Re: [PATCH v2] io_uring: export cq overflow status to userspace

[Jens Axboe <axboe@kernel.dk>]

On 7/8/20 7:15 PM, 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 by adding new
> IORING_SQ_CQ_OVERFLOW flag, then helper functions() in liburing, such as
> io_uring_peek_cqe, can be aware of this cq overflow and do flush accordingly.

Applied, thanks.

-- 
Jens Axboe