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

From: Stefano Garzarella
Subject: Re: [PATCH] block/io_uring: resubmit when result is -EAGAIN
Date: Wed, 28 Jul 2021 15:02:22 +0200

On Wed, Jul 28, 2021 at 02:09:40PM +0200, Philippe Mathieu-Daudé wrote:
Cc'ing the maintainers for you. See

$ ./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.


  #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>

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