Re: Allow IORING_OP_ASYNC_CANCEL to cancel requests on other rings

[Jens Axboe <axboe@kernel.dk>]

On 7/5/23 10:44?AM, Artyom Pavlov wrote:
> Greetings!
> 
> Right now when I want to cancel request which runs on a different ring
> I have to use IORING_OP_MSG_RING with a special len value. CQEs with
> res equal to this special value get intercepted by my code and
> IORING_OP_ASYNC_CANCEL SQE gets created in the receiver ring with
> user_data taken from the received message. This approach kind of
> works, but not efficient (it requires additional round trip through
> the ring) and somewhat fragile (it relies on lack of collisions
> between the special value and potential error codes).
> 
> I think it should be possible to add support for cancelling requests
> on other rings to IORING_OP_ASYNC_CANCEL by introducing a new flag. If
> the flag is enabled, then the fd field would be interpreted as fd of
> another ring to which cancellation request should be sent. Using the
> fd field would mean that the new flag would conflict with
> IORING_ASYNC_CANCEL_FD, so it could be worth to use a different field
> for receiver ring fd.

This could certainly work, though I think it'd be a good idea to use a
reserved field for the "other ring fd". As of right now, the
'splice_fd_in' descriptor field is not applicable to cancel requests, so
that'd probably be the right place to put it.

Some complications around locking here, as we'd need to grab the other
ring lock. If ring A and ring B both cancel requests for each other,
then there would be ordering concerns. But nothing that can't be worked
around.

Let me take a quick look at that.

-- 
Jens Axboe