Re: [PATCH-next v3] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT

[Waiman Long]

On 12/14/21 23:36, Davidlohr Bueso wrote:
> On Tue, 14 Dec 2021, Waiman Long wrote:
>
> > @@ -2189,7 +2194,7 @@ static void drain_local_stock(struct 
> > work_struct *dummy)
> >      * drain_stock races is that we always operate on local CPU stock
> >      * here with IRQ disabled
> >      */
> > -    local_irq_save(flags);
> > +    local_lock_irqsave(&memcg_stock.lock, flags);
> >
> >     stock = this_cpu_ptr(&memcg_stock);
> >     drain_obj_stock(&stock->irq_obj);
>
> So here there is still the problem that you can end up taking sleeping 
> locks
> with irqs disabled via obj_cgroup_put() >> obj_cgroup_release() - ie: the
> percpu_ref_switch_lock and css_set_lock. It had occurred to me to promote
> the former to a raw spinlock, but doubt we can get away with the latter.

Yes, it is tricky to avoid all these. One possibility to to delay 
obj_cgroup_release() through RCU. However, this is outside the scope of 
this patch.

Cheers,
Longman