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

[Stefano Garzarella]

On Wed, Jul 28, 2021 at 02:09:40PM +0200, Philippe Mathieu-Daudé wrote:
> Cc'ing the maintainers for you. See
> https://wiki.qemu.org/Contribute/SubmitAPatch#CC_the_relevant_maintainer
>
> $ ./scripts/get_maintainer.pl -f block/io_uring.c
> Aarushi Mehta <mehta.aaru20@gmail.com> (maintainer:Linux io_uring)
> Julia Suvorova <jusual@redhat.com> (maintainer:Linux io_uring)
> Stefan Hajnoczi <stefanha@redhat.com> (maintainer:Linux io_uring)
> Kevin Wolf <kwolf@redhat.com> (supporter:Block layer core)
> Max Reitz <mreitz@redhat.com> (supporter:Block layer core)
> qemu-block@nongnu.org (open list:Linux io_uring)
> qemu-devel@nongnu.org (open list:All patches CC here)
>
> Also Cc'ing Stefano for commit b4e44c9944e ("io_uring: retry
> io_uring_submit() if it fails with errno=EINTR").

Thanks Phil!

> (Stefano, you might want to add yourself a R: tag in MAINTAINERS).

Yep, I'll send a patch for that.

> On 7/28/21 12:35 PM, Fabian Ebner wrote:
> > Quoting from [0]:
> >
> >  Some setups, like SCSI, can throw spurious -EAGAIN off the softirq
> >  completion path. Normally we expect this to happen inline as part
> >  of submission, but apparently SCSI has a weird corner case where it
> >  can happen as part of normal completions.
> >
> > Host kernels without patch [0] can panic when this happens [1], and
> > resubmitting makes the panic more likely. On the other hand, for
> > kernels with patch [0], resubmitting ensures that a block job is not
> > aborted just because of such spurious errors.
> >
> > [0]: 
> > https://lore.kernel.org/io-uring/20210727165811.284510-3-axboe@kernel.dk/T/#u
> >
> > [1]:
> >   #9 [ffffb732000c8b70] asm_exc_page_fault at ffffffffa4800ade
> >  #10 [ffffb732000c8bf8] io_prep_async_work at ffffffffa3d89c16
> >  #11 [ffffb732000c8c50] io_rw_reissue at ffffffffa3d8b2e1
> >  #12 [ffffb732000c8c78] io_complete_rw at ffffffffa3d8baa8
> >  #13 [ffffb732000c8c98] blkdev_bio_end_io at ffffffffa3d62a80
> >  #14 [ffffb732000c8cc8] bio_endio at ffffffffa3f4e800
> >  #15 [ffffb732000c8ce8] dec_pending at ffffffffa432f854
> >  #16 [ffffb732000c8d30] clone_endio at ffffffffa433170c
> >  #17 [ffffb732000c8d70] bio_endio at ffffffffa3f4e800
> >  #18 [ffffb732000c8d90] blk_update_request at ffffffffa3f53a37
> >  #19 [ffffb732000c8dd0] scsi_end_request at ffffffffa4233a5c
> >  #20 [ffffb732000c8e08] scsi_io_completion at ffffffffa423432c
> >  #21 [ffffb732000c8e58] scsi_finish_command at ffffffffa422c527
> >  #22 [ffffb732000c8e88] scsi_softirq_done at ffffffffa42341e4
> >
> > Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
> > ---
> >
> > I'm new to this code and io_uring, so I don't know what all the
> > implications are, but retrying upon EAGAIN does not sound like
> > a bad thing to my inexperienced ears.

Yeah, that doesn't sound bad.

For kernels that don't have the patch applied, I don't think there's 
much we can do about it, so this change seems okay to me:

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>