Re: Race condition in block layer runtime PM init and scsi disk driver

[sthumma@codeaurora.org]

> Hi Aaron,
>
> I found a race condition with the block layer runtime PM due to which
> the q->nr_pending is decremented to less than zero (0xFFFF_FFFF (-1))
> and hence the blk pre-runtime suspend always returns -EBUSY.
>
>
> The issue is easily reproduced with a scsi disk with disabled tagged
> command queuing
>
> sd_probe_async() ->
> 	add_disk() ->
> 		disk_add_event() ->
> 			schedule(disk_events_workfn)
> 	sd_revalidate_disk()
> 	blk_pm_runtime_init()
> return;
>
> Let's say the disk_events_workfn() calls sd_check_events() which tries
> to send test_unit_ready() and because of sd_revalidate_disk() trying to
> send another commands the test_unit_ready() might be re-queued as the
> tagged command queuing is disabled.
>
> So the race condition is -
>
> Thread 1 			  |		Thread 2
> sd_revalidate_disk()		  |	sd_check_events()
> ...nr_pending = 0 as q->dev = NULL|	scsi_queue_insert()
> blk_runtime_pm_init()		  | 	blk_pm_requeue_request() ->
> 				  |	nr_pending = -1 since
> 				  |	q->dev != NULL
>
> Do you have any suggestions on how to fix this issue?
>
>
> --
> Regards,
> Sujit
>