mac80211: use RCU for RX aggregation

Currently we allocate some memory for each RX
aggregation session and additionally keep a
flag indicating whether or not it is valid.
By using RCU to protect the pointer and making
sure that the memory is fully set up before it
becomes visible to the RX path, we can remove
the need for the bool that indicates validity,
as well as for locking on the RX path since it
is always synchronised against itself, and we
can guarantee that all other modifications are
done when the structure is not visible to the
RX path.

The net result is that since we remove locking
requirements from the RX path, we can in the
future use any kind of lock for the setup and
teardown code paths.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 15 insertions, 19 deletions

diff --git a/net/mac80211/rx.c b/net/mac80211/rx.c
index 84f1173..ee01dac 100644
--- a/net/mac80211/rx.c
+++ b/net/mac80211/rx.c
@@ -719,16 +719,13 @@ static void ieee80211_rx_reorder_ampdu(struct ieee80211_rx_data *rx,
 
 	tid = *ieee80211_get_qos_ctl(hdr) & IEEE80211_QOS_CTL_TID_MASK;
 
-	spin_lock(&sta->lock);
-
-	if (!sta->ampdu_mlme.tid_active_rx[tid])
-		goto dont_reorder_unlock;
-
-	tid_agg_rx = sta->ampdu_mlme.tid_rx[tid];
+	tid_agg_rx = rcu_dereference(sta->ampdu_mlme.tid_rx[tid]);
+	if (!tid_agg_rx)
+		goto dont_reorder;
 
 	/* qos null data frames are excluded */
 	if (unlikely(hdr->frame_control & cpu_to_le16(IEEE80211_STYPE_NULLFUNC)))
-		goto dont_reorder_unlock;
+		goto dont_reorder;
 
 	/* new, potentially un-ordered, ampdu frame - process it */
 
@@ -740,20 +737,22 @@ static void ieee80211_rx_reorder_ampdu(struct ieee80211_rx_data *rx,
 	/* if this mpdu is fragmented - terminate rx aggregation session */
 	sc = le16_to_cpu(hdr->seq_ctrl);
 	if (sc & IEEE80211_SCTL_FRAG) {
-		spin_unlock(&sta->lock);
 		skb->pkt_type = IEEE80211_SDATA_QUEUE_TYPE_FRAME;
 		skb_queue_tail(&rx->sdata->skb_queue, skb);
 		ieee80211_queue_work(&local->hw, &rx->sdata->work);
 		return;
 	}
 
-	if (ieee80211_sta_manage_reorder_buf(hw, tid_agg_rx, skb, frames)) {
-		spin_unlock(&sta->lock);
+	/*
+	 * No locking needed -- we will only ever process one
+	 * RX packet at a time, and thus own tid_agg_rx. All
+	 * other code manipulating it needs to (and does) make
+	 * sure that we cannot get to it any more before doing
+	 * anything with it.
+	 */
+	if (ieee80211_sta_manage_reorder_buf(hw, tid_agg_rx, skb, frames))
 		return;
-	}
 
- dont_reorder_unlock:
-	spin_unlock(&sta->lock);
  dont_reorder:
 	__skb_queue_tail(frames, skb);
 }
@@ -1830,13 +1829,11 @@ ieee80211_rx_h_ctrl(struct ieee80211_rx_data *rx, struct sk_buff_head *frames)
 				  &bar_data, sizeof(bar_data)))
 			return RX_DROP_MONITOR;
 
-		spin_lock(&rx->sta->lock);
 		tid = le16_to_cpu(bar_data.control) >> 12;
-		if (!rx->sta->ampdu_mlme.tid_active_rx[tid]) {
-			spin_unlock(&rx->sta->lock);
+
+		tid_agg_rx = rcu_dereference(rx->sta->ampdu_mlme.tid_rx[tid]);
+		if (!tid_agg_rx)
 			return RX_DROP_MONITOR;
-		}
-		tid_agg_rx = rx->sta->ampdu_mlme.tid_rx[tid];
 
 		start_seq_num = le16_to_cpu(bar_data.start_seq_num) >> 4;
 
@@ -1849,7 +1846,6 @@ ieee80211_rx_h_ctrl(struct ieee80211_rx_data *rx, struct sk_buff_head *frames)
 		ieee80211_release_reorder_frames(hw, tid_agg_rx, start_seq_num,
 						 frames);
 		kfree_skb(skb);
-		spin_unlock(&rx->sta->lock);
 		return RX_QUEUED;
 	}