Re: [PATCH for-next 5/7] io_uring: post msg_ring CQE in task context

[Dylan Yudaken <dylany@meta.com>]

On Mon, 2022-12-05 at 04:53 -0700, Jens Axboe wrote:
> On 12/4/22 7:44?PM, Pavel Begunkov wrote:
> > We want to limit post_aux_cqe() to the task context when -
> > >task_complete
> > is set, and so we can't just deliver a IORING_OP_MSG_RING CQE to
> > another
> > thread. Instead of trying to invent a new delayed CQE posting
> > mechanism
> > push them into the overflow list.
> 
> This is really the only one out of the series that I'm not a big fan
> of.
> If we always rely on overflow for msg_ring, then that basically
> removes
> it from being usable in a higher performance setting.
> 
> The natural way to do this would be to post the cqe via task_work for
> the target, ring, but we also don't any storage available for that.
> Might still be better to alloc something ala
> 
> struct tw_cqe_post {
>         struct task_work work;
>         s32 res;
>         u32 flags;
>         u64 user_data;
> }
> 
> and post it with that?
> 

It might work to post the whole request to the target, post the cqe,
and then return the request back to the originating ring via tw for the
msg_ring CQE and cleanup.