Re: task_group unthrottling and removal race (was Re: [PATCH] sched/fair: Use rq->lock when checking cfs_rq list) presence

[Michal Koutný]

On Wed, Nov 03, 2021 at 11:51:12AM +0100, Mathias Krause <minipli@xxxxxxxxxxxxxx> wrote:
> Adding synchronize_rcu() here will ensure all concurrent RCU "readers"
> will have finished what they're doing, so we can unlink safely. That
> was, apparently, the missing piece.

What reader(s) are you referring to here? The
list_for_each_entry_rcu(cfs_rq, &cfs_b->throttled_cfs_rq,
throttled_list) in distribute_cfs_runtime()?

I'm think (not sure) that wouldn't work since the unthrottle_cfs_rq can
still be called after this synchronize_rcu() but before
free_fair_sched_group().

(But if you considered update_blocked_averages() as the reader(s) and
synchronize_rcu() within free_fair_sched_group(), that may ensure UBA
won't step on a free'd cfs_rq (+unlinking would need to happen only in
free_fair_sched_group() too.))

Michal