Re: [PATCH v3 10/10] selftests/io_uring: add bpf io_uring selftests

[Ming Lei <ming.lei@redhat.com>]

On Wed, Nov 19, 2025 at 07:00:41PM +0000, Pavel Begunkov wrote:
> On 11/14/25 13:08, Ming Lei wrote:
> > On Thu, Nov 13, 2025 at 11:59:47AM +0000, Pavel Begunkov wrote:
> ...
> > > +	bpf_printk("queue nop request, data %lu\n", (unsigned long)reqs_to_run);
> > > +	sqe = &sqes[sq_hdr->tail & (SQ_ENTRIES - 1)];
> > > +	sqe->user_data = reqs_to_run;
> > > +	sq_hdr->tail++;
> > 
> > Looks this way turns io_uring_enter() into pthread-unsafe, does it need to
> > be documented?
> 
> Assuming you mean parallel io_uring_enter() calls modifying the SQ,
> it's not different from how it currently is. If you're sharing an
> io_uring, threads need to sync the use of SQ/CQ.

Please see the example:

thread_fn(struct io_uring *ring)
{
	while (true) {
		pthread_mutex_lock(sqe_mutex);
		sqe = io_uring_get_sqe(ring);
		io_uring_prep_op(sqe);
		pthread_mutex_unlock(sqe_mutex);

		io_uring_enter(ring);

		pthread_mutex_lock(cqe_mutex);
		io_uring_wait_cqe(ring, &cqe);
		io_uring_cqe_seen(ring, cqe);
		pthread_mutex_unlock(cqe_mutex);
	}
}

`thread_fn` is supposed to work concurrently from >1 pthreads:

1) io_uring_enter() is claimed as pthread safe

2) because of userspace lock protection, there is single code path for
producing sqe for SQ at same time, and single code path for consuming sqe
from io_uring_enter().

With bpf controlled io_uring patches, sqe can be produced from io_uring_enter(),
and cqe can be consumed in io_uring_enter() too, there will be race between
bpf prog(producing sqe, or consuming cqe) and userspace lock-protected
code block.


Thanks,
Ming