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    SubjectRe: [PATCH net-next v2] net/smc: align the connect behaviour with TCP
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    On 24/05/2022 04:59, Guangguan Wang wrote:
    >
    >
    > On 2022/5/23 20:24, Karsten Graul wrote:
    >> On 13/05/2022 04:24, Guangguan Wang wrote:
    >>> Connect with O_NONBLOCK will not be completed immediately
    >>> and returns -EINPROGRESS. It is possible to use selector/poll
    >>> for completion by selecting the socket for writing. After select
    >>> indicates writability, a second connect function call will return
    >>> 0 to indicate connected successfully as TCP does, but smc returns
    >>> -EISCONN. Use socket state for smc to indicate connect state, which
    >>> can help smc aligning the connect behaviour with TCP.
    >>>
    >>> Signed-off-by: Guangguan Wang <guangguan.wang@linux.alibaba.com>
    >>> Acked-by: Karsten Graul <kgraul@linux.ibm.com>
    >>> ---
    >>> net/smc/af_smc.c | 50 ++++++++++++++++++++++++++++++++++++++++++++----
    >>> 1 file changed, 46 insertions(+), 4 deletions(-)
    >>>
    >>> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
    >>> index fce16b9d6e1a..5f70642a8044 100644
    >>> --- a/net/smc/af_smc.c
    >>> +++ b/net/smc/af_smc.c
    >>> @@ -1544,9 +1544,29 @@ static int smc_connect(struct socket *sock, struct sockaddr *addr,
    >>> goto out_err;
    >>>
    >>> lock_sock(sk);
    >>> + switch (sock->state) {
    >>> + default:
    >>> + rc = -EINVAL;
    >>> + goto out;
    >>> + case SS_CONNECTED:
    >>> + rc = sk->sk_state == SMC_ACTIVE ? -EISCONN : -EINVAL;
    >>> + goto out;
    >>> + case SS_CONNECTING:
    >>> + if (sk->sk_state == SMC_ACTIVE)
    >>> + goto connected;
    >>
    >> I stumbled over this when thinking about the fallback processing. If for whatever reason
    >> fallback==true during smc_connect(), the "if (smc->use_fallback)" below would set sock->state
    >> to e.g. SS_CONNECTED. But in the fallback case sk_state keeps SMC_INIT. So during the next call
    >> the SS_CONNECTING case above would break because sk_state in NOT SMC_ACTIVE, and we would end
    >> up calling kernel_connect() again. Which seems to be no problem when kernel_connect() returns
    >> -EISCONN and we return this to the caller. But is this how it should work, or does it work by chance?
    >>
    >
    > Since the sk_state keeps SMC_INIT and does not correctly indicate the state of clcsock, it should end
    > up calling kernel_connect() again to get the actual connection state of clcsock.
    >
    > And I'm sorry there is a problem that if sock->state==SS_CONNECTED and sk_state==SMC_INIT, further call
    > of smc_connect will return -EINVAL where -EISCONN is preferred.
    > The steps to reproduce:
    > 1)switch fallback before connect, such as setsockopt TCP_FASTOPEN
    > 2)connect with noblocking and returns -EINPROGRESS. (sock->state changes to SS_CONNECTING)
    > 3) end up calling connect with noblocking again and returns 0. (kernel_connect() returns 0 and sock->state changes to
    > SS_CONNECTED but sk->sk_state stays SMC_INIT)
    > 4) call connect again, maybe by mistake, will return -EINVAL, but -EISCONN is preferred.
    >
    > What do you think about if we synchronize the sk_state to SMC_ACTIVE instead of keeping SMC_INIT when clcsock
    > connected successfully in fallback case described above.
    >
    > ...

    I start thinking that the fix in 86434744 introduced a problem. Before that fix a connect with
    fallback always reached __smc_connect() and on top of that function in case of fallback
    smc_connect_fallback() is called, which itself sets sk_state to SMC_ACTIVE.

    86434744 removed that code path and I wonder what it actually fixed, because at this time the
    fallback check in __smc_connect() was already present.

    Without that "goto out;" the state would be set correctly in smc_connect_fallback(), and the
    socket close processing would work as expected.

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