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-ax...@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.eb...@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. Some more context, leading up to this patch: We had some users reporting issues after we switched to using io_uring by default. Namely, kernel panics [2] for some, and failing block jobs [3] (with a custom backup mechanism we implemented on top of QEMU's block layer) for others. I had luck and managed to reprouce the issue, and it was a failed block job about half of the time and a kernel panic the other half. When using a host kernel with [0], it's a failed block job all the time, and this patch attempts to fix that, by resubmitting instead of bubbling up the error. [2]: https://forum.proxmox.com/threads/kernel-panic-whole-server-crashes-about-every-day.91803/post-404382 [3]: https://forum.proxmox.com/threads/backup-job-failed-with-err-11-on-2-of-6-vms.92568/ block/io_uring.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/block/io_uring.c b/block/io_uring.c index 00a3ee9fb8..77d162cb24 100644 --- a/block/io_uring.c +++ b/block/io_uring.c @@ -165,7 +165,7 @@ static void luring_process_completions(LuringState *s) total_bytes = ret + luringcb->total_read; if (ret < 0) { - if (ret == -EINTR) { + if (ret == -EINTR || ret == -EAGAIN) { luring_resubmit(s, luringcb); continue; } -- 2.30.2
