iwlwifi: fix skb usage after free

Since

commit a120e912eb51e347f36c71b60a1d13af74d30e83
Author: Stanislaw Gruszka <[e-mail]>
Date: Fri Feb 19 15:47:33 2010 -0800

iwlwifi: sanity check before counting number of tfds can be free

we use skb->data after calling ieee80211_tx_status_irqsafe(), which
could free skb instantly.

On current kernels I do not observe practical problems related with
bug, but on 2.6.35.y it cause random system hangs when stressing
wireless link.

Cc: [e-mail] # 2.6.32+
Signed-off-by: Stanislaw Gruszka <[e-mail]>
Acked-by: Wey-Yi Guy <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>

drivers/net/wireless/iwlwifi/iwl-agn-tx.c

···
 	     q->read_ptr = iwl_queue_inc_wrap(q->read_ptr, q->n_bd)) {
 
 		tx_info = &txq->txb[txq->q.read_ptr];
-		iwlagn_tx_status(priv, tx_info,
-				 txq_id >= IWLAGN_FIRST_AMPDU_QUEUE);
+
+		if (WARN_ON_ONCE(tx_info->skb == NULL))
+			continue;
 
 		hdr = (struct ieee80211_hdr *)tx_info->skb->data;
-		if (hdr && ieee80211_is_data_qos(hdr->frame_control))
+		if (ieee80211_is_data_qos(hdr->frame_control))
 			nfreed++;
+
+		iwlagn_tx_status(priv, tx_info,
+				 txq_id >= IWLAGN_FIRST_AMPDU_QUEUE);
 		tx_info->skb = NULL;
 
 		if (priv->cfg->ops->lib->txq_inval_byte_cnt_tbl)