Re: [PATCH v2 0/5] io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER

[Jens Axboe <axboe@kernel.dk>]

On 9/10/25 5:57 AM, Pavel Begunkov wrote:
> On 9/9/25 14:35, Jens Axboe wrote:
>>
>> On Thu, 04 Sep 2025 11:08:57 -0600, Caleb Sander Mateos wrote:
>>> As far as I can tell, setting IORING_SETUP_SINGLE_ISSUER when creating
>>> an io_uring doesn't actually enable any additional optimizations (aside
>>> from being a requirement for IORING_SETUP_DEFER_TASKRUN). This series
>>> leverages IORING_SETUP_SINGLE_ISSUER's guarantee that only one task
>>> submits SQEs to skip taking the uring_lock mutex in the submission and
>>> task work paths.
>>>
>>> [...]
>>
>> Applied, thanks!
>>
>> [1/5] io_uring: don't include filetable.h in io_uring.h
>>        commit: 5d4c52bfa8cdc1dc1ff701246e662be3f43a3fe1
>> [2/5] io_uring/rsrc: respect submitter_task in io_register_clone_buffers()
>>        commit: 2f076a453f75de691a081c89bce31b530153d53b
>> [3/5] io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP_SQPOLL
>>        commit: 6f5a203998fcf43df1d43f60657d264d1918cdcd
>> [4/5] io_uring: factor out uring_lock helpers
>>        commit: 7940a4f3394a6af801af3f2bcd1d491a71a7631d
>> [5/5] io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER
>>        commit: 4cc292a0faf1f0755935aebc9b288ce578d0ced2
> 
> FWIW, from a glance that should be quite broken, there is a bunch of
> bits protected from parallel use by the lock. I described this
> optimisation few years back around when first introduced SINGLE_ISSUER
> and the DEFER_TASKRUN locking model, but to this day think it's not
> worth it as it'll be a major pain for any future changes. It would've
> been more feasible if links wasn't a thing. Though, none of it is
> my problem anymore, and I'm not insisting.

Hmm yes, was actually pondering this last night as well and was going
to take a closer look today as I have a flight coming up. I'll leave
them in there for now just to see if syzbot finds anything, and take
that closer look and see if it's salvageable for now or if we just need
a new revised take on this.

-- 
Jens Axboe