Re: [PATCH v3 4/4] scsi: scsi_core: Fix IO hang when device removing

[Mike Christie]

On 10/15/23 9:03 PM, Wenchao Hao wrote:
> shost_for_each_device() would skip devices which is in progress of
> removing, so scsi_run_queue() for these devices would be skipped in
> scsi_run_host_queues() after blocking hosts' IO.
> 
> IO hang would be caused if return true when state is SDEV_CANCEL with
> following order:
> 
> T1:					    T2:scsi_error_handler
> __scsi_remove_device()
>   scsi_device_set_state(sdev, SDEV_CANCEL)
>   ...
>   sd_remove()
>   del_gendisk()
>   blk_mq_freeze_queue_wait()
>   					    scsi_eh_flush_done_q()
> 					      scsi_queue_insert(scmd,...)
> 
> scsi_queue_insert() would not kick device's queue since commit
> 8b566edbdbfb ("scsi: core: Only kick the requeue list if necessary")
> 
> After scsi_unjam_host(), the scsi error handler would call
> scsi_run_host_queues() to trigger run queue for devices, while it
> would not run queue for devices which is in progress of removing
> because shost_for_each_device() would skip them.
> 
> So the requests added to these queues would not be handled any more,
> and the removing device process would hang too.
> 
> Fix this issue by using shost_for_each_device_include_deleted() in
> scsi_run_host_queues() to trigger a run queue for devices in removing.
> 
> Signed-off-by: Wenchao Hao <haowenchao2@xxxxxxxxxx>
> ---
>  drivers/scsi/scsi_lib.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 195ca80667d0..40f407ffd26f 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -466,7 +466,7 @@ void scsi_run_host_queues(struct Scsi_Host *shost)
>  {
>  	struct scsi_device *sdev;
>  
> -	shost_for_each_device(sdev, shost)
> +	shost_for_each_device_include_deleted(sdev, shost)
>  		scsi_run_queue(sdev->request_queue);

What happens if there were no commands for the device that
was destroyed and we race with this code and device deletion?

So thread1 has set the device state tp SDEV_DEL and has finished
blk_mq_destroy_queue because there were no commands running.

The above eh thread, then is calling:

scsi_run_queue -> blk_mq_kick_requeue_list

and that queues the requeue work.

blk_mq_destroy_queue had done blk_mq_cancel_work_sync but
blk_mq_kick_requeue_list just added it back on the kblockd_workqueue.

When __scsi_iterate_devices does scsi_device_put it would call
scsi_device_dev_release and call blk_put_queue which frees the
request_queue while it's requeue work might still be queued on
kblockd_workqueue.