Re: Fwd: Waking up from resume locks up on sr device

[Damien Le Moal]

On 6/12/23 15:09, Hannes Reinecke wrote:
> On 6/12/23 05:09, Damien Le Moal wrote:
>> On 6/11/23 00:03, Bart Van Assche wrote:
>>> On 6/10/23 06:27, Bagas Sanjaya wrote:
>>>> On 6/10/23 15:55, Pavel Machek wrote:
>>>>>>> #regzbot introduced: v5.0..v6.4-rc5 https://bugzilla.kernel.org/show_bug.cgi?id=217530
>>>>>>> #regzbot title: Waking up from resume locks up on SCSI CD/DVD drive
>>>>>>>
>>>>>> The reporter had found the culprit (via bisection), so:
>>>>>>
>>>>>> #regzbot introduced: a19a93e4c6a98c
>>>>> Maybe cc the authors of that commit?
>>>>
>>>> Ah! I forgot to do that! Thanks anyway.
>>>
>>> Hi Damien,
>>>
>>> Why does the ATA code call scsi_rescan_device() before system resume has
>>> finished? Would ATA devices still work with the patch below applied?
>>
>> I do not know the PM code well at all, need to dig into it. But your patch
>> worries me as it seems it would prevent rescan of the device on a resume, which
>> can be an issue if the device has changed.
>>
>> I am not yet 100% clear on the root cause for this, but I think it comes from
>> the fact that ata_port_pm_resume() runs before the sci device resume is done, so
>> with scsi_dev->power.is_suspended still true. And ata_port_pm_resume() calls
>> ata_port_resume_async() which triggers EH (which will do reset + rescan)
>> asynchronously. So it looks like we have scsi device resume and libata EH for
>> rescan fighting each others for the scan mutex and device lock, leading to deadlock.
>>
>> Trying to recreate this issue now to confirm and debug further. But I suspect
>> the solution to this may be best implemented in libata, not in scsi.
>> This looks definitely related to this thread:
>>
>> https://lore.kernel.org/linux-scsi/7b553268-69d3-913a-f9de-28f8d45bdb1e@xxxxxxx/
>>
>> Similaraly to your comment on that thread, having to look at
>> dev->power.is_suspended is not ideal I think. What we need is to have ata and
>> scsi pm resume be synchronized, but I am not yet 100% clear on the scsi layer side.
>>
> Which is my feeling, too.
> libata runs rescan as part of the device discovery, so really it will 
> run after resume. And consequently resume really cannot wait for rescan 
> to finish.
> 
> What I would be looking at is to decouple resume from libata device 
> rescan, and have resume to complete before libata EH runs.

That is the case now, for the ata port at least, even though that is not super
explicit, and not reliable. See ata_port_pm_resume(): I think that the call to
EH in ata_port_pm_resume() -> ata_port_resume_async() -> ata_port_request_pm()
-> ata_port_schedule_eh() should instead use a sync resume, leading to a sync EH
call.

That EH execution essentially does ata_eh_handle_port_resume(), which calls into
the adapter resume operation. That in itself does not do much beside some
registers accesses to wakeup the port. There should be no issues doing that
synchronously.

The problem is that after that is done, ata EH calls ata_std_error_handler() ->
ata_do_eh() -> ata_eh_recover() -> ata_eh_revalidate_and_attach() ->
schedule_work(&(ap->scsi_rescan_task)). And the rescan work calls
scsi_rescan_device() (yet in another context than EH) which causes the problem
when the scsi disk device has not been resumed yet (dev->power_is_suspended
still true).

So it really looks like the solution should be to have ata_scsi_dev_rescan()
wait for the scsi device to resume first, but not sure how to do that with the
pm API. Digging...

> 
> Cheers,
> 
> Hannes

-- 
Damien Le Moal
Western Digital Research