Question about asynchronous blk-mq tag waiting for io_uring

[Maks <mi9992828@gmail.com>]

From: Maks Ivanov <mi9992828@gmail.com>

Hi,

I read your comment in liburing issue #1492 about callback-based retry
when storage tags are exhausted:

https://github.com/axboe/liburing/issues/1492#issuecomment-4696489936

I'm considering this direction for my bachelor's thesis and would like
to clarify its scope and design.

The initial scope would be io_uring direct reads on raw block devices.
For requests without explicit RWF_NOWAIT, the mechanism would register
a continuation when tag allocation cannot complete immediately, then
retry submission when a tag becomes available.
Explicit RWF_NOWAIT requests would retain their existing
behaviour when tag allocation would require waiting.

Could you clarify a few points?

- Is this still an open direction worth investigating?
- Is there related work or an existing approach I should look at first?
- Which layer should own the continuation: io_uring, the block submission
  path, or blk-mq?
- What lifetime and cancellation constraints would such a mechanism need
  to satisfy?

I would also appreciate pointers to other open problems in io_uring or
the block layer that could suit a focused implementation and validation
project. Existing bug reports, design discussions, or TODOs would be
very helpful.

Thanks,
Maks Ivanov