Hello,


I got a kernel trace when unplugging the power supply, switching to battery
mode. I get the same kernel trace when booting on battery.
Both making the system unusable or breaking the boot.

The kernel call trace with symbols:

? blk_mq_requeue_request+0x...
? __scsi_queue_insert+0x...
? ata_scsi_var_len_cdb_xlat+0x
? __blk_mq_complete_request+0x...
? ata_scsi_translate+0x...
? ata_scsi_queuecmd+0x...
? scsi_dispatch_cmd+0x...
? scsi_queue_rq+0x...
? blk_mq_dispatch_rq_list+0x...
? kyber_dispatch_cur_domain+0x...
? kyber_completed_request+0x...
? blk_mq_sched_dispatch_requests+0x...
? __ blk_mq_run_hw_queue+0x...
? __blk_mq_delay_run_hw_queue+0x...
? blk_mq_run_hw_queue+0x...
? blk_mq_run_hw_queues+0x...
? blk_mq_requeue_work+0x...
? process_one_work+0x...
? worker_thread+0x...
? process_one_work+0x...
? kthread+0x...
? kthread_flush_work_fn+0x...
? ret_from_fork+0x...
Code: ...
RIP: sbitmap_queue_clear+0x...

Screenshot: 	https://ibin.co/4D34Ej3DWsqI.jpg
Kernel config: 	https://bpaste.net/show/870004e55123

Kernel: 4.17.11-ck

Setup:

btrfs-on-bcache-on-luks
btrfs options (rw,noatime,nodiratime,compress-
force=lzo,nossd,noacl,space_cache,autodefrag)

Using mq bfq scheduler for the hdd backing and kyber for the ssd caching device


Failed tests:
Tested many kernel down to 4.13.2 with Gentoo or Ck patchset. Sorry for not
including the vanilla sources in the test, I can provide if required.
Skipping services in the boot process didn't help, any next service leads to the
same trace.
Switching off the laptop-mode-tools daemon didn't help.
Switching all devices to "none" scheduler did not help.


Workaround:
After some tests and due to the *mq* call stack I was able to workaround by
disabling CONFIG_SCSI_MQ_DEFAULT and CONFIG_DM_MQ_DEFAULT and switching all
devices to cfq scheduler.
However with the MQ enabled kernel, only bfq, kyber and none are possible, while
the non-mq kernel can only set cfq. I guess this is intentional as the current
bfq implementation is a MQ only version and CFQ is a non-mq only version?

Best regards,
Massimo