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Date:   Mon, 17 Jul 2023 09:49:29 +0200
From:   Marc Kleine-Budde <mkl@pengutronix.de>
To:     Michal Sojka <michal.sojka@cvut.cz>
Cc:     Maxime Jayat <maxime.jayat@mobile-devices.fr>,
        Oliver Hartkopp <socketcan@hartkopp.net>,
        linux-can@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        "Dae R. Jeong" <threeearcat@gmail.com>,
        Hillf Danton <hdanton@sina.com>
Subject: Re: can: isotp: epoll breaks isotp_sendmsg
Message-ID: <20230717-disbelief-catalyst-bcff471e0433-mkl@pengutronix.de>
References: <11328958-453f-447f-9af8-3b5824dfb041@munic.io>
 <87cz1czihl.fsf@steelpick.2x.cz>
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On 01.07.2023 00:35:18, Michal Sojka wrote:
> Hi Maxime,
>=20
> On Fri, Jun 30 2023, Maxime Jayat wrote:
> > Hi,
> >
> > There is something not clear happening with the non-blocking behavior
> > of ISO-TP sockets in the TX path, but more importantly, using epoll now
> > completely breaks isotp_sendmsg.
> > I believe it is related to
> > 79e19fa79c ("can: isotp: isotp_ops: fix poll() to not report false=20
> > EPOLLOUT events"),
> > but actually is probably deeper than that.
> >
> > I don't completely understand what is exactly going on, so I am sharing
> > the problem I face:
> >
> > With an ISO-TP socket in non-blocking mode, using epoll seems to make
> > isotp_sendmsg always return -EAGAIN.
>=20
> That's definitely not expected behavior. I tested the patch only with
> poll, hoping that epoll would behave the same.
>=20
> [...]
>=20
> >
> > By reverting 79e19fa79c, I get better results but still incorrect:
>=20
> [...]
>=20
> > It is then possible to write on the socket but the write is blocking,
> > which is not the expected behavior for a non-blocking socket.
>=20
> Yes, incorrect behavior was why we made the commit in question, however
> we saw write() returning -EAGAIN when it shouldn't.
>=20
> > I don't know how to solve the problem. To me, using wq_has_sleeper seem=
s=20
> > weird.
>=20
> Agreed. I've never tried to understand how synchronization works here.
> Hopefully, Oliver knows more.
>=20
> > The implementation of isotp_poll feels weird too (calling both=20
> > datagram_poll and
> > poll_wait?). But I am not sure what would be the correct
> > implementation.
>=20
> I understand it as follows (which might be wrong - someone, please
> correct me), isotp_poll() should register the file with all waitqueues
> it can wait on. so->wait is one and sock->sq.wait (used by
> datagram_poll) is another. The former is definitely used for TX, the
> latter is probably used because skb_recv_datagram() is called for RX.
> But so->wait is also used for RX and there might proabbly be be some
> inconsistency between those.
>=20
> > My actual use-case is in Async Rust using tokio.
>=20
> Our initial motivation was also Rust and tokio however than I did
> testing only with simple C programs. I'm definitely interested in having
> this working.
>=20
> I'll try to look at this in more detail during the weekend. It's too
> late for me today.

Any progress on this issue?

Marc

--=20
Pengutronix e.K.                 | Marc Kleine-Budde          |
Embedded Linux                   | https://www.pengutronix.de |
Vertretung N=C3=BCrnberg              | Phone: +49-5121-206917-129 |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-9   |

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