Re: [PATCH] sched/fair: Clear ->h_load_next after hierarchical load

[Peng Wang <peng_wang@linux.alibaba.com>]

On Wed, Oct 15, 2025 at 02:44:22PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 15, 2025 at 08:19:50PM +0800, Peng Wang wrote:
> 
> > We found that the task_group corresponding to the problematic se
> > is not in the parent task_group’s children list, indicating that
> > h_load_next points to an invalid address. Consider the following
> > cgroup and task hierarchy:
> > 
> >          A
> >         / \
> >        /   \
> >       B     E
> >      / \    |
> >     /   \   t2
> >    C     D
> >    |     |
> >    t0    t1
> > 
> > Here follows a timing sequence that may be responsible for triggering
> > the problem:
> > 
> > CPU X                   CPU Y                   CPU Z
> > wakeup t0
> > set list A->B->C
> > traverse A->B->C
> > t0 exits
> > destroy C
> >                         wakeup t2
> >                         set list A->E           wakeup t1
> >                                                 set list A->B->D
> >                         traverse A->B->C
> >                         panic
> > 
> > CPU Z sets ->h_load_next list to A->B->D, but due to arm64 weaker memory
> > ordering, Y may observe A->B before it sees B->D, then in this time window,
> > it can traverse A->B->C and reach an invalid se.
> 
> Hmm, I rather think we should ensure update_cfs_rq_h_load() is
> serialized against unregister_fair_sched_group().

I might be mistaken, but it seems that, even with RCU protection around
update_cfs_rq_h_load(), there remains a risk of reading stale values.


 CPU X                   CPU Y                   CPU Z

 wakeup t0
 rcu_read_lock()
 set list A->B->C
 traverse A->B->C
 rcu_read_unlock()
 t0 exits
 destroy C

After the prior RCU grace period has elapsed, C has already been reclaimed,
yet the stale A->B->C remains.


                         wakeup t2
                         rcu_read_lock()
                         set list A->E           wakeup t1
                                                 rcu_read_lock()
                                                 set list A->B->D
                                                      ...
                         traverse A->B->C
                         panic

A subsequent rcu_read_lock() only guarantees that A/B/D/E will not be
reclaimed while the list is being traversed; C had already been freed
before the next grace period even began.

> 
> And I'm thinking that really shouldn't be hard; note how
> sched_unregister_group() already has an RCU grace period. So all we need
> to ensure is that task_h_load() is called in a context that stops RCU
> grace periods (rcu_read_lock(), preempt_disable(), local_irq_disable(),
> local_bh_disable()).
> 
> A very quick scan makes me think at the very least the usage in
> 
>   task_numa_migrate()
>     task_numa_find_cpu()
>       task_h_load()
> 
> fails here; probably more.