Re: Feature request: Please implement IORING_OP_TEE

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

On 27/04/2020 23:02, Jens Axboe wrote:
> On 4/27/20 12:22 PM, Jann Horn wrote:
>> On Mon, Apr 27, 2020 at 5:56 PM Jens Axboe <axboe@kernel.dk> wrote:
>>> On 4/27/20 9:40 AM, Clay Harris wrote:
>>>> I was excited to see IORING_OP_SPLICE go in, but disappointed that tee
>>>> didn't go in at the same time.  It would be very useful to copy pipe
>>>> buffers in an async program.
>>>
>>> Pavel, care to wire up tee? From a quick look, looks like just exposing
>>> do_tee() and calling that, so should be trivial.
>>
>> Just out of curiosity:
>>
>> What's the purpose of doing that via io_uring? Non-blocking sys_tee()
>> just shoves around some metadata, it doesn't do any I/O, right? Is
>> this purely for syscall-batching reasons? (And does that mean that you
>> would also add syscalls like epoll_wait() and futex() to io_uring?) Or
>> is this because you're worried about blocking on the pipe mutex?
> 
> Right, it doesn't do any IO. It does potentially block on the inode
> mutex, but that's about it. I think the reasons are mainly:

Good catch, the waiting probably can happen with splice as well.
I need to read it through, but looks strange that it just ignores O_NONBLOCK,
is there some upper bound for holding it or something?

> 
> - Keep the interfaces the same, instead of using both sync and async
>   calls.
> - Bundling/batch reasons, either in same submission, or chained.
> 
> Some folks have talked about futex, and epoll_wait would also be a
> natural extension as well, since we already have the ctl part.

-- 
Pavel Begunkov