On Fri, Dec 01, 2017 at 06:04:29PM +0100, Alban Browaeys wrote:

I initially reported as https://bugzilla.kernel.org/show_bug.cgi?id=198
023 .

I have now bisected this issue to commit a6a252e6491443c1c1 "blk-mq-
sched: decide how to handle flush rq via RQF_FLUSH_SEQ".

This is with an USB stick Sandisk Cruzer (USB Version:  2.10) I
regressed with.
systemctl restart systemd-udevd restores sanity.

PS: With an USB3 Lexar (USB Version:  3.00) I get more severe an issue
(not bisected) where I find no way out of reboot. My report to bugzilla
has logs when I was swapping between the these keys. The logs attached
there mixes what looks like two different behaviors.

Hi Paolo,

From both Alban's trace and my trace, looks this issue is in BFQ,

since request can't be retrieved via e->type->ops.mq.dispatch_request()
in blk_mq_do_dispatch_sched() after it is inserted into BFQ's queue.

        https://bugzilla.kernel.org/show_bug.cgi?id=198023#c4
        https://marc.info/?l=linux-block&m=151214241518562&w=2

BTW, I have tried to reproduce the issue with scsi_debug, but not succeed,
and it can't be reproduced with other schedulers(mq-deadline, none) too.

So could you take a look?

Thanks,
Ming