• Jon Paul Maloy's avatar
    tipc: resolve connection flow control compatibility problem · 6998cc6e
    Jon Paul Maloy authored
    In commit 10724cc7 ("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: default avatarJon Maloy <jon.maloy@ericsson.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    6998cc6e
socket.c 71.8 KB