[PATCH net 1/1] tipc: resolve connection flow control compatibility problem

[PATCH net 1/1] tipc: resolve connection flow control compatibility problem

[Jon Maloy]

In commit 10724cc7bb78 ("tipc: redesign connection-level flow control")
we replaced the previous message based flow control with one based on
1k blocks. In order to ensure backwards compatibility the mechanism
falls back to using message as base unit when it senses that the peer
doesn't support the new algorithm. The default flow control window,
i.e., how many units can be sent before the sender blocks and waits
for an acknowledge (aka advertisement) is 512. This was tested against
the previous version, which uses an acknowledge frequency of on ack per
256 received message, and found to work fine.

However, we missed the fact that versions older than Linux 3.15 use an
acknowledge frequency of 512, which is exactly the limit where a 4.6+
sender will stop and wait for acknowledge. This would also work fine if
it weren't for the fact that if the first sent message on a 4.6+ server
side is an empty SYNACK, this one is also is counted as a sent message,
while it is not counted as a received message on a legacy 3.15-receiver.
This leads to the sender always being one step ahead of the receiver, a
scenario causing the sender to block after 512 sent messages, while the
receiver only has registered 511 read messages. Hence, the legacy
receiver is not trigged to send an acknowledge, with a permanently
blocked sender as result.

We solve this deadlock by simply allowing the sender to send one more
message before it blocks, i.e., by a making minimal change to the
condition used for determining connection congestion.

Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
---
 net/tipc/socket.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index db32777..41f0138 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -186,7 +186,7 @@ static struct tipc_sock *tipc_sk(const struct sock *sk)
 
 static bool tsk_conn_cong(struct tipc_sock *tsk)
 {
-	return tsk->snt_unacked >= tsk->snd_win;
+	return tsk->snt_unacked > tsk->snd_win;
 }
 
 /* tsk_blocks(): translate a buffer size in bytes to number of
-- 
2.7.4

Re: [PATCH net 1/1] tipc: resolve connection flow control compatibility problem

[David Miller]

From: Jon Maloy <jon.maloy@ericsson.com>
Date: Thu, 24 Nov 2016 18:47:07 -0500

> In commit 10724cc7bb78 ("tipc: redesign connection-level flow control")
> we replaced the previous message based flow control with one based on
> 1k blocks. In order to ensure backwards compatibility the mechanism
> falls back to using message as base unit when it senses that the peer
> doesn't support the new algorithm. The default flow control window,
> i.e., how many units can be sent before the sender blocks and waits
> for an acknowledge (aka advertisement) is 512. This was tested against
> the previous version, which uses an acknowledge frequency of on ack per
> 256 received message, and found to work fine.
> 
> However, we missed the fact that versions older than Linux 3.15 use an
> acknowledge frequency of 512, which is exactly the limit where a 4.6+
> sender will stop and wait for acknowledge. This would also work fine if
> it weren't for the fact that if the first sent message on a 4.6+ server
> side is an empty SYNACK, this one is also is counted as a sent message,
> while it is not counted as a received message on a legacy 3.15-receiver.
> This leads to the sender always being one step ahead of the receiver, a
> scenario causing the sender to block after 512 sent messages, while the
> receiver only has registered 511 read messages. Hence, the legacy
> receiver is not trigged to send an acknowledge, with a permanently
> blocked sender as result.
> 
> We solve this deadlock by simply allowing the sender to send one more
> message before it blocks, i.e., by a making minimal change to the
> condition used for determining connection congestion.
> 
> Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>

Applied, thanks Jon.