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Date: Mon, 28 Mar 2022 12:12:59 -0700 From: Roman Gushchin <roman.gushchin@...ux.dev> To: Muchun Song <songmuchun@...edance.com> Cc: Waiman Long <longman@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Linux Memory Management List <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node() On Wed, Mar 23, 2022 at 09:06:03AM +0800, Muchun Song wrote: > On Wed, Mar 9, 2022 at 10:40 PM Waiman Long <longman@...hat.com> wrote: > > > > Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node() > > 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. > > Hi Waiman, > > 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. Thank you for bringing this up, really cool race!
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