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

On 2021-12-10 11:29:31 [-0500], Waiman Long wrote:

On 12/10/21 08:01, Sebastian Andrzej Siewior wrote:

quoted

On 2021-12-09 21:52:28 [-0500], Waiman Long wrote:
…

quoted

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

…

quoted

@@ -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