Re: [PATCH] io_uring: reset -EBUSY error when io sq thread is waken up

Jens Axboe
2020-05-20T13:03:41Z

On 5/19/20 10:40 PM, Xiaoguang Wang wrote:
> In io_sq_thread(), currently if we get an -EBUSY error, we will
> won't clear it again, which will result in io_sq_thread() will
> never have a chance to submit sqes again. Below test program test.c
> can reveal this bug:
>
> int main(int argc, char *argv[])
> {
> struct io_uring ring;
> int i, fd, ret;
> struct io_uring_sqe *sqe;
> struct io_uring_cqe *cqe;
> struct iovec *iovecs;
> void *buf;
> struct io_uring_params p;
>
> if (argc < 2) {
> printf("%s: file\n", argv[0]);
> return 1;
> }
>
> memset(&p, 0, sizeof(p));
> p.flags = IORING_SETUP_SQPOLL;
> ret = io_uring_queue_init_params(4, &ring, &p);
> if (ret < 0) {
> fprintf(stderr, "queue_init: %s\n", strerror(-ret));
> return 1;
> }
>
> fd = open(argv[1], O_RDONLY | O_DIRECT);
> if (fd < 0) {
> perror("open");
> return 1;
> }
>
> iovecs = calloc(10, sizeof(struct iovec));
> for (i = 0; i < 10; i++) {
> if (posix_memalign(&buf, 4096, 4096))
> return 1;
> iovecs[i].iov_base = buf;
> iovecs[i].iov_len = 4096;
> }
>
> ret = io_uring_register_files(&ring, &fd, 1);
> if (ret < 0) {
> fprintf(stderr, "%s: register %d\n", __FUNCTION__, ret);
> return ret;
> }
>
> for (i = 0; i < 10; i++) {
> sqe = io_uring_get_sqe(&ring);
> if (!sqe)
> break;
>
> io_uring_prep_readv(sqe, 0, &iovecs[i], 1, 0);
> sqe->flags |= IOSQE_FIXED_FILE;
>
> ret = io_uring_submit(&ring);
> sleep(1);
> printf("submit %d\n", i);
> }
>
> for (i = 0; i < 10; i++) {
> io_uring_wait_cqe(&ring, &cqe);
> printf("receive: %d\n", i);
> if (cqe->res != 4096) {
> fprintf(stderr, "ret=%d, wanted 4096\n", cqe->res);
> ret = 1;
> }
> io_uring_cqe_seen(&ring, cqe);
> }
>
> close(fd);
> io_uring_queue_exit(&ring);
> return 0;
> }
> sudo ./test testfile
> above command will hang on the tenth request, to fix this bug, when io
> sq_thread is waken up, we reset the variable 'ret' to be zero.
>
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
> ---
> fs/io_uring.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 6e51140a5722..747de5cdf38a 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -6045,6 +6045,8 @@ static int io_sq_thread(void *data)
> finish_wait(&ctx->sqo_wait, &wait);
>
> ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
> + if (ret == -EBUSY)
> + ret = 0;
> continue;
> }
> finish_wait(&ctx->sqo_wait, &wait);

How about just unconditionally clearing it? There's nothing we
need to preserve at this point, it should just get set to zero
regardless of the value.

--
Jens Axboe