Clarification on cancellation

[vasama]

The documentation on cancellation seems a little lacking.

• [ ] 1. How do IORING_OP_ASYNC_CANCEL and IORING_OP_LINK_TIMEOUT affect the result of the target operation?
• [x] 2. What is the effect of IORING_OP_ASYNC_CANCEL when there are multiple operations in flight with the specified user data?
• [ ] 3. How is an IORING_OP_LINK_TIMEOUT operation affected by the successful cancellation via IORING_OP_ASYNC_CANCEL of the operation to which it is linked?
• [ ] 4. What is the effect of IORING_OP_LINK_TIMEOUT on subsequent operations in a chain, after the successful completion of the operation to which it is linked?

[cmazakas]

I've actually wondered about these things myself.

Most specifically:

What is the effect of IORING_OP_ASYNC_CANCEL when there are multiple operations in flight with the specified user data?

An answer to this would be really appreciated.

[axboe]

It depends on if match all is set or not. If set, it'll match all of them and try to cancel them. If not, it'll match the first one it finds.

[davidzeng0]

An area of note is cancelling a poll request.

Using IORING_OP_POLL_REMOVE, the poll request returns, then the cancel request. Using IORING_OP_ASYNC_CANCEL, the cancel request returns, then the poll request. (if I remember correctly, I may have the order mixed up)

I was wondering if this was intentional. I would guess that "async" means the request being cancelled finishes asynchronously so it returns after.

[DylanZA]

I suspect this is going to be an implementation detail, and subject to different states of the ring

I wouldnt make any assumptions on the ordering of completions for different requests