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

Waiman Long
2021-12-15T16:58:45Z

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