mac80211: AMPDU rx reorder timeout timer

This patch introduces a new timer, which will release
queued-up MPDUs from the reorder buffer, whenever
they've waited for more than HT_RX_REORDER_BUF_TIMEOUT
(which is at around 100 ms).

The advantage of having a dedicated timer, instead of
relying on a constant stream of freshly arriving aMPDUs
to release the old ones, is particularly observable when
even a small fraction of MPDUs are forever lost at
low network speeds.

Previously under these circumstances frames would become
stuck in the reorder buffer and the network stack of both
HT peers throttled back, instead of revving up and
gunning the pipes.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 11 insertions, 5 deletions

diff --git a/net/mac80211/sta_info.h b/net/mac80211/sta_info.h
index 54262e72376d..810c5ce98316 100644
--- a/net/mac80211/sta_info.h
+++ b/net/mac80211/sta_info.h
@@ -103,6 +103,7 @@ struct tid_ampdu_tx {
  * @reorder_buf: buffer to reorder incoming aggregated MPDUs
  * @reorder_time: jiffies when skb was added
  * @session_timer: check if peer keeps Tx-ing on the TID (by timeout value)
+ * @reorder_timer: releases expired frames from the reorder buffer.
  * @head_seq_num: head sequence number in reordering buffer.
  * @stored_mpdu_num: number of MPDUs in reordering buffer
  * @ssn: Starting Sequence Number expected to be aggregated.
@@ -110,20 +111,25 @@ struct tid_ampdu_tx {
  * @timeout: reset timer value (in TUs).
  * @dialog_token: dialog token for aggregation session
  * @rcu_head: RCU head used for freeing this struct
+ * @reorder_lock: serializes access to reorder buffer, see below.
  *
  * This structure is protected by RCU and the per-station
  * spinlock. Assignments to the array holding it must hold
- * the spinlock, only the RX path can access it under RCU
- * lock-free. The RX path, since it is single-threaded,
- * can even modify the structure without locking since the
- * only other modifications to it are done when the struct
- * can not yet or no longer be found by the RX path.
+ * the spinlock.
+ *
+ * The @reorder_lock is used to protect the variables and
+ * arrays such as @reorder_buf, @reorder_time, @head_seq_num,
+ * @stored_mpdu_num and @reorder_time from being corrupted by
+ * concurrent access of the RX path and the expired frame
+ * release timer.
  */
 struct tid_ampdu_rx {
         struct rcu_head rcu_head;
+        spinlock_t reorder_lock;
         struct sk_buff **reorder_buf;
         unsigned long *reorder_time;
         struct timer_list session_timer;
+        struct timer_list reorder_timer;
         u16 head_seq_num;
         u16 stored_mpdu_num;
         u16 ssn;