cxgb4: Avoids race and deadlock while freeing tx descriptor

There could be race between t4_eth_xmit() and t4_free_sge_resources() while
freeing tx descriptors, take txq lock in t4_free_sge_resources(). We need
to stop the xmit frame path which runs in bottom half context while
unloading the driver using _bh variant of the lock. This is to prevent
the deadlock between xmit and driver unload.
Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/chelsio/cxgb4/sge.c

@@ -3006,7 +3006,9 @@ void t4_free_sge_resources(struct adapter *adap)
 		if (etq->q.desc) {
 			t4_eth_eq_free(adap, adap->mbox, adap->pf, 0,
 				       etq->q.cntxt_id);
+			__netif_tx_lock_bh(etq->txq);
 			free_tx_desc(adap, &etq->q, etq->q.in_use, true);
+			__netif_tx_unlock_bh(etq->txq);
 			kfree(etq->q.sdesc);
 			free_txq(adap, &etq->q);
 		}