[PATCH wireless 1/2] wifi: mt76: mt7921: fix lock inversion between dev->mutex and iflist_mtx

[Devin Wittmayer]

mt7921_config() holds dev->mutex across
ieee80211_iterate_active_interfaces(), which takes local->iflist_mtx.
mac80211 uses the opposite order: ieee80211_del_virtual_monitor() holds
iflist_mtx while calling drv_unassign_vif_chanctx(), and
mt792x_unassign_vif_chanctx() takes dev->mutex.

Deleting a monitor interface that has a channel assigned reaches the
second order and lockdep reports the cycle. The driver side of it:

  -> #1 (&local->iflist_mtx){+.+.}-{4:4}:
         __mutex_lock+0xda/0x1070
         ieee80211_iterate_interfaces+0x34/0x70 [mac80211]
         mt7921_config+0xb8/0xe0 [mt7921_common]
         drv_config+0x63/0x1d0 [mac80211]

.config is only reached through drv_config(), which asserts the wiphy
mutex, so ieee80211_iterate_active_interfaces_mtx() can be used. It walks
the same list under the wiphy mutex and never takes iflist_mtx, which
drops the driver's dev->mutex -> iflist_mtx edge.

Fixes: cbaa0a404f8d ("mt76: mt7921: fix up the monitor mode")
Link: https://lore.kernel.org/linux-wireless/20260730053916.109426-1-lucid_duck@justthetip.ca/
Signed-off-by: Devin Wittmayer <lucid_duck at justthetip.ca>
---
 drivers/net/wireless/mediatek/mt76/mt7921/main.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/wireless/mediatek/mt76/mt7921/main.c b/drivers/net/wireless/mediatek/mt76/mt7921/main.c
index 3480205d5fb9..a3d06992cc17 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7921/main.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7921/main.c
@@ -652,9 +652,9 @@ static int mt7921_config(struct ieee80211_hw *hw, int radio_idx, u32 changed)
 	}
 
 	if (changed & IEEE80211_CONF_CHANGE_MONITOR) {
-		ieee80211_iterate_active_interfaces(hw,
-						    IEEE80211_IFACE_ITER_RESUME_ALL,
-						    mt7921_sniffer_interface_iter, dev);
+		ieee80211_iterate_active_interfaces_mtx(hw,
+							IEEE80211_IFACE_ITER_RESUME_ALL,
+							mt7921_sniffer_interface_iter, dev);
 	}
 
 out:
-- 
2.55.0