Re: [RFC PATCH v4 13/16] io_uring: add io_recvzc request

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/14/24 16:14, Jens Axboe wrote:
[...]
>>>> @@ -1053,6 +1058,85 @@ struct io_zc_rx_ifq *io_zc_verify_sock(struct io_kiocb *req,
>>>>        return ifq;
>>>>    }
>>>>    +int io_recvzc_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>>> +{
>>>> +    struct io_recvzc *zc = io_kiocb_to_cmd(req, struct io_recvzc);
>>>> +
>>>> +    /* non-iopoll defer_taskrun only */
>>>> +    if (!req->ctx->task_complete)
>>>> +        return -EINVAL;
>>>
>>> What's the reasoning behind this?
>>
>> CQ locking, see the comment a couple lines below
> 
> My question here was more towards "is this something we want to do".
> Maybe this is just a temporary work-around and it's nothing to discuss,
> but I'm not sure we want to have opcodes only work on certain ring
> setups.

I don't think it's that unreasonable restricting it. It's hard to
care about !DEFER_TASKRUN for net workloads, it makes CQE posting a bit
cleaner, and who knows where the single task part would become handy.
Thinking about ifq termination, which should better cancel and wait
for all corresponding zc requests, it's should be easier without
parallel threads. E.g. what if another thread is in the enter syscall
using ifq, or running task_work and not cancellable. Then apart
from (non-atomic) refcounting, we'd need to somehow wait for it,
doing wake ups on the zc side, and so on.

The CQ side is easy to support though, put conditional locking
around the posting like fill/post_cqe does with the todays
patchset.

-- 
Pavel Begunkov