Re: [PATCH net-next v2] net/smc: align the connect behaviour with TCP

liuyacan
2022-05-24T12:57:39Z

> >
> >
> > 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.

I think it is OK without that "goto out;". And I guess the purpose of "goto out;" is to avoid calling __smc_connect(),
because it is impossible to establish an rdma channel at this time.