Re: IOSQE_IO_LINK vs. short send of SOCK_STREAM

Ming Lei
2023-01-12T03:29:02Z

Hi Stefan and Jens,

Thanks for the help.

BTW, the issue is observed when I write ublk-nbd:

https://github.com/ming1/ubdsrv/commits/nbd

and it isn't completed yet(multiple send sqe chains not serialized
yet), the issue is triggered when writing big chunk data to ublk-nbd.

On Wed, Jan 11, 2023 at 05:32:00PM +0100, Stefan Metzmacher wrote:
> Hi Ming,
>
> > Per my understanding, a short send on SOCK_STREAM should terminate the
> > remainder of the SQE chain built by IOSQE_IO_LINK.
> >
> > But from my observation, this point isn't true when using io_sendmsg or
> > io_sendmsg_zc on TCP socket, and the other remainder of the chain still
> > can be completed after one short send is found. MSG_WAITALL is off.
>
> This is due to legacy reasons, you need pass MSG_WAITALL explicitly
> in order to a retry or an error on a short write...
> It should work for send, sendmsg, sendmsg_zc, recv and recvmsg.

Turns out there is another application bug in which recv sqe may cut in the
send sqe chain.

After the issue is fixed, if MSG_WAITALL is set, short send can't be
observed any more. But if MSG_WAITALL isn't set, short send can be
observed and the send io chain still won't be terminated.

So if MSG_WAITALL is set, will io_uring be responsible for retry in case
of short send, and application needn't to take care of it?

>
> For recv and recvmsg MSG_WAITALL also fails the link for MSG_TRUNC and MSG_CTRUNC.

OK, thanks for the sharing of recvmsg MSG_WAITALL.

Thanks,
Ming