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

[Damien Le Moal <dlemoal@kernel.org>]

On 6/12/23 16:36, Kai-Heng Feng wrote:
> On Mon, Jun 12, 2023 at 3:22 PM Damien Le Moal <dlemoal@kernel.org> wrote:
>>
>> 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@acm.org/
>>>>
>>>> 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...
> 
> Probably use dpm_wait_for_children()? Right now it's an internal PM API.

But I am not sure if there is a relationship between ata_device and its
scsi_device (dev->sdev)... Need to clarify that.
> 
> Rafael,
> What do you think?
> 
> Kai-Heng
> 
>>
>>>
>>> Cheers,
>>>
>>> Hannes
>>
>> --
>> Damien Le Moal
>> Western Digital Research
>>

-- 
Damien Le Moal
Western Digital Research