[PATCH] wifi: mac80211: defer link RX stats percpu free to RCU

[Maoyi Xie <maoyixie.tju@gmail.com>]

sta_remove_link() frees a removed MLO link's RX stats percpu buffer right
away, but defers only the link container to RCU:

	sta_info_free_link(&alloc->info);
	kfree_rcu(alloc, rcu_head);

The RX fast path reads link_sta under rcu_read_lock and writes the percpu
stats. A reader that resolved link_sta before the removal keeps the
pointer. The container stays alive from the kfree_rcu, so the read still
works. But the percpu block it points to is already freed. This needs
uses_rss. That is when pcpu_rx_stats exists.

The full STA teardown frees the deflink stats only after
synchronize_net(). The link removal path had no such barrier. The race is
hard to win in practice, but the free should still wait for RCU.

Free the link together with its data from a single RCU callback, so the
percpu block is reclaimed only after readers drain.

Fixes: c71420db653a ("wifi: mac80211: RCU-ify link STA pointers")
Link: https://lore.kernel.org/r/20260626080158.3589711-1-maoyixie.tju@gmail.com
Suggested-by: Johannes Berg <johannes@sipsolutions.net>
Co-developed-by: Kaixuan Li <kaixuan.li@ntu.edu.sg>
Signed-off-by: Kaixuan Li <kaixuan.li@ntu.edu.sg>
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
---
The earlier inquiry pointed at cb71f1d136a6, which added sta_remove_link().
The regression is c71420db653a though. It moved the link pointers to RCU
and deferred the container free. The percpu free stayed synchronous.

 net/mac80211/sta_info.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
index 4c31ef8817ce0..6d58571a364d1 100644
--- a/net/mac80211/sta_info.c
+++ b/net/mac80211/sta_info.c
@@ -355,6 +355,15 @@ static void sta_info_free_link(struct link_sta_info *link_sta)
 	free_percpu(link_sta->pcpu_rx_stats);
 }

+static void sta_link_free_rcu(struct rcu_head *head)
+{
+	struct sta_link_alloc *alloc =
+		container_of(head, struct sta_link_alloc, rcu_head);
+
+	sta_info_free_link(&alloc->info);
+	kfree(alloc);
+}
+
 static void sta_accumulate_removed_link_stats(struct sta_info *sta, int link_id)
 {
 	struct link_sta_info *link_sta = wiphy_dereference(sta->local->hw.wiphy,
@@ -439,10 +448,8 @@ static void sta_remove_link(struct sta_info *sta, unsigned int link_id,

 	RCU_INIT_POINTER(sta->link[link_id], NULL);
 	RCU_INIT_POINTER(sta->sta.link[link_id], NULL);
-	if (alloc) {
-		sta_info_free_link(&alloc->info);
-		kfree_rcu(alloc, rcu_head);
-	}
+	if (alloc)
+		call_rcu(&alloc->rcu_head, sta_link_free_rcu);

 	ieee80211_sta_recalc_aggregates(&sta->sta);
 }