[PATCH] mm/memcontrol: publish null pointer before dropping css reference

[Joshua Hahn <joshua.hahnjy@gmail.com>]

struct memcg_stock_pcp hold references to cgroups that it tracks to make
sure that the memcg never gets freed with unaccounted charges still
living in the stock. The cached stock can be cleared either via
drain_stock() or consume_stock(), which drop the reference if the
stock goes to 0, and then clears the pointer from the stock.

If that was the last reference, the memcg is freed after an RCU grace
period, but this can happen while the pointer still points to the
to-be-freed memcg. Then a reader that enters its read-side critical
section after the grace period has started is not waited for, and then
can dereference it after the memcg gets freed.

In practice, this is very rare and I have not been able to reproduce
the issue in my own testing.

Switch the order of the reference drop and the publishing so that
we first publish the NULL pointer, then drop the reference.

This is based on a report from Sashiko, who found this pre-existing
issue in my page_counter_stock series [1].

[1] https://sashiko.dev/#/patchset/20260928192349.3432886-1-joshua.hahnjy%40gmail.com

Fixes: e1a366be5cb4 ("mm: memcontrol: switch to rcu protection in drain_all_stock()")
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
---
 mm/memcontrol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e7d7db5718f9a..a05d9a8baa1b6 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2172,8 +2172,8 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 			stock_pages -= nr_pages;
 			WRITE_ONCE(stock->nr_pages[i], stock_pages);
 			if (!stock_pages) {
-				css_put(&memcg->css);
 				WRITE_ONCE(stock->cached[i], NULL);
+				css_put(&memcg->css);
 			}
 			ret = true;
 		}
@@ -2209,8 +2209,8 @@ static void drain_stock(struct memcg_stock_pcp *stock, int i)
 		WRITE_ONCE(stock->nr_pages[i], 0);
 	}
 
-	css_put(&old->css);
 	WRITE_ONCE(stock->cached[i], NULL);
+	css_put(&old->css);
 }
 
 static void drain_stock_fully(struct memcg_stock_pcp *stock)
-- 
2.53.0-Meta