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

[Jens Axboe <axboe@kernel.dk>]

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