsystems()

[Tejun Heo]

(resending, I messed up the message header, sorry)

Hello,

On Fri, Jul 15, 2022 at 01:59:38PM +0200, Michal Koutný wrote:
> The css->rstat_css_node should not be modified if there are possible RCU
> readers elsewhere.
> One way to fix this would be to insert synchronize_rcu() after
> list_del_rcu() and before list_add_rcu().
> (A further alternative (I've heard about) would be to utilize 'nulls'
> RCU lists [1] to make the move between lists detectable.)
> 
> But as I'm looking at it from distance, it may be simpler and sufficient
> to just take cgroup_rstat_lock around the list migration (the nesting
> under cgroup_mutex that's held with rebind_subsystems() is fine).

synchronize_rcu() prolly is the better fit here given how that list_node's
usage, but yeah, great find.

Thanks.

-- 
tejun