tcp: do not block bh during prequeue processing

AFAIK, nothing in current TCP stack absolutely wants BH
being disabled once socket is owned by a thread running in
process context.

As mentioned in my prior patch ("tcp: give prequeue mode some care"),
processing a batch of packets might take time, better not block BH
at all.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1449,12 +1449,8 @@ static void tcp_prequeue_process(struct sock *sk)
 
 	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPREQUEUED);
 
-	/* RX process wants to run with disabled BHs, though it is not
-	 * necessary */
-	local_bh_disable();
 	while ((skb = __skb_dequeue(&tp->ucopy.prequeue)) != NULL)
 		sk_backlog_rcv(sk, skb);
-	local_bh_enable();
 
 	/* Clear memory counter. */
 	tp->ucopy.memory = 0;

net/ipv4/tcp_input.c

@@ -4611,14 +4611,12 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
 
 			__set_current_state(TASK_RUNNING);
 
-			local_bh_enable();
 			if (!skb_copy_datagram_msg(skb, 0, tp->ucopy.msg, chunk)) {
 				tp->ucopy.len -= chunk;
 				tp->copied_seq += chunk;
 				eaten = (chunk == skb->len);
 				tcp_rcv_space_adjust(sk);
 			}
-			local_bh_disable();
 		}
 
 		if (eaten <= 0) {
@@ -5134,7 +5132,6 @@ static int tcp_copy_to_iovec(struct sock *sk, struct sk_buff *skb, int hlen)
 	int chunk = skb->len - hlen;
 	int err;
 
-	local_bh_enable();
 	if (skb_csum_unnecessary(skb))
 		err = skb_copy_datagram_msg(skb, hlen, tp->ucopy.msg, chunk);
 	else
@@ -5146,32 +5143,9 @@ static int tcp_copy_to_iovec(struct sock *sk, struct sk_buff *skb, int hlen)
 		tcp_rcv_space_adjust(sk);
 	}
 
-	local_bh_disable();
 	return err;
 }
 
-static __sum16 __tcp_checksum_complete_user(struct sock *sk,
-					    struct sk_buff *skb)
-{
-	__sum16 result;
-
-	if (sock_owned_by_user(sk)) {
-		local_bh_enable();
-		result = __tcp_checksum_complete(skb);
-		local_bh_disable();
-	} else {
-		result = __tcp_checksum_complete(skb);
-	}
-	return result;
-}
-
-static inline bool tcp_checksum_complete_user(struct sock *sk,
-					     struct sk_buff *skb)
-{
-	return !skb_csum_unnecessary(skb) &&
-	       __tcp_checksum_complete_user(sk, skb);
-}
-
 /* Does PAWS and seqno based validation of an incoming segment, flags will
  * play significant role here.
  */
@@ -5386,7 +5360,7 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
 				}
 			}
 			if (!eaten) {
-				if (tcp_checksum_complete_user(sk, skb))
+				if (tcp_checksum_complete(skb))
 					goto csum_error;
 
 				if ((int)skb->truesize > sk->sk_forward_alloc)
@@ -5430,7 +5404,7 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
 	}
 
 slow_path:
-	if (len < (th->doff << 2) || tcp_checksum_complete_user(sk, skb))
+	if (len < (th->doff << 2) || tcp_checksum_complete(skb))
 		goto csum_error;
 
 	if (!th->ack && !th->rst && !th->syn)