io_uring_for_each_cqe can overflow the cqe ring with SQPOLL

[MarkReedZ]

A server using for_each_cqe with SQPOLL set will overflow the cqe ring given enough connections / time such that we keep getting new CQEs added onto the end before we complete the loop.

As peek/wait is significantly slower than this loop does anyone have a better method of cq polling in this case? Queue'ing the events and doing a second loop is a 5% slowdown.

    io_uring_for_each_cqe(&uring, head, cqe) {
        handle_read()
        write_response()
    }
    io_uring_cq_advance(&uring, n);

This is the for_each macro. Perhaps saving the tail prior to the loop would be okay?

  for (head = *(ring)->cq.khead;          \
       (cqe = (head != io_uring_smp_load_acquire((ring)->cq.ktail) ? \
    &(ring)->cq.cqes[io_uring_cqe_index(ring, head, (ring)->cq.ring_mask)] : NULL)); \
       head++)  

[MarkReedZ]

I went with breaking out of the loop after N iterations.

[axboe]

I really liked your idea of using a tail assigned at the start of the loop though, I think that's a nice general thing to do and it would solve your issue as well.