Re: Feature request: Please implement IORING_OP_TEE

[Jens Axboe <axboe@kernel.dk>]

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:

- 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.

-- 
Jens Axboe