• Sowmini Varadhan's avatar
    RDS: TCP: Force every connection to be initiated by numerically smaller IP address · 1a0e100f
    Sowmini Varadhan authored
    When 2 RDS peers initiate an RDS-TCP connection simultaneously,
    there is a potential for "duelling syns" on either/both sides.
    See commit 241b2719 ("RDS-TCP: Reset tcp callbacks if re-using an
    outgoing socket in rds_tcp_accept_one()") for a description of this
    condition, and the arbitration logic which ensures that the
    numerically large IP address in the TCP connection is bound to the
    RDS_TCP_PORT ("canonical ordering").
    
    The rds_connection should not be marked as RDS_CONN_UP until the
    arbitration logic has converged for the following reason. The sender
    may start transmitting RDS datagrams as soon as RDS_CONN_UP is set,
    and since the sender removes all datagrams from the rds_connection's
    cp_retrans queue based on TCP acks. If the TCP ack was sent from
    a tcp socket that got reset as part of duel aribitration (but
    before data was delivered to the receivers RDS socket layer),
    the sender may end up prematurely freeing the datagram, and
    the datagram is no longer reliably deliverable.
    
    This patch remedies that condition by making sure that, upon
    receipt of 3WH completion state change notification of TCP_ESTABLISHED
    in rds_tcp_state_change, we mark the rds_connection as RDS_CONN_UP
    if, and only if, the IP addresses and ports for the connection are
    canonically ordered. In all other cases, rds_tcp_state_change will
    force an rds_conn_path_drop(), and rds_queue_reconnect() on
    both peers will restart the connection to ensure canonical ordering.
    
    A side-effect of enforcing this condition in rds_tcp_state_change()
    is that rds_tcp_accept_one_path() can now be refactored for simplicity.
    It is also no longer possible to encounter an RDS_CONN_UP connection in
    the arbitration logic in rds_tcp_accept_one().
    Signed-off-by: default avatarSowmini Varadhan <sowmini.varadhan@oracle.com>
    Acked-by: default avatarSantosh Shilimkar <santosh.shilimkar@oracle.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    1a0e100f
connection.c 19.5 KB