Re: [v6.4 Regression] rust/io_uring: tests::net::test_tcp_recv_multi hangs

[Jens Axboe <axboe@kernel.dk>]

On 10/19/23 2:06 AM, Ming Lei wrote:
> Hello Jens,
> 
> Guang Wu found that tests::net::test_tcp_recv_multi in rust:io_uring
> hangs, and no such issue in RH test kernel.
> 
> - git clone https://github.com/tokio-rs/io-uring.git
> - cd io-uring
> - cargo run --package io-uring-test
> 
> I figured out that it is made by missing the last CQE with -ENOBUFS,
> which is caused by commit a2741c58ac67 ("io_uring/net: don't retry recvmsg()
> unnecessarily").
> 
> I am not sure if the last CQE should be returned and that depends how normal
> recv_multi is written, but IORING_CQE_F_MORE in the previous CQE shouldn't be
> returned at least.

Is this because it depends on this spurious retry? IOW, it adds N
buffers and triggers N receives, then depends on an internal extra retry
which would then yield -ENOBUFS? Because that sounds like a broken test.
As long as the recv triggers successfully, IORING_CQE_F_MORE will be
set. Only if it his some terminating condition would it trigger a CQE
without the MORE flag set. If it remains armed and ready to trigger
again, it will have MORE set. I'll take a look, this is pure guesswork
on my side right now.

We've done quite a lot of testing with recv multishot with this change,
and haven't had any issues. Which is why I'm a bit skeptical.

-- 
Jens Axboe