[PATCH v2 net-next 0/5] mptcp: re-enable sndbuf autotune

[PATCH v2 net-next 0/5] mptcp: re-enable sndbuf autotune

[Paolo Abeni]

The sendbuffer autotuning was unintentionally disabled as a
side effect of the recent workqueue removal refactor. These
patches re-enable id, with some extra care: with autotuning
enable/large send buffer we need a more accurate packet
scheduler to be able to use efficiently the available
subflow bandwidth, especially when the subflows have
different capacities.

The first patch cleans-up subflow socket handling, making
the actual re-enable (patch 2) simpler.

Patches 3 and 4 improve the packet scheduler, to better cope
with non trivial scenarios and large send buffer. 

Finally patch 5 adds and uses some infrastructure to avoid
the workqueue usage for the packet scheduler operations introduced
by the previous patches.

v1 -> v2:
 - fix a UaF in patch 5, see patch changelog for the details 

Paolo Abeni (5):
  mptcp: always graft subflow socket to parent
  mptcp: re-enable sndbuf autotune
  mptcp: do not queue excessive data on subflows
  mptcp: schedule work for better snd subflow selection
  mptcp: implement delegated actions

 net/mptcp/protocol.c | 179 ++++++++++++++++++++++++++++---------------
 net/mptcp/protocol.h |  87 +++++++++++++++++++++
 net/mptcp/subflow.c  |  55 ++++++++++++-
 3 files changed, 258 insertions(+), 63 deletions(-)

-- 
2.26.2

[PATCH v2 net-next 1/5] mptcp: always graft subflow socket to parent

[Paolo Abeni]

Currently, incoming subflows link to the parent socket,
while outgoing ones link to a per subflow socket. The latter
is not really needed, except at the initial connect() time and
for the first subflow.

Always graft the outgoing subflow to the parent socket and
free the unneeded ones early.

This allows some code cleanup, reduces the amount of memory
used and will simplify the next patch

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.c | 36 ++++++++++--------------------------
 net/mptcp/protocol.h |  1 +
 net/mptcp/subflow.c  |  3 +++
 3 files changed, 14 insertions(+), 26 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index f998a077c7dd0..c5c80f9253832 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -114,11 +114,7 @@ static int __mptcp_socket_create(struct mptcp_sock *msk)
 	list_add(&subflow->node, &msk->conn_list);
 	sock_hold(ssock->sk);
 	subflow->request_mptcp = 1;
-
-	/* accept() will wait on first subflow sk_wq, and we always wakes up
-	 * via msk->sk_socket
-	 */
-	RCU_INIT_POINTER(msk->first->sk_wq, &sk->sk_socket->wq);
+	mptcp_sock_graft(msk->first, sk->sk_socket);
 
 	return 0;
 }
@@ -2116,9 +2112,6 @@ static struct sock *mptcp_subflow_get_retrans(const struct mptcp_sock *msk)
 void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 		       struct mptcp_subflow_context *subflow)
 {
-	bool dispose_socket = false;
-	struct socket *sock;
-
 	list_del(&subflow->node);
 
 	lock_sock_nested(ssk, SINGLE_DEPTH_NESTING);
@@ -2126,11 +2119,8 @@ void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 	/* if we are invoked by the msk cleanup code, the subflow is
 	 * already orphaned
 	 */
-	sock = ssk->sk_socket;
-	if (sock) {
-		dispose_socket = sock != sk->sk_socket;
+	if (ssk->sk_socket)
 		sock_orphan(ssk);
-	}
 
 	subflow->disposable = 1;
 
@@ -2148,8 +2138,6 @@ void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 		__sock_put(ssk);
 	}
 	release_sock(ssk);
-	if (dispose_socket)
-		iput(SOCK_INODE(sock));
 
 	sock_put(ssk);
 }
@@ -2536,6 +2524,12 @@ static void __mptcp_destroy_sock(struct sock *sk)
 
 	pr_debug("msk=%p", msk);
 
+	/* dispose the ancillatory tcp socket, if any */
+	if (msk->subflow) {
+		iput(SOCK_INODE(msk->subflow));
+		msk->subflow = NULL;
+	}
+
 	/* be sure to always acquire the join list lock, to sync vs
 	 * mptcp_finish_join().
 	 */
@@ -2586,20 +2580,10 @@ static void mptcp_close(struct sock *sk, long timeout)
 	inet_csk(sk)->icsk_mtup.probe_timestamp = tcp_jiffies32;
 	list_for_each_entry(subflow, &mptcp_sk(sk)->conn_list, node) {
 		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
-		bool slow, dispose_socket;
-		struct socket *sock;
+		bool slow = lock_sock_fast(ssk);
 
-		slow = lock_sock_fast(ssk);
-		sock = ssk->sk_socket;
-		dispose_socket = sock && sock != sk->sk_socket;
 		sock_orphan(ssk);
 		unlock_sock_fast(ssk, slow);
-
-		/* for the outgoing subflows we additionally need to free
-		 * the associated socket
-		 */
-		if (dispose_socket)
-			iput(SOCK_INODE(sock));
 	}
 	sock_orphan(sk);
 
@@ -3041,7 +3025,7 @@ void mptcp_finish_connect(struct sock *ssk)
 	mptcp_rcv_space_init(msk, ssk);
 }
 
-static void mptcp_sock_graft(struct sock *sk, struct socket *parent)
+void mptcp_sock_graft(struct sock *sk, struct socket *parent)
 {
 	write_lock_bh(&sk->sk_callback_lock);
 	rcu_assign_pointer(sk->sk_wq, &parent->wq);
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index d6400ad2d6156..65d200a1072bf 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -473,6 +473,7 @@ void mptcp_subflow_shutdown(struct sock *sk, struct sock *ssk, int how);
 void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 		       struct mptcp_subflow_context *subflow);
 void mptcp_subflow_reset(struct sock *ssk);
+void mptcp_sock_graft(struct sock *sk, struct socket *parent);
 
 /* called with sk socket lock held */
 int __mptcp_subflow_connect(struct sock *sk, const struct mptcp_addr_info *loc,
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 278cbe3e539ea..22313710d7696 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1159,6 +1159,9 @@ int __mptcp_subflow_connect(struct sock *sk, const struct mptcp_addr_info *loc,
 	if (err && err != -EINPROGRESS)
 		goto failed_unlink;
 
+	/* discard the subflow socket */
+	mptcp_sock_graft(ssk, sk->sk_socket);
+	iput(SOCK_INODE(sf));
 	return err;
 
 failed_unlink:
-- 
2.26.2

[PATCH v2 net-next 2/5] mptcp: re-enable sndbuf autotune

[Paolo Abeni]

After commit 6e628cd3a8f7 ("mptcp: use mptcp release_cb for
delayed tasks"), MPTCP never sets the flag bit SOCK_NOSPACE
on its subflow. As a side effect, autotune never takes place,
as it happens inside tcp_new_space(), which in turn is called
only when the mentioned bit is set.

Let's sendmsg() set the subflows NOSPACE bit when looking for
more memory and use the subflow write_space callback to propagate
the snd buf update and wake-up the user-space.

Additionally, this allows dropping a bunch of duplicate code and
makes the SNDBUF_LIMITED chrono relevant again for MPTCP subflows.

Fixes: 6e628cd3a8f7 ("mptcp: use mptcp release_cb for delayed tasks")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.c | 52 +++++++++++++++++---------------------------
 net/mptcp/protocol.h | 19 ++++++++++++++++
 net/mptcp/subflow.c  |  7 +++++-
 3 files changed, 45 insertions(+), 33 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index c5c80f9253832..d07e60330df56 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -730,10 +730,14 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
 
 void __mptcp_flush_join_list(struct mptcp_sock *msk)
 {
+	struct mptcp_subflow_context *subflow;
+
 	if (likely(list_empty(&msk->join_list)))
 		return;
 
 	spin_lock_bh(&msk->join_list_lock);
+	list_for_each_entry(subflow, &msk->join_list, node)
+		mptcp_propagate_sndbuf((struct sock *)msk, mptcp_subflow_tcp_sock(subflow));
 	list_splice_tail_init(&msk->join_list, &msk->conn_list);
 	spin_unlock_bh(&msk->join_list_lock);
 }
@@ -1033,13 +1037,6 @@ static void __mptcp_clean_una(struct sock *sk)
 			__mptcp_update_wmem(sk);
 			sk_mem_reclaim_partial(sk);
 		}
-
-		if (sk_stream_is_writeable(sk)) {
-			/* pairs with memory barrier in mptcp_poll */
-			smp_mb();
-			if (test_and_clear_bit(MPTCP_NOSPACE, &msk->flags))
-				sk_stream_write_space(sk);
-		}
 	}
 
 	if (snd_una == READ_ONCE(msk->snd_nxt)) {
@@ -1358,8 +1355,7 @@ struct subflow_send_info {
 	u64 ratio;
 };
 
-static struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk,
-					   u32 *sndbuf)
+static struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk)
 {
 	struct subflow_send_info send_info[2];
 	struct mptcp_subflow_context *subflow;
@@ -1370,24 +1366,17 @@ static struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk,
 
 	sock_owned_by_me((struct sock *)msk);
 
-	*sndbuf = 0;
 	if (__mptcp_check_fallback(msk)) {
 		if (!msk->first)
 			return NULL;
-		*sndbuf = msk->first->sk_sndbuf;
 		return sk_stream_memory_free(msk->first) ? msk->first : NULL;
 	}
 
 	/* re-use last subflow, if the burst allow that */
 	if (msk->last_snd && msk->snd_burst > 0 &&
 	    sk_stream_memory_free(msk->last_snd) &&
-	    mptcp_subflow_active(mptcp_subflow_ctx(msk->last_snd))) {
-		mptcp_for_each_subflow(msk, subflow) {
-			ssk =  mptcp_subflow_tcp_sock(subflow);
-			*sndbuf = max(tcp_sk(ssk)->snd_wnd, *sndbuf);
-		}
+	    mptcp_subflow_active(mptcp_subflow_ctx(msk->last_snd)))
 		return msk->last_snd;
-	}
 
 	/* pick the subflow with the lower wmem/wspace ratio */
 	for (i = 0; i < 2; ++i) {
@@ -1400,7 +1389,6 @@ static struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk,
 			continue;
 
 		nr_active += !subflow->backup;
-		*sndbuf = max(tcp_sk(ssk)->snd_wnd, *sndbuf);
 		if (!sk_stream_memory_free(subflow->tcp_sock))
 			continue;
 
@@ -1430,6 +1418,7 @@ static struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk,
 				       sk_stream_wspace(msk->last_snd));
 		return msk->last_snd;
 	}
+
 	return NULL;
 }
 
@@ -1450,7 +1439,6 @@ static void mptcp_push_pending(struct sock *sk, unsigned int flags)
 	};
 	struct mptcp_data_frag *dfrag;
 	int len, copied = 0;
-	u32 sndbuf;
 
 	while ((dfrag = mptcp_send_head(sk))) {
 		info.sent = dfrag->already_sent;
@@ -1461,12 +1449,7 @@ static void mptcp_push_pending(struct sock *sk, unsigned int flags)
 
 			prev_ssk = ssk;
 			__mptcp_flush_join_list(msk);
-			ssk = mptcp_subflow_get_send(msk, &sndbuf);
-
-			/* do auto tuning */
-			if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK) &&
-			    sndbuf > READ_ONCE(sk->sk_sndbuf))
-				WRITE_ONCE(sk->sk_sndbuf, sndbuf);
+			ssk = mptcp_subflow_get_send(msk);
 
 			/* try to keep the subflow socket lock across
 			 * consecutive xmit on the same socket
@@ -1533,11 +1516,6 @@ static void __mptcp_subflow_push_pending(struct sock *sk, struct sock *ssk)
 		while (len > 0) {
 			int ret = 0;
 
-			/* do auto tuning */
-			if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK) &&
-			    ssk->sk_sndbuf > READ_ONCE(sk->sk_sndbuf))
-				WRITE_ONCE(sk->sk_sndbuf, ssk->sk_sndbuf);
-
 			if (unlikely(mptcp_must_reclaim_memory(sk, ssk))) {
 				__mptcp_update_wmem(sk);
 				sk_mem_reclaim_partial(sk);
@@ -1575,6 +1553,15 @@ static void __mptcp_subflow_push_pending(struct sock *sk, struct sock *ssk)
 	}
 }
 
+static void mptcp_set_nospace(struct sock *sk)
+{
+	/* enable autotune */
+	set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
+
+	/* will be cleared on avail space */
+	set_bit(MPTCP_NOSPACE, &mptcp_sk(sk)->flags);
+}
+
 static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);
@@ -1676,7 +1663,7 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		continue;
 
 wait_for_memory:
-		set_bit(MPTCP_NOSPACE, &msk->flags);
+		mptcp_set_nospace(sk);
 		mptcp_push_pending(sk, msg->msg_flags);
 		ret = sk_stream_wait_memory(sk, &timeo);
 		if (ret)
@@ -3268,6 +3255,7 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 
 		mptcp_copy_inaddrs(newsk, msk->first);
 		mptcp_rcv_space_init(msk, msk->first);
+		mptcp_propagate_sndbuf(newsk, msk->first);
 
 		/* set ssk->sk_socket of accept()ed flows to mptcp socket.
 		 * This is needed so NOSPACE flag can be set from tcp stack.
@@ -3308,7 +3296,7 @@ static __poll_t mptcp_check_writeable(struct mptcp_sock *msk)
 	if (sk_stream_is_writeable(sk))
 		return EPOLLOUT | EPOLLWRNORM;
 
-	set_bit(MPTCP_NOSPACE, &msk->flags);
+	mptcp_set_nospace(sk);
 	smp_mb__after_atomic(); /* msk->flags is changed by write_space cb */
 	if (sk_stream_is_writeable(sk))
 		return EPOLLOUT | EPOLLWRNORM;
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 65d200a1072bf..871534df6140f 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -522,6 +522,25 @@ static inline bool mptcp_data_fin_enabled(const struct mptcp_sock *msk)
 	       READ_ONCE(msk->write_seq) == READ_ONCE(msk->snd_nxt);
 }
 
+static inline bool mptcp_propagate_sndbuf(struct sock *sk, struct sock *ssk)
+{
+	if ((sk->sk_userlocks & SOCK_SNDBUF_LOCK) || ssk->sk_sndbuf <= READ_ONCE(sk->sk_sndbuf))
+		return false;
+
+	WRITE_ONCE(sk->sk_sndbuf, ssk->sk_sndbuf);
+	return true;
+}
+
+static inline void mptcp_write_space(struct sock *sk)
+{
+	if (sk_stream_is_writeable(sk)) {
+		/* pairs with memory barrier in mptcp_poll */
+		smp_mb();
+		if (test_and_clear_bit(MPTCP_NOSPACE, &mptcp_sk(sk)->flags))
+			sk_stream_write_space(sk);
+	}
+}
+
 void mptcp_destroy_common(struct mptcp_sock *msk);
 
 void __init mptcp_token_init(void);
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 22313710d7696..1ca0c82b0dbde 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -343,6 +343,7 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 	if (subflow->conn_finished)
 		return;
 
+	mptcp_propagate_sndbuf(parent, sk);
 	subflow->rel_write_seq = 1;
 	subflow->conn_finished = 1;
 	subflow->ssn_offset = TCP_SKB_CB(skb)->seq;
@@ -1040,7 +1041,10 @@ static void subflow_data_ready(struct sock *sk)
 
 static void subflow_write_space(struct sock *ssk)
 {
-	/* we take action in __mptcp_clean_una() */
+	struct sock *sk = mptcp_subflow_ctx(ssk)->conn;
+
+	mptcp_propagate_sndbuf(sk, ssk);
+	mptcp_write_space(sk);
 }
 
 static struct inet_connection_sock_af_ops *
@@ -1302,6 +1306,7 @@ static void subflow_state_change(struct sock *sk)
 	__subflow_state_change(sk);
 
 	if (subflow_simultaneous_connect(sk)) {
+		mptcp_propagate_sndbuf(parent, sk);
 		mptcp_do_fallback(sk);
 		mptcp_rcv_space_init(mptcp_sk(parent), sk);
 		pr_fallback(mptcp_sk(parent));
-- 
2.26.2

[PATCH v2 net-next 3/5] mptcp: do not queue excessive data on subflows

[Paolo Abeni]

The current packet scheduler can enqueue up to sndbuf
data on each subflow. If the send buffer is large and
the subflows are not symmetric, this could lead to
suboptimal aggregate bandwidth utilization.

Limit the amount of queued data to the maximum send
window.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index d07e60330df56..e741201acc98f 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1389,7 +1389,7 @@ static struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk)
 			continue;
 
 		nr_active += !subflow->backup;
-		if (!sk_stream_memory_free(subflow->tcp_sock))
+		if (!sk_stream_memory_free(subflow->tcp_sock) || !tcp_sk(ssk)->snd_wnd)
 			continue;
 
 		pace = READ_ONCE(ssk->sk_pacing_rate);
@@ -1415,7 +1415,7 @@ static struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk)
 	if (send_info[0].ssk) {
 		msk->last_snd = send_info[0].ssk;
 		msk->snd_burst = min_t(int, MPTCP_SEND_BURST_SIZE,
-				       sk_stream_wspace(msk->last_snd));
+				       tcp_sk(msk->last_snd)->snd_wnd);
 		return msk->last_snd;
 	}
 
-- 
2.26.2

[PATCH v2 net-next 4/5] mptcp: schedule work for better snd subflow selection

[Paolo Abeni]

Otherwise the packet scheduler policy will not be
enforced when pushing pending data at MPTCP-level
ack reception time.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index e741201acc98f..8cb582eee2862 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2242,6 +2242,7 @@ static void mptcp_worker(struct work_struct *work)
 	if (unlikely(state == TCP_CLOSE))
 		goto unlock;
 
+	mptcp_push_pending(sk, 0);
 	mptcp_check_data_fin_ack(sk);
 	__mptcp_flush_join_list(msk);
 
@@ -2899,10 +2900,14 @@ void __mptcp_check_push(struct sock *sk, struct sock *ssk)
 	if (!mptcp_send_head(sk))
 		return;
 
-	if (!sock_owned_by_user(sk))
-		__mptcp_subflow_push_pending(sk, ssk);
-	else
+	if (!sock_owned_by_user(sk)) {
+		if (mptcp_subflow_get_send(mptcp_sk(sk)) == ssk)
+			__mptcp_subflow_push_pending(sk, ssk);
+		else
+			mptcp_schedule_work(sk);
+	} else {
 		set_bit(MPTCP_PUSH_PENDING, &mptcp_sk(sk)->flags);
+	}
 }
 
 #define MPTCP_DEFERRED_ALL (TCPF_WRITE_TIMER_DEFERRED)
-- 
2.26.2

[PATCH v2 net-next 5/5] mptcp: implement delegated actions

[Paolo Abeni]

On MPTCP-level ack reception, the packet scheduler
may select a subflow other then the current one.

Prior to this commit we rely on the workqueue to trigger
action on such subflow.

This changeset introduces an infrastructure that allows
any MPTCP subflow to schedule actions (MPTCP xmit) on
others subflows without resorting to (multiple) process
reschedule.

A dummy NAPI instance is used instead. When MPTCP needs to
trigger action an a different subflow, it enqueues the target
subflow on the NAPI backlog and schedule such instance as needed.

The dummy NAPI poll method walks the sockets backlog and tries
to acquire the (BH) socket lock on each of them. If the socket
is owned by the user space, the action will be completed by
the sock release cb, otherwise push is started.

This change leverages the delegated action infrastructure
to avoid invoking the MPTCP worker to spool the pending data,
when the packet scheduler picks a subflow other then the one
currently processing the incoming MPTCP-level ack.

Additionally we further refine the subflow selection
invoking the packet scheduler for each chunk of data
even inside __mptcp_subflow_push_pending().

v1 -> v2:
 - fix possible UaF at shutdown time, resetting sock ops
   after removing the ulp context

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.c | 86 +++++++++++++++++++++++++++++++++++++++++---
 net/mptcp/protocol.h | 67 ++++++++++++++++++++++++++++++++++
 net/mptcp/subflow.c  | 45 +++++++++++++++++++++++
 3 files changed, 194 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 8cb582eee2862..a033bf9c26ee1 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -45,6 +45,9 @@ static struct percpu_counter mptcp_sockets_allocated;
 static void __mptcp_destroy_sock(struct sock *sk);
 static void __mptcp_check_send_data_fin(struct sock *sk);
 
+DEFINE_PER_CPU(struct mptcp_delegated_action, mptcp_delegated_actions);
+static struct net_device mptcp_napi_dev;
+
 /* If msk has an initial subflow socket, and the MP_CAPABLE handshake has not
  * completed yet or has failed, return the subflow socket.
  * Otherwise return NULL.
@@ -1506,7 +1509,9 @@ static void __mptcp_subflow_push_pending(struct sock *sk, struct sock *ssk)
 	struct mptcp_sock *msk = mptcp_sk(sk);
 	struct mptcp_sendmsg_info info;
 	struct mptcp_data_frag *dfrag;
+	struct sock *xmit_ssk;
 	int len, copied = 0;
+	bool first = true;
 
 	info.flags = 0;
 	while ((dfrag = mptcp_send_head(sk))) {
@@ -1516,6 +1521,18 @@ static void __mptcp_subflow_push_pending(struct sock *sk, struct sock *ssk)
 		while (len > 0) {
 			int ret = 0;
 
+			/* the caller already invoked the packet scheduler,
+			 * check for a different subflow usage only after
+			 * spooling the first chunk of data
+			 */
+			xmit_ssk = first ? ssk : mptcp_subflow_get_send(mptcp_sk(sk));
+			if (!xmit_ssk)
+				goto out;
+			if (xmit_ssk != ssk) {
+				mptcp_subflow_delegate(mptcp_subflow_ctx(xmit_ssk));
+				goto out;
+			}
+
 			if (unlikely(mptcp_must_reclaim_memory(sk, ssk))) {
 				__mptcp_update_wmem(sk);
 				sk_mem_reclaim_partial(sk);
@@ -1534,6 +1551,7 @@ static void __mptcp_subflow_push_pending(struct sock *sk, struct sock *ssk)
 			msk->tx_pending_data -= ret;
 			copied += ret;
 			len -= ret;
+			first = false;
 		}
 		WRITE_ONCE(msk->first_pending, mptcp_send_next(sk));
 	}
@@ -2242,7 +2260,6 @@ static void mptcp_worker(struct work_struct *work)
 	if (unlikely(state == TCP_CLOSE))
 		goto unlock;
 
-	mptcp_push_pending(sk, 0);
 	mptcp_check_data_fin_ack(sk);
 	__mptcp_flush_join_list(msk);
 
@@ -2901,10 +2918,12 @@ void __mptcp_check_push(struct sock *sk, struct sock *ssk)
 		return;
 
 	if (!sock_owned_by_user(sk)) {
-		if (mptcp_subflow_get_send(mptcp_sk(sk)) == ssk)
+		struct sock *xmit_ssk = mptcp_subflow_get_send(mptcp_sk(sk));
+
+		if (xmit_ssk == ssk)
 			__mptcp_subflow_push_pending(sk, ssk);
-		else
-			mptcp_schedule_work(sk);
+		else if (xmit_ssk)
+			mptcp_subflow_delegate(mptcp_subflow_ctx(xmit_ssk));
 	} else {
 		set_bit(MPTCP_PUSH_PENDING, &mptcp_sk(sk)->flags);
 	}
@@ -2955,6 +2974,20 @@ static void mptcp_release_cb(struct sock *sk)
 	}
 }
 
+void mptcp_subflow_process_delegated(struct sock *ssk)
+{
+	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
+	struct sock *sk = subflow->conn;
+
+	mptcp_data_lock(sk);
+	if (!sock_owned_by_user(sk))
+		__mptcp_subflow_push_pending(sk, ssk);
+	else
+		set_bit(MPTCP_PUSH_PENDING, &mptcp_sk(sk)->flags);
+	mptcp_data_unlock(sk);
+	mptcp_subflow_delegated_done(subflow);
+}
+
 static int mptcp_hash(struct sock *sk)
 {
 	/* should never be called,
@@ -3365,13 +3398,58 @@ static struct inet_protosw mptcp_protosw = {
 	.flags		= INET_PROTOSW_ICSK,
 };
 
+static int mptcp_napi_poll(struct napi_struct *napi, int budget)
+{
+	struct mptcp_delegated_action *delegated;
+	struct mptcp_subflow_context *subflow;
+	int work_done = 0;
+
+	delegated = container_of(napi, struct mptcp_delegated_action, napi);
+	while ((subflow = mptcp_subflow_delegated_next(delegated)) != NULL) {
+		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+
+		bh_lock_sock_nested(ssk);
+		if (!sock_owned_by_user(ssk) &&
+		    mptcp_subflow_has_delegated_action(subflow))
+			mptcp_subflow_process_delegated(ssk);
+		/* ... elsewhere tcp_release_cb_override already processed
+		 * the action or will do at next release_sock().
+		 * In both case must dequeue the subflow here - on the same
+		 * CPU that scheduled it.
+		 */
+		bh_unlock_sock(ssk);
+		sock_put(ssk);
+
+		if (++work_done == budget)
+			return budget;
+	}
+
+	/* always provide a 0 'work_done' argument, so that napi_complete_done
+	 * will not try accessing the NULL napi->dev ptr
+	 */
+	napi_complete_done(napi, 0);
+	return work_done;
+}
+
 void __init mptcp_proto_init(void)
 {
+	struct mptcp_delegated_action *delegated;
+	int cpu;
+
 	mptcp_prot.h.hashinfo = tcp_prot.h.hashinfo;
 
 	if (percpu_counter_init(&mptcp_sockets_allocated, 0, GFP_KERNEL))
 		panic("Failed to allocate MPTCP pcpu counter\n");
 
+	init_dummy_netdev(&mptcp_napi_dev);
+	for_each_possible_cpu(cpu) {
+		delegated = per_cpu_ptr(&mptcp_delegated_actions, cpu);
+		INIT_LIST_HEAD(&delegated->head);
+		netif_tx_napi_add(&mptcp_napi_dev, &delegated->napi, mptcp_napi_poll,
+				  NAPI_POLL_WEIGHT);
+		napi_enable(&delegated->napi);
+	}
+
 	mptcp_subflow_init();
 	mptcp_pm_init();
 	mptcp_token_init();
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 871534df6140f..1460705aaad05 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -378,6 +378,15 @@ enum mptcp_data_avail {
 	MPTCP_SUBFLOW_OOO_DATA
 };
 
+struct mptcp_delegated_action {
+	struct napi_struct napi;
+	struct list_head head;
+};
+
+DECLARE_PER_CPU(struct mptcp_delegated_action, mptcp_delegated_actions);
+
+#define MPTCP_DELEGATE_SEND		0
+
 /* MPTCP subflow context */
 struct mptcp_subflow_context {
 	struct	list_head node;/* conn_list of subflows */
@@ -415,6 +424,9 @@ struct mptcp_subflow_context {
 	u8	local_id;
 	u8	remote_id;
 
+	long	delegated_status;
+	struct	list_head delegated_node;   /* link into delegated_action, protected by local BH */
+
 	struct	sock *tcp_sock;	    /* tcp sk backpointer */
 	struct	sock *conn;	    /* parent mptcp_sock */
 	const	struct inet_connection_sock_af_ops *icsk_af_ops;
@@ -463,6 +475,61 @@ static inline void mptcp_add_pending_subflow(struct mptcp_sock *msk,
 	spin_unlock_bh(&msk->join_list_lock);
 }
 
+void mptcp_subflow_process_delegated(struct sock *ssk);
+
+static inline void mptcp_subflow_delegate(struct mptcp_subflow_context *subflow)
+{
+	struct mptcp_delegated_action *delegated;
+	bool schedule;
+
+	/* The implied barrier pairs with mptcp_subflow_delegated_done(), and
+	 * ensures the below list check sees list updates done prior to status
+	 * bit changes
+	 */
+	if (!test_and_set_bit(MPTCP_DELEGATE_SEND, &subflow->delegated_status)) {
+		/* still on delegated list from previous scheduling */
+		if (!list_empty(&subflow->delegated_node))
+			return;
+
+		/* the caller held the subflow bh socket lock */
+		lockdep_assert_in_softirq();
+
+		delegated = this_cpu_ptr(&mptcp_delegated_actions);
+		schedule = list_empty(&delegated->head);
+		list_add_tail(&subflow->delegated_node, &delegated->head);
+		sock_hold(mptcp_subflow_tcp_sock(subflow));
+		if (schedule)
+			napi_schedule(&delegated->napi);
+	}
+}
+
+static inline struct mptcp_subflow_context *
+mptcp_subflow_delegated_next(struct mptcp_delegated_action *delegated)
+{
+	struct mptcp_subflow_context *ret;
+
+	if (list_empty(&delegated->head))
+		return NULL;
+
+	ret = list_first_entry(&delegated->head, struct mptcp_subflow_context, delegated_node);
+	list_del_init(&ret->delegated_node);
+	return ret;
+}
+
+static inline bool mptcp_subflow_has_delegated_action(const struct mptcp_subflow_context *subflow)
+{
+	return test_bit(MPTCP_DELEGATE_SEND, &subflow->delegated_status);
+}
+
+static inline void mptcp_subflow_delegated_done(struct mptcp_subflow_context *subflow)
+{
+	/* pairs with mptcp_subflow_delegate, ensures delegate_node is updated before
+	 * touching the status bit
+	 */
+	smp_wmb();
+	clear_bit(MPTCP_DELEGATE_SEND, &subflow->delegated_status);
+}
+
 int mptcp_is_enabled(struct net *net);
 unsigned int mptcp_get_add_addr_timeout(struct net *net);
 void mptcp_subflow_fully_established(struct mptcp_subflow_context *subflow,
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 1ca0c82b0dbde..721059916c968 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -18,12 +18,15 @@
 #include <net/tcp.h>
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
 #include <net/ip6_route.h>
+#include <net/transp_v6.h>
 #endif
 #include <net/mptcp.h>
 #include <uapi/linux/mptcp.h>
 #include "protocol.h"
 #include "mib.h"
 
+static void mptcp_subflow_ops_undo_override(struct sock *ssk);
+
 static void SUBFLOW_REQ_INC_STATS(struct request_sock *req,
 				  enum linux_mptcp_mib_field field)
 {
@@ -428,6 +431,7 @@ static int subflow_v4_conn_request(struct sock *sk, struct sk_buff *skb)
 static struct tcp_request_sock_ops subflow_request_sock_ipv6_ops;
 static struct inet_connection_sock_af_ops subflow_v6_specific;
 static struct inet_connection_sock_af_ops subflow_v6m_specific;
+static struct proto tcpv6_prot_override;
 
 static int subflow_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 {
@@ -509,6 +513,8 @@ static void subflow_ulp_fallback(struct sock *sk,
 	icsk->icsk_ulp_ops = NULL;
 	rcu_assign_pointer(icsk->icsk_ulp_data, NULL);
 	tcp_sk(sk)->is_mptcp = 0;
+
+	mptcp_subflow_ops_undo_override(sk);
 }
 
 static void subflow_drop_ctx(struct sock *ssk)
@@ -682,6 +688,7 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 }
 
 static struct inet_connection_sock_af_ops subflow_specific;
+static struct proto tcp_prot_override;
 
 enum mapping_status {
 	MAPPING_OK,
@@ -1203,6 +1210,25 @@ static void mptcp_attach_cgroup(struct sock *parent, struct sock *child)
 #endif /* CONFIG_SOCK_CGROUP_DATA */
 }
 
+static void mptcp_subflow_ops_override(struct sock *ssk)
+{
+#if IS_ENABLED(CONFIG_MPTCP_IPV6)
+	if (ssk->sk_prot == &tcpv6_prot)
+		ssk->sk_prot = &tcpv6_prot_override;
+	else
+#endif
+		ssk->sk_prot = &tcp_prot_override;
+}
+
+static void mptcp_subflow_ops_undo_override(struct sock *ssk)
+{
+#if IS_ENABLED(CONFIG_MPTCP_IPV6)
+	if (ssk->sk_prot == &tcpv6_prot_override)
+		ssk->sk_prot = &tcpv6_prot;
+	else
+#endif
+		ssk->sk_prot = &tcp_prot;
+}
 int mptcp_subflow_create_socket(struct sock *sk, struct socket **new_sock)
 {
 	struct mptcp_subflow_context *subflow;
@@ -1258,6 +1284,7 @@ int mptcp_subflow_create_socket(struct sock *sk, struct socket **new_sock)
 	*new_sock = sf;
 	sock_hold(sk);
 	subflow->conn = sk;
+	mptcp_subflow_ops_override(sf->sk);
 
 	return 0;
 }
@@ -1274,6 +1301,7 @@ static struct mptcp_subflow_context *subflow_create_ctx(struct sock *sk,
 
 	rcu_assign_pointer(icsk->icsk_ulp_data, ctx);
 	INIT_LIST_HEAD(&ctx->node);
+	INIT_LIST_HEAD(&ctx->delegated_node);
 
 	pr_debug("subflow=%p", ctx);
 
@@ -1386,6 +1414,7 @@ static void subflow_ulp_release(struct sock *ssk)
 		sock_put(sk);
 	}
 
+	mptcp_subflow_ops_undo_override(ssk);
 	if (release)
 		kfree_rcu(ctx, rcu);
 }
@@ -1439,6 +1468,16 @@ static void subflow_ulp_clone(const struct request_sock *req,
 	}
 }
 
+static void tcp_release_cb_override(struct sock *ssk)
+{
+	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
+
+	if (mptcp_subflow_has_delegated_action(subflow))
+		mptcp_subflow_process_delegated(ssk);
+
+	tcp_release_cb(ssk);
+}
+
 static struct tcp_ulp_ops subflow_ulp_ops __read_mostly = {
 	.name		= "mptcp",
 	.owner		= THIS_MODULE,
@@ -1479,6 +1518,9 @@ void __init mptcp_subflow_init(void)
 	subflow_specific.syn_recv_sock = subflow_syn_recv_sock;
 	subflow_specific.sk_rx_dst_set = subflow_finish_connect;
 
+	tcp_prot_override = tcp_prot;
+	tcp_prot_override.release_cb = tcp_release_cb_override;
+
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
 	subflow_request_sock_ipv6_ops = tcp_request_sock_ipv6_ops;
 	subflow_request_sock_ipv6_ops.route_req = subflow_v6_route_req;
@@ -1494,6 +1536,9 @@ void __init mptcp_subflow_init(void)
 	subflow_v6m_specific.net_header_len = ipv4_specific.net_header_len;
 	subflow_v6m_specific.mtu_reduced = ipv4_specific.mtu_reduced;
 	subflow_v6m_specific.net_frag_header_len = 0;
+
+	tcpv6_prot_override = tcpv6_prot;
+	tcpv6_prot_override.release_cb = tcp_release_cb_override;
 #endif
 
 	mptcp_diag_subflow_init(&subflow_ulp_ops);
-- 
2.26.2

Re: [PATCH v2 net-next 5/5] mptcp: implement delegated actions

[Jakub Kicinski]

On Wed, 20 Jan 2021 15:39:14 +0100 Paolo Abeni wrote:
> On MPTCP-level ack reception, the packet scheduler
> may select a subflow other then the current one.
> 
> Prior to this commit we rely on the workqueue to trigger
> action on such subflow.
> 
> This changeset introduces an infrastructure that allows
> any MPTCP subflow to schedule actions (MPTCP xmit) on
> others subflows without resorting to (multiple) process
> reschedule.

If your work doesn't reschedule there should not be multiple 
rescheds, no?

> A dummy NAPI instance is used instead. When MPTCP needs to
> trigger action an a different subflow, it enqueues the target
> subflow on the NAPI backlog and schedule such instance as needed.
> 
> The dummy NAPI poll method walks the sockets backlog and tries
> to acquire the (BH) socket lock on each of them. If the socket
> is owned by the user space, the action will be completed by
> the sock release cb, otherwise push is started.
> 
> This change leverages the delegated action infrastructure
> to avoid invoking the MPTCP worker to spool the pending data,
> when the packet scheduler picks a subflow other then the one
> currently processing the incoming MPTCP-level ack.
> 
> Additionally we further refine the subflow selection
> invoking the packet scheduler for each chunk of data
> even inside __mptcp_subflow_push_pending().

Is there much precedence for this sort of hijacking of NAPI 
for protocol work? Do you need it because of locking?

Re: [PATCH v2 net-next 5/5] mptcp: implement delegated actions

[Paolo Abeni]

Hi,

On Thu, 2021-01-21 at 17:34 -0800, Jakub Kicinski wrote:
> On Wed, 20 Jan 2021 15:39:14 +0100 Paolo Abeni wrote:
> > On MPTCP-level ack reception, the packet scheduler
> > may select a subflow other then the current one.
> > 
> > Prior to this commit we rely on the workqueue to trigger
> > action on such subflow.
> > 
> > This changeset introduces an infrastructure that allows
> > any MPTCP subflow to schedule actions (MPTCP xmit) on
> > others subflows without resorting to (multiple) process
> > reschedule.
> 
> If your work doesn't reschedule there should not be multiple 
> rescheds, no?

Thank you for looking into this.

With the workqueue it would be:

<running process> -> BH -> (process scheduler) -> MPTCP workqueue ->
(process scheduler) -> <running process>

With this infra is:

<running process> -> BH -> BH -> <running process>

> > A dummy NAPI instance is used instead. When MPTCP needs to
> > trigger action an a different subflow, it enqueues the target
> > subflow on the NAPI backlog and schedule such instance as needed.
> > 
> > The dummy NAPI poll method walks the sockets backlog and tries
> > to acquire the (BH) socket lock on each of them. If the socket
> > is owned by the user space, the action will be completed by
> > the sock release cb, otherwise push is started.
> > 
> > This change leverages the delegated action infrastructure
> > to avoid invoking the MPTCP worker to spool the pending data,
> > when the packet scheduler picks a subflow other then the one
> > currently processing the incoming MPTCP-level ack.
> > 
> > Additionally we further refine the subflow selection
> > invoking the packet scheduler for each chunk of data
> > even inside __mptcp_subflow_push_pending().
> 
> Is there much precedence for this sort of hijacking of NAPI 
> for protocol work? 

AFAICS, xfrm is using a similar trick in the receive path.

Note that we uses TX-only NAPIs, so this does not pollute the napi hash
table.

> Do you need it because of locking?

This infrastructure is used to avoid the workqueue usage in the MPTCP
receive path (to push pending data). With many mptcp connections
established that would be very bad for tput and latency. This
infrastructure is not strictly needed from a functinal PoV, but I was
unable to find any other way to avoid the workqueue usage.

Please let me know if the above is clear enough!

Thanks,

Paolo

Re: [PATCH v2 net-next 5/5] mptcp: implement delegated actions

[Jakub Kicinski]

On Fri, 22 Jan 2021 09:25:07 +0100 Paolo Abeni wrote:
> On Thu, 2021-01-21 at 17:34 -0800, Jakub Kicinski wrote:
> > On Wed, 20 Jan 2021 15:39:14 +0100 Paolo Abeni wrote:  
> > > On MPTCP-level ack reception, the packet scheduler
> > > may select a subflow other then the current one.
> > > 
> > > Prior to this commit we rely on the workqueue to trigger
> > > action on such subflow.
> > > 
> > > This changeset introduces an infrastructure that allows
> > > any MPTCP subflow to schedule actions (MPTCP xmit) on
> > > others subflows without resorting to (multiple) process
> > > reschedule.  
> > 
> > If your work doesn't reschedule there should not be multiple 
> > rescheds, no?  
> 
> Thank you for looking into this.
> 
> With the workqueue it would be:
> 
> <running process> -> BH -> (process scheduler) -> MPTCP workqueue ->
> (process scheduler) -> <running process>
> 
> With this infra is:
> 
> <running process> -> BH -> BH -> <running process>

You assume your process is running when you got the Rx IRQ but also
is RPS enabled? For MPTCP there is no way to do reuseport to match Rx
CPU to processing thread, right?

> > > A dummy NAPI instance is used instead. When MPTCP needs to
> > > trigger action an a different subflow, it enqueues the target
> > > subflow on the NAPI backlog and schedule such instance as needed.
> > > 
> > > The dummy NAPI poll method walks the sockets backlog and tries
> > > to acquire the (BH) socket lock on each of them. If the socket
> > > is owned by the user space, the action will be completed by
> > > the sock release cb, otherwise push is started.
> > > 
> > > This change leverages the delegated action infrastructure
> > > to avoid invoking the MPTCP worker to spool the pending data,
> > > when the packet scheduler picks a subflow other then the one
> > > currently processing the incoming MPTCP-level ack.
> > > 
> > > Additionally we further refine the subflow selection
> > > invoking the packet scheduler for each chunk of data
> > > even inside __mptcp_subflow_push_pending().  
> > 
> > Is there much precedence for this sort of hijacking of NAPI 
> > for protocol work?   
> 
> AFAICS, xfrm is using a similar trick in the receive path.
> 
> Note that we uses TX-only NAPIs, so this does not pollute the napi hash
> table.

Ack, I think what you have should work with busy polling and threaded
IRQs as well. Just another piece of the puzzle to keep in one's head
when thinking about NAPI processing :)

> > Do you need it because of locking?  
> 
> This infrastructure is used to avoid the workqueue usage in the MPTCP
> receive path (to push pending data). With many mptcp connections
> established that would be very bad for tput and latency. This
> infrastructure is not strictly needed from a functinal PoV, but I was
> unable to find any other way to avoid the workqueue usage.

But it is due to locking or is it not? Because you're running the
callback in the same context, so otherwise why not just call the
function directly? Can't be batching, it's after GRO so we won't 
batch much more.

> Please let me know if the above is clear enough!

Sure thing, it just looks like interesting work. I'll apply end 
of the day if nobody has any comments.

Re: [PATCH v2 net-next 0/5] mptcp: re-enable sndbuf autotune

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net-next.git (refs/heads/master):

On Wed, 20 Jan 2021 15:39:09 +0100 you wrote:
> The sendbuffer autotuning was unintentionally disabled as a
> side effect of the recent workqueue removal refactor. These
> patches re-enable id, with some extra care: with autotuning
> enable/large send buffer we need a more accurate packet
> scheduler to be able to use efficiently the available
> subflow bandwidth, especially when the subflows have
> different capacities.
> 
> [...]

Here is the summary with links:
  - [v2,net-next,1/5] mptcp: always graft subflow socket to parent
    https://git.kernel.org/netdev/net-next/c/866f26f2a9c3
  - [v2,net-next,2/5] mptcp: re-enable sndbuf autotune
    https://git.kernel.org/netdev/net-next/c/5cf92bbadc58
  - [v2,net-next,3/5] mptcp: do not queue excessive data on subflows
    https://git.kernel.org/netdev/net-next/c/ec369c3a337f
  - [v2,net-next,4/5] mptcp: schedule work for better snd subflow selection
    https://git.kernel.org/netdev/net-next/c/40dc9416cc95
  - [v2,net-next,5/5] mptcp: implement delegated actions
    https://git.kernel.org/netdev/net-next/c/b19bc2945b40

You are awesome, thank you!
--
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

Re: [PATCH v2 net-next 5/5] mptcp: implement delegated actions

[Paolo Abeni]

On Fri, 2021-01-22 at 15:23 -0800, Jakub Kicinski wrote:
> On Fri, 22 Jan 2021 09:25:07 +0100 Paolo Abeni wrote:
> > > Do you need it because of locking?  
> > 
> > This infrastructure is used to avoid the workqueue usage in the MPTCP
> > receive path (to push pending data). With many mptcp connections
> > established that would be very bad for tput and latency. This
> > infrastructure is not strictly needed from a functinal PoV, but I was
> > unable to find any other way to avoid the workqueue usage.
> 
> But it is due to locking or is it not? Because you're running the
> callback in the same context, so otherwise why not just call the
> function directly? Can't be batching, it's after GRO so we won't 
> batch much more.

Thank you for the feedback. 

Let me try to elaborate a bit more on this. When processing the input
packet (MPTCP data ack) on the MPTCP subflow A, under the subflow A
socket lock, we possibly need to push some data via a different subflow
B - depending on the MPTCP packet scheduler decision. We can't try to
acquire the B subflow socket lock due to ABBA deadlock.

Either the workqueue usage and this infra avoid the deadlock breaking
the locks chain.

Should not have any bad iteraction with threaded NAPI nor busy polling,
but I don't have experimented yet. Placing that on my TODO list.

Thanks!

Paolo