Commit 9f950415 authored May 29, 2015 by Neal Cardwell Committed by David S. Miller May 31, 2015

tcp: fix child sockets to use system default congestion control if not set



Linux 3.17 and earlier are explicitly engineered so that if the app
doesn't specifically request a CC module on a listener before the SYN
arrives, then the child gets the system default CC when the connection
is established. See tcp_init_congestion_control() in 3.17 or earlier,
which says "if no choice made yet assign the current value set as
default". The change ("net: tcp: assign tcp cong_ops when tcp sk is
created") altered these semantics, so that children got their parent
listener's congestion control even if the system default had changed
after the listener was created.

This commit returns to those original semantics from 3.17 and earlier,
since they are the original semantics from 2007 in 4d4d3d1e ("[TCP]:
Congestion control initialization."), and some Linux congestion
control workflows depend on that.

In summary, if a listener socket specifically sets TCP_CONGESTION to
"x", or the route locks the CC module to "x", then the child gets
"x". Otherwise the child gets current system default from
net.ipv4.tcp_congestion_control. That's the behavior in 3.17 and
earlier, and this commit restores that.

Fixes: 55d8694f ("net: tcp: assign tcp cong_ops when tcp sk is created")
Cc: Florian Westphal <fw@strlen.de>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: Glenn Judd <glenn.judd@morganstanley.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent beb39db5

include/net/inet_connection_sock.h

+2 −1

Original line number	Diff line number	Diff line
		@@ -98,7 +98,8 @@ struct inet_connection_sock {
		const struct tcp_congestion_ops *icsk_ca_ops;
		const struct inet_connection_sock_af_ops *icsk_af_ops;
		unsigned int (icsk_sync_mss)(struct sock sk, u32 pmtu);
		__u8 icsk_ca_state:7,
		__u8 icsk_ca_state:6,
		icsk_ca_setsockopt:1,
		icsk_ca_dst_locked:1;
		__u8 icsk_retransmits;
		__u8 icsk_pending;

net/ipv4/tcp_cong.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -187,6 +187,7 @@ static void tcp_reinit_congestion_control(struct sock *sk,

		tcp_cleanup_congestion_control(sk);
		icsk->icsk_ca_ops = ca;
		icsk->icsk_ca_setsockopt = 1;

		if (sk->sk_state != TCP_CLOSE && icsk->icsk_ca_ops->init)
		icsk->icsk_ca_ops->init(sk);
		@@ -335,8 +336,10 @@ int tcp_set_congestion_control(struct sock sk, const char name)
		rcu_read_lock();
		ca = __tcp_ca_find_autoload(name);
		/* No change asking for existing value */
		if (ca == icsk->icsk_ca_ops)
		if (ca == icsk->icsk_ca_ops) {
		icsk->icsk_ca_setsockopt = 1;
		goto out;
		}
		if (!ca)
		err = -ENOENT;
		else if (!((ca->flags & TCP_CONG_NON_RESTRICTED) \|\|

net/ipv4/tcp_minisocks.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -420,7 +420,10 @@ void tcp_ca_openreq_child(struct sock sk, const struct dst_entry dst)
		rcu_read_unlock();
		}

		if (!ca_got_dst && !try_module_get(icsk->icsk_ca_ops->owner))
		/* If no valid choice made yet, assign current system default ca. */
		if (!ca_got_dst &&
		(!icsk->icsk_ca_setsockopt \|\|
		!try_module_get(icsk->icsk_ca_ops->owner)))
		tcp_assign_congestion_control(sk);

		tcp_set_ca_state(sk, TCP_CA_Open);

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