Re: [PATCH] io_uring: consistently use rcu semantics with sqpoll thread

[Jens Axboe <axboe@kernel.dk>]

On 6/10/25 2:20 PM, Keith Busch wrote:
> On Tue, Jun 10, 2025 at 02:04:41PM -0600, Jens Axboe wrote:
>> On 6/10/25 1:30 PM, Keith Busch wrote:
>>> From: Keith Busch <kbusch@kernel.org>
>>>
>>> It is already dereferenced with rcu read protection, so it needs to be
>>> annotated as such, and consistently use rcu helpers for access and
>>> assignment.
>>
>> There are some bits in io_uring.c that access it, which probably need
>> some attention too I think. One of them a bit trickier.
> 
> Oh, sure is. I just ran 'make C=1' on the originally affected files, but
> should have ran it on all of io_uring/.
> 
> I think the below should clear up the new warnings. I think it's safe to
> hold the rcu read lock for the tricky one as io_wq_cancel_cb() doesn't
> appear to make any blocking calls.

It _probably_ is, but that's entirely untested. Right now it looks fine,
for a variety of reasons like submitting work that's marked cancel
should not be doing anything with it really. But it doesn't feel me with
joy, particularly as only the somewhat uncommon SQPOLL is the one that
will do it.

The io_sq_thread_park() parts in the patch also look broken, as an
rcu_access_pointer() is being passed into wake_up_process(). It should
all be fine, but it's now a case of instrumentation actively making the
code more confusing to read.

I think we might be better off leaving the sparse warnings and doing a
proper io_sq_data accessor thing for this, rather than try and paper
over the sparse warnings.

-- 
Jens Axboe