netdev - Re: [PATCH net-next 3/3] net/tcp-fastopen: Add new API support

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Date:   Mon, 23 Jan 2017 23:01:21 +0100
From:   Willy Tarreau <w@....eu>
To:     Wei Wang <weiwan@...gle.com>
Cc:     Wei Wang <tracywwnj@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next 3/3] net/tcp-fastopen: Add new API support

On Mon, Jan 23, 2017 at 10:37:32PM +0100, Willy Tarreau wrote:
> On Mon, Jan 23, 2017 at 01:28:53PM -0800, Wei Wang wrote:
> > Hi Willy,
> > 
> > True. If you call connect() multiple times on a socket which already has
> > cookie without a write(), the second and onward connect() call will return
> > EINPROGRESS.
> > It is basically because the following code block in __inet_stream_connect()
> > can't distinguish if it is the first time connect() is called or not:
> > 
> > case SS_CONNECTING:
> >                 if (inet_sk(sk)->defer_connect)  <----- defer_connect will
> > be 0 only after a write() is called
> >                         err = -EINPROGRESS;
> >                 else
> >                         err = -EALREADY;
> >                 /* Fall out of switch with err, set for this state */
> >                 break;
> 
> Ah OK that totally makes sense, thanks for the explanation!
> 
> > I guess we can add some extra logic here to address this issue. So the
> > second connect() and onwards will return EALREADY.

Thinking about it a bit more, I really think it would make more
sense to return -EISCONN here if we want to match the semantics
of a connect() returning zero on the first call. This way the
caller knows it can write whenever it wants and can disable
write polling until needed.

I'm currently rebuilding a kernel with this change to see if it
behaves any better :

-                        err = -EINPROGRESS;
+                        err = -EISCONN;

I'll keep you updated.

Thanks,
Willy