[PATCH] mm/list_lru: Fix possible race in memcg_reparent_list_lru_node()

[Waiman Long <longman@redhat.com>]

Muchun Song found out there could be a race between list_lru_add()
and memcg_reparent_list_lru_node() causing the later function to miss
reparenting of a lru entry as shown below:

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++

Though it is not likely that a list_lru_node that has 0 item suddenly
has a newly added lru entry at the end of its life. The race is still
theoretically possible.

Adding a spin_is_locked() check will likely be enough for x86, but it
is less certain for other arches with a more relaxed memory semantics
like arcm64 and ppc. To avoid race, this patch moves the nr_items check
to within the lock critical section.

Fixes: 405cc51fc104 ("mm/list_lru: optimize memcg_reparent_list_lru_node()")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 mm/list_lru.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index c669d87001a6..8aec8ebd5995 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -394,18 +394,18 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid,
 	int dst_idx = dst_memcg->kmemcg_id;
 	struct list_lru_one *src, *dst;
 
-	/*
-	 * If there is no lru entry in this nlru, we can skip it immediately.
-	 */
-	if (!READ_ONCE(nlru->nr_items))
-		return;
-
 	/*
 	 * Since list_lru_{add,del} may be called under an IRQ-safe lock,
 	 * we have to use IRQ-safe primitives here to avoid deadlock.
 	 */
 	spin_lock_irq(&nlru->lock);
 
+	/*
+	 * If there is no lru entry in this nlru, we can skip it immediately.
+	 */
+	if (!nlru->nr_items)
+		goto out;
+
 	src = list_lru_from_memcg_idx(lru, nid, src_idx);
 	if (!src)
 		goto out;
-- 
2.27.0