On 7/7/20 10:29 AM, Xiaoguang Wang wrote:
> hi,
>
>> On 7/7/20 7:24 AM, Xiaoguang Wang wrote:
>>> For those applications which are not willing to use io_uring_enter()
>>> to reap and handle cqes, they may completely rely on liburing's
>>> io_uring_peek_cqe(), but if cq ring has overflowed, currently because
>>> io_uring_peek_cqe() is not aware of this overflow, it won't enter
>>> kernel to flush cqes, below test program can reveal this bug:
>>>
>>> static void test_cq_overflow(struct io_uring *ring)
>>> {
>>> struct io_uring_cqe *cqe;
>>> struct io_uring_sqe *sqe;
>>> int issued = 0;
>>> int ret = 0;
>>>
>>> do {
>>> sqe = io_uring_get_sqe(ring);
>>> if (!sqe) {
>>> fprintf(stderr, "get sqe failed\n");
>>> break;;
>>> }
>>> ret = io_uring_submit(ring);
>>> if (ret <= 0) {
>>> if (ret != -EBUSY)
>>> fprintf(stderr, "sqe submit failed: %d\n", ret);
>>> break;
>>> }
>>> issued++;
>>> } while (ret > 0);
>>> assert(ret == -EBUSY);
>>>
>>> printf("issued requests: %d\n", issued);
>>>
>>> while (issued) {
>>> ret = io_uring_peek_cqe(ring, &cqe);
>>> if (ret) {
>>> if (ret != -EAGAIN) {
>>> fprintf(stderr, "peek completion failed: %s\n",
>>> strerror(ret));
>>> break;
>>> }
>>> printf("left requets: %d\n", issued);
>>> continue;
>>> }
>>> io_uring_cqe_seen(ring, cqe);
>>> issued--;
>>> printf("left requets: %d\n", issued);
>>> }
>>> }
>>>
>>> int main(int argc, char *argv[])
>>> {
>>> int ret;
>>> struct io_uring ring;
>>>
>>> ret = io_uring_queue_init(16, &ring, 0);
>>> if (ret) {
>>> fprintf(stderr, "ring setup failed: %d\n", ret);
>>> return 1;
>>> }
>>>
>>> test_cq_overflow(&ring);
>>> return 0;
>>> }
>>>
>>> To fix this issue, export cq overflow status to userspace, then
>>> helper functions() in liburing, such as io_uring_peek_cqe, can be
>>> aware of this cq overflow and do flush accordingly.
>>
>> Is there any way we can accomplish the same without exporting
>> another set of flags?
> I understand your concerns and will try to find some better methods
> later, but not sure there're some better :)
Ignore this one and look at the one I just sent, hopefully that'll be
better for you.
--
Jens Axboe
