Re: Sending CQE to a different ring

[Artyom Pavlov <newpavlov@gmail.com>]

After thinking about it a bit, I think this approach has one serious 
disadvantage: you lose successful result value of the initial request. 
Imagine we submit IORING_OP_READ and link IORING_OP_WAKEUP_RING to it. 
If the request is completed successfully, both ring1 and ring2 will lose 
number of read bytes.

10.03.2022 07:14, Jens Axboe wrote:
> On 3/9/22 9:03 PM, Jens Axboe wrote:
>> I'll mull over this a bit...
> 
> One idea... You issue the request as you normally would for ring1, and
> you mark that request A with IOSQE_CQE_SKIP_SUCCESS. Then you link an
> IORING_OP_WAKEUP_RING to request A, with the fd for it set to ring2, and
> also mark that with IOSQE_CQE_SKIP_SUCCESS.
> 
> We'd need to have sqe->addr (or whatever field) in the WAKEUP_RING
> request be set to the user_data of request A, so we can propagate it.
> 
> The end result is that ring1 won't see any completions for request A or
> the linked WAKEUP, unless one of them fails. If that happens, then you
> get to process things locally still, but given that this is a weird
> error case, shouldn't affect things in practice. ring2 will get the CQE
> posted once request A has completed, with the user_data filled in from
> request A. Which is really what we care about here, as far as I
> understand.
> 
> This basically works right now with what I posted, and without needing
> to rearchitect a bunch of stuff. And it's pretty efficient. Only thing
> we'd need to add is passing in the target cqe user_data for the WAKEUP
> request. Would just need to be wrapped in something that allows you to
> do this easily, as it would be useful for others too potentially.
>