On Wed, Mar 9, 2022 at 10:40 PM Waiman Long <longman@xxxxxxxxxx> wrote:
Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node()Hi Waiman,
to be race free"), we are tracking the total number of lru
entries in a list_lru_node in its nr_items field. In the case of
memcg_reparent_list_lru_node(), there is nothing to be done if nr_items
is 0. We don't even need to take the nlru->lock as no new lru entry
could be added by a racing list_lru_add() to the draining src_idx memcg
at this point.
Sorry for the late reply. Quick question: what if there is an inflight
list_lru_add()? How about the following race?
CPU0: CPU1:
list_lru_add()
spin_lock(&nlru->lock)
l = list_lru_from_kmem(memcg)
memcg_reparent_objcgs(memcg)
memcg_reparent_list_lrus(memcg)
memcg_reparent_list_lru()
memcg_reparent_list_lru_node()
if (!READ_ONCE(nlru->nr_items))
// Miss reparenting
return
// Assume 0->1
l->nr_items++
// Assume 0->1
nlru->nr_items++
IIUC, we use nlru->lock to serialise this scenario.