From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Date: Tue, 11 Apr 2017 21:51:06 +0000
Subject: Re: SCTP getting EAGAIN but poll or select return socket is writable
Message-Id: <20170411215106.GH911@localhost.localdomain>
List-Id: <linux-sctp.vger.kernel.org>
References: <45198365-50CE-456C-8F4A-8BDA672B97F7@lurchi.franken.de>
In-Reply-To: <45198365-50CE-456C-8F4A-8BDA672B97F7@lurchi.franken.de>
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To: linux-sctp@vger.kernel.org

On Wed, Apr 12, 2017 at 01:54:41AM +0530, Deepak Khandelwal wrote:
> Hi,
> 

Please don't top post..

> 
> App act as client (1-1 Style SOCK_STREAM)
> App try to send 278 bytes of message.

Okay.

> 
> So you mean that, if socket has space for few bytes (say 1 or 2
> bytes), lksctp will set POLLOUT event for fd,
> but since user application doesn;t know the underlying socket space
> available, it can write more than that but will fail with EAGAIN ?

Was reading the code again on this, I actually don't think this can
happen. The sleep on waiting for sndbuf doesn't check for msgsize, just
if there is any amount of buffer available or not.

sctp_sendmsg()
{
...
        timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
        if (!sctp_wspace(asoc)) {
                err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len);
                if (err)
                        goto out_free;
        }
...
}

For returning -EAGAIN due to sndbuf, it has to match !sctp_wspace, which
is only about the soc/asoc and not the message. The EAGAIN in this case
comes from inside sctp_wait_for_sndbuf().

That leads us to my other question, are you binding/connecting this
socket before sending the message?

> 
> 
> Best Regards,
> Deepak
> 
> On Wed, Apr 12, 2017 at 12:44 AM, Michael Tuexen
> <Michael.Tuexen@lurchi.franken.de> wrote:
> >> On 11. Apr 2017, at 21:08, Deepak Khandelwal <dazz.87@gmail.com> wrote:
> >>
> >> Hi,
> >>
> >> I am using 4.4.x kernel.
> >>
> >> in context of LKSCTP, what could be the possible reasons that poll or
> >> select on fd return false positive events, causing application do
> >> write on fd but failed with EAGAIN ?
> > I'm just guessing here... What if you can write something, but not
> > something big as you try...
> > So how big is the message you are trying so send?
> >
> > Best regards
> > Michael
> >>
> >>
> >> Best Regards,
> >> Deepak
> >> --
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