Re: [PATCH v2] block/io_uring: resubmit when result is -EAGAIN

[Kevin Wolf <kwolf@redhat.com>]

Am 29.07.2021 um 11:10 hat Fabian Ebner geschrieben:
> Linux SCSI can throw spurious -EAGAIN in some corner cases in its
> completion path, which will end up being the result in the completed
> io_uring request.
> 
> Resubmitting such requests should allow block jobs to complete, even
> if such spurious errors are encountered.
> 
> Co-authored-by: Stefan Hajnoczi <stefanha@gmail.com>
> Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
> Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
> ---
> 
> Changes from v1:
>     * Focus on what's relevant for the patch itself in the commit
>       message.
>     * Add Stefan's comment.
>     * Add Stefano's R-b tag (I hope that's fine, since there was no
>       change code-wise).
> 
>  block/io_uring.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/block/io_uring.c b/block/io_uring.c
> index 00a3ee9fb8..dfa475cc87 100644
> --- a/block/io_uring.c
> +++ b/block/io_uring.c
> @@ -165,7 +165,21 @@ static void luring_process_completions(LuringState *s)
>          total_bytes = ret + luringcb->total_read;
>  
>          if (ret < 0) {
> -            if (ret == -EINTR) {
> +            /*
> +             * Only writev/readv/fsync requests on regular files or host block
> +             * devices are submitted. Therefore -EAGAIN is not expected but it's
> +             * known to happen sometimes with Linux SCSI. Submit again and hope
> +             * the request completes successfully.
> +             *
> +             * For more information, see:
> +             * https://lore.kernel.org/io-uring/20210727165811.284510-3-axboe@kernel.dk/T/#u
> +             *
> +             * If the code is changed to submit other types of requests in the
> +             * future, then this workaround may need to be extended to deal with
> +             * genuine -EAGAIN results that should not be resubmitted
> +             * immediately.
> +             */
> +            if (ret == -EINTR || ret == -EAGAIN) {
>                  luring_resubmit(s, luringcb);
>                  continue;
>              }

Reviewed-by: Kevin Wolf <kwolf@redhat.com>

Question about the preexisting code, though: luring_resubmit() requires
that the caller calls ioq_submit() later so that the request doesn't
just sit in a queue without getting any attention, but actually gets
submitted to the kernel.

In the call chain ioq_submit() -> luring_process_completions() ->
luring_resubmit(), who takes care of that?

Kevin