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bes2600: Patch D — atomicize ba_lock counters, drop the spinlock

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The block-ack policy uses 4 int counters (ba_acc, ba_cnt, ba_acc_rx,
ba_cnt_rx) bumped per data frame in the TX and RX hot paths under
spin_lock_bh(&hw_priv->ba_lock).  The lock was the heaviest per-frame
synchronization cost remaining after Patch C v3 (which fixed the
sdio_rx_work relay).  Per the Opus structural critique (PR #8), this
pattern matches mac80211 driver convention for per-frame statistics:
atomic_t suffices, no lock needed.

Field-by-field changes in struct bes2600_common:
  ba_acc, ba_cnt, ba_acc_rx, ba_cnt_rx: int -> atomic_t
  ba_armed:                              new atomic_t (timer-arm flag)
  ba_ena:                                bool -> atomic_t
  ba_lock:                               removed (spinlock_t deleted)
  ba_hist:                               int (single-writer = ba_timer)

Producer hot path (txrx.c TX submit + RX receive):
  - atomic_add for the byte accumulator
  - atomic_inc for the frame counter
  - atomic_cmpxchg(&ba_armed, 0, 1) to claim the once-per-window
    mod_timer arm — at most ONE producer succeeds; race-free
  - no spin_lock_bh

Consumer paths (sta.c bes2600_ba_timer, sta.c disconnect-reset, sta.c
bes2600_ba_work, debug.c debugfs reader):
  - atomic_read snapshots all 4 counters into locals; the threshold
    predicate (acc/cnt >= THLD) tolerates approximate snapshots — the
    timer fires periodically, a single misclassification just delays
    the policy update by one tick
  - atomic_set zeroes the counters at end of timer-callback window;
    racing producer increments after the snapshot are lost (acceptable
    for stats; same approximation the original lock allowed under
    contention)
  - atomic_set(&ba_armed, 0) re-enables the next window's arm

Followup-amenable simplification: ba_hist remains int because only
the single ba_timer callback writes it; multiple writers would need
to upgrade it too.

This patch follows the cw1200-mainline-idiom established by Patch C v3
(structural fix, not bandaid).  The cw1200 reference doesn't have a
similar lock to compare; bes2600 inherited this from a later
Bestechnic addition rather than the upstream tree.
This commit is contained in:
Markus Fritsche
2026-05-08 00:17:46 +02:00
parent 979d5436ee
commit 44b296647b
5 changed files with 85 additions and 56 deletions
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bes2600/main.c
+1 -1
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@@ -490,7 +490,7 @@ static struct ieee80211_hw *bes2600_init_common(size_t hw_priv_data_len)
INIT_LIST_HEAD(&hw_priv->event_queue);
INIT_WORK(&hw_priv->event_handler, bes2600_event_handler);
INIT_WORK(&hw_priv->ba_work, bes2600_ba_work);
spin_lock_init(&hw_priv->ba_lock);
/* Patch D: ba_lock removed; ba_acc/ba_cnt/etc are atomic_t. */
timer_setup(&hw_priv->ba_timer, bes2600_ba_timer, 0);
if (unlikely(bes2600_queue_stats_init(&hw_priv->tx_queue_stats,