Re: [PATCH-next v2] mm/memcg: Properly handle memcg_stock access for PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2021-12-10 11:29:31 [-0500], Waiman Long wrote:
> 
> On 12/10/21 08:01, Sebastian Andrzej Siewior wrote:
> > On 2021-12-09 21:52:28 [-0500], Waiman Long wrote:
> > …
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > …
> > > @@ -2210,7 +2211,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> > >   	struct memcg_stock_pcp *stock;
> > >   	unsigned long flags;
> > > -	local_irq_save(flags);
> > > +	local_lock_irqsave(&memcg_stock.lock, flags);
> > Why is this one using the lock? It isn't accessing irq_obj, right?
> Well, the lock isn't just for irq_obj. It protects the whole memcg_stock
> structure which include irq_obj. Sometimes, data in irq_obj (or task_obj)
> will get transfer to nr_pages and vice versa. So it is easier to use one
> single lock for the whole thing.

This needs way better documentation what protects what any why.
I don't like the quick slapping for RT only usage without any kind of
explanation. Once you think you know it is irq_obj only you end up here
where you have the lock again for no obvious reason.

> 
> Cheers,
> Longman

Sebastian