mac80211: rework locking for txq scheduling / airtime fairness

Holding the lock around the entire duration of tx scheduling can create
some nasty lock contention, especially when processing airtime information
from the tx status or the rx path.
Improve locking by only holding the active_txq_lock for lookups / scheduling
list modifications.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -3649,16 +3649,17 @@ EXPORT_SYMBOL(ieee80211_tx_dequeue);
 struct ieee80211_txq *ieee80211_next_txq(struct ieee80211_hw *hw, u8 ac)
 {
 	struct ieee80211_local *local = hw_to_local(hw);
+	struct ieee80211_txq *ret = NULL;
 	struct txq_info *txqi = NULL;
 
-	lockdep_assert_held(&local->active_txq_lock[ac]);
+	spin_lock_bh(&local->active_txq_lock[ac]);
 
  begin:
 	txqi = list_first_entry_or_null(&local->active_txqs[ac],
 					struct txq_info,
 					schedule_order);
 	if (!txqi)
-		return NULL;
+		goto out;
 
 	if (txqi->txq.sta) {
 		struct sta_info *sta = container_of(txqi->txq.sta,
@@ -3675,21 +3676,25 @@ struct ieee80211_txq *ieee80211_next_txq(struct ieee80211_hw *hw, u8 ac)
 
 
 	if (txqi->schedule_round == local->schedule_round[ac])
-		return NULL;
+		goto out;
 
 	list_del_init(&txqi->schedule_order);
 	txqi->schedule_round = local->schedule_round[ac];
-	return &txqi->txq;
+	ret = &txqi->txq;
+
+out:
+	spin_unlock_bh(&local->active_txq_lock[ac]);
+	return ret;
 }
 EXPORT_SYMBOL(ieee80211_next_txq);
 
-void ieee80211_return_txq(struct ieee80211_hw *hw,
-			  struct ieee80211_txq *txq)
+void ieee80211_schedule_txq(struct ieee80211_hw *hw,
+			    struct ieee80211_txq *txq)
 {
 	struct ieee80211_local *local = hw_to_local(hw);
 	struct txq_info *txqi = to_txq_info(txq);
 
-	lockdep_assert_held(&local->active_txq_lock[txq->ac]);
+	spin_lock_bh(&local->active_txq_lock[txq->ac]);
 
 	if (list_empty(&txqi->schedule_order) &&
 	    (!skb_queue_empty(&txqi->frags) || txqi->tin.backlog_packets)) {
@@ -3709,17 +3714,7 @@ void ieee80211_return_txq(struct ieee80211_hw *hw,
 			list_add_tail(&txqi->schedule_order,
 				      &local->active_txqs[txq->ac]);
 	}
-}
-EXPORT_SYMBOL(ieee80211_return_txq);
 
-void ieee80211_schedule_txq(struct ieee80211_hw *hw,
-			    struct ieee80211_txq *txq)
-	__acquires(txq_lock) __releases(txq_lock)
-{
-	struct ieee80211_local *local = hw_to_local(hw);
-
-	spin_lock_bh(&local->active_txq_lock[txq->ac]);
-	ieee80211_return_txq(hw, txq);
 	spin_unlock_bh(&local->active_txq_lock[txq->ac]);
 }
 EXPORT_SYMBOL(ieee80211_schedule_txq);
@@ -3732,7 +3727,7 @@ bool ieee80211_txq_may_transmit(struct ieee80211_hw *hw,
 	struct sta_info *sta;
 	u8 ac = txq->ac;
 
-	lockdep_assert_held(&local->active_txq_lock[ac]);
+	spin_lock_bh(&local->active_txq_lock[ac]);
 
 	if (!txqi->txq.sta)
 		goto out;
@@ -3762,34 +3757,27 @@ bool ieee80211_txq_may_transmit(struct ieee80211_hw *hw,
 
 	sta->airtime[ac].deficit += sta->airtime_weight;
 	list_move_tail(&txqi->schedule_order, &local->active_txqs[ac]);
+	spin_unlock_bh(&local->active_txq_lock[ac]);
 
 	return false;
 out:
 	if (!list_empty(&txqi->schedule_order))
 		list_del_init(&txqi->schedule_order);
+	spin_unlock_bh(&local->active_txq_lock[ac]);
 
 	return true;
 }
 EXPORT_SYMBOL(ieee80211_txq_may_transmit);
 
 void ieee80211_txq_schedule_start(struct ieee80211_hw *hw, u8 ac)
-	__acquires(txq_lock)
 {
 	struct ieee80211_local *local = hw_to_local(hw);
 
 	spin_lock_bh(&local->active_txq_lock[ac]);
 	local->schedule_round[ac]++;
-}
-EXPORT_SYMBOL(ieee80211_txq_schedule_start);
-
-void ieee80211_txq_schedule_end(struct ieee80211_hw *hw, u8 ac)
-	__releases(txq_lock)
-{
-	struct ieee80211_local *local = hw_to_local(hw);
-
 	spin_unlock_bh(&local->active_txq_lock[ac]);
 }
-EXPORT_SYMBOL(ieee80211_txq_schedule_end);
+EXPORT_SYMBOL(ieee80211_txq_schedule_start);
 
 void __ieee80211_subif_start_xmit(struct sk_buff *skb,
 				  struct net_device *dev,