Re: can: isotp: epoll breaks isotp_sendmsg

[Michal Sojka <michal.sojka@cvut.cz>]

Hi Maxime,

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 
> 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.

That's definitely not expected behavior. I tested the patch only with
poll, hoping that epoll would behave the same.

[...]

>
> By reverting 79e19fa79c, I get better results but still incorrect:

[...]

> 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.

Yes, incorrect behavior was why we made the commit in question, however
we saw write() returning -EAGAIN when it shouldn't.

> I don't know how to solve the problem. To me, using wq_has_sleeper seems 
> weird.

Agreed. I've never tried to understand how synchronization works here.
Hopefully, Oliver knows more.

> The implementation of isotp_poll feels weird too (calling both 
> datagram_poll and
> poll_wait?). But I am not sure what would be the correct
> implementation.

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.

> My actual use-case is in Async Rust using tokio.

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.

I'll try to look at this in more detail during the weekend. It's too
late for me today.

Best regards,
-Michal