bes2600: take pending_record_lock with _bh() to fix SOFTIRQ-safe → -unsafe inversion (besser#18)

PROVE_LOCKING reports:

  WARNING: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected
  kworker/u16:1 is trying to acquire:
    &hw_priv->tx_loop.pending_record_lock at bes2600_queue_clear+0x80
  and this task is already holding:
    &queue->lock at bes2600_queue_clear+0x60

  which would create a new lock dependency:
    (&queue->lock){+.-.}   -> (&hw_priv->tx_loop.pending_record_lock){+.+.}

  but this new dependency connects a SOFTIRQ-irq-safe lock:
    (&queue->lock){+.-.}
  ... which became SOFTIRQ-irq-safe at:
    bes2600_tx -> ieee80211_handle_wake_tx_queue -> tasklet_action
  to a SOFTIRQ-irq-unsafe lock:
    (&hw_priv->tx_loop.pending_record_lock){+.+.}
  ... which became SOFTIRQ-irq-unsafe at:
    bes2600_queue_get_skb -> bes2600_join_work -> process_one_work

queue->lock is taken consistently with spin_lock_bh() at 22 sites;
the nested acquisition of pending_record_lock at queue.c:289 (inside
the outer queue->lock_bh held at line 285) had it implicitly BH-safe
via the outer scope. But pending_record_lock is ALSO taken from
non-BH-disabled contexts:

  bes2600_queue_get_skb  (queue.c:832)  — process context via
    bes2600_join_work (workqueue), no outer queue->lock held
  bes2600_tx_loop_item_pending_check (tx_loop.c:112)
                                     — TX-loop context, no outer
                                     queue->lock held

When CPU0 holds pending_record_lock from one of those non-BH paths
and a softirq fires that wants queue->lock, and CPU1 in softirq has
queue->lock and is about to acquire pending_record_lock — classic AB-BA
SOFTIRQ deadlock.

The fix is the conservative one: take pending_record_lock with _bh()
at every site that's not already inside a queue->lock_bh-held scope.
That makes the lock consistently SOFTIRQ-safe, eliminating the
inversion. queue.c:289/295 stays as plain spin_lock because BH is
already disabled by the outer queue->lock_bh acquired at queue.c:285.

Five sites converted:
  bes2600/queue.c:832 -- spin_lock      -> spin_lock_bh
  bes2600/queue.c:839 -- spin_unlock    -> spin_unlock_bh
  bes2600/queue.c:844 -- spin_unlock    -> spin_unlock_bh
  bes2600/tx_loop.c:112 -- spin_lock    -> spin_lock_bh
  bes2600/tx_loop.c:114 -- spin_unlock  -> spin_unlock_bh

Contract:
  - Documentation/locking/locktypes.rst spelling: spin_lock_bh() is
    the canonical way to make a non-IRQ spinlock safe against
    softirq preemption that might re-enter the same lock.
  - Same shape as queue->lock in this driver and as is_drv->lock
    in the cw1200 ancestor.

Closes: besser#18
Fixes: <bes2600 base import>
Signed-off-by: Markus Fritsche <fritsche.markus@gmail.com>

bes2600/queue.c

@@ -829,19 +829,19 @@ int bes2600_queue_get_skb(struct bes2600_queue *queue, u32 packetID,
 	bes2600_queue_parse_id(packetID, &queue_generation, &queue_id,
 				&item_generation, &item_id, &if_id, &link_id);
 
-	spin_lock(&queue->stats->hw_priv->tx_loop.pending_record_lock);
+	spin_lock_bh(&queue->stats->hw_priv->tx_loop.pending_record_lock);
 	if (!list_empty(&queue->stats->hw_priv->tx_loop.pending_record_list)) {
 		list_for_each_entry_safe(record_item, temp_record_item, &queue->stats->hw_priv->tx_loop.pending_record_list, head) {
 			if (record_item->packetID == packetID) {
 				list_del(&record_item->head);
 				dev_kfree_skb(record_item->skb);
 				kfree(record_item);
-				spin_unlock(&queue->stats->hw_priv->tx_loop.pending_record_lock);
+				spin_unlock_bh(&queue->stats->hw_priv->tx_loop.pending_record_lock);
 				return -EINVAL;
 			}
 		}
 	}
-	spin_unlock(&queue->stats->hw_priv->tx_loop.pending_record_lock);
+	spin_unlock_bh(&queue->stats->hw_priv->tx_loop.pending_record_lock);
 
 	item = &queue->pool[item_id];