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

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On 3/27/22 20:57, Waiman Long wrote:
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()")

Sorry, I should have added

Reported-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
  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;
Cheers,
Longman





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