From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Mon, 21 Aug 2017 15:09:13 +0200
Subject: net: take the tcp_sk_lock lock with BH disabled
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: 71eea2c581729cf6e156a44681fc6f8499a857fb
Patch-mainline: Queued in subsystem maintainer repository
References: SLE12 Realtime Extension

Lockdep may complain about an unsafe locking scenario:
|      CPU0                    CPU1
|      ----                    ----
| lock((tcp_sk_lock).lock);
|                              lock(&per_cpu(local_softirq_locks[i], __cpu).lock);
|                              lock((tcp_sk_lock).lock);
| lock(&per_cpu(local_softirq_locks[i], __cpu).lock);

in the call paths:
	do_current_softirqs -> tcp_v4_send_ack()
vs
	tcp_v4_send_reset -> do_current_softirqs().

This should not happen since local_softirq_locks is per CPU. Reversing
the order makes lockdep happy.

Reported-by: Jacek Konieczny <jajcus@jajcus.net>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 net/ipv4/tcp_ipv4.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -709,8 +709,8 @@ static void tcp_v4_send_reset(const stru
 
 	arg.tos = ip_hdr(skb)->tos;
 	arg.uid = sock_net_uid(net, sk && sk_fullsock(sk) ? sk : NULL);
-	local_lock(tcp_sk_lock);
 	local_bh_disable();
+	local_lock(tcp_sk_lock);
 	ip_send_unicast_reply(*this_cpu_ptr(net->ipv4.tcp_sk),
 			      skb, &TCP_SKB_CB(skb)->header.h4.opt,
 			      ip_hdr(skb)->saddr, ip_hdr(skb)->daddr,
@@ -718,8 +718,8 @@ static void tcp_v4_send_reset(const stru
 
 	__TCP_INC_STATS(net, TCP_MIB_OUTSEGS);
 	__TCP_INC_STATS(net, TCP_MIB_OUTRSTS);
-	local_bh_enable();
 	local_unlock(tcp_sk_lock);
+	local_bh_enable();
 
 #ifdef CONFIG_TCP_MD5SIG
 out:
@@ -797,16 +797,16 @@ static void tcp_v4_send_ack(const struct
 		arg.bound_dev_if = oif;
 	arg.tos = tos;
 	arg.uid = sock_net_uid(net, sk_fullsock(sk) ? sk : NULL);
-	local_lock(tcp_sk_lock);
 	local_bh_disable();
+	local_lock(tcp_sk_lock);
 	ip_send_unicast_reply(*this_cpu_ptr(net->ipv4.tcp_sk),
 			      skb, &TCP_SKB_CB(skb)->header.h4.opt,
 			      ip_hdr(skb)->saddr, ip_hdr(skb)->daddr,
 			      &arg, arg.iov[0].iov_len);
 
 	__TCP_INC_STATS(net, TCP_MIB_OUTSEGS);
-	local_bh_enable();
 	local_unlock(tcp_sk_lock);
+	local_bh_enable();
 }
 
 static void tcp_v4_timewait_ack(struct sock *sk, struct sk_buff *skb)