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

[Damien Le Moal]

On 6/14/23 23:26, Alan Stern wrote:
> On Wed, Jun 14, 2023 at 04:35:50PM +0900, Damien Le Moal wrote:
>> On 6/14/23 15:57, Hannes Reinecke wrote:
>>> On 6/14/23 06:49, Damien Le Moal wrote:
>>>> On 6/11/23 18:05, Joe Breuer wrote:
>>>>> I'm the reporter of this issue.
>>>>>
>>>>> I just tried this patch against 6.3.4, and it completely fixes my
>>>>> suspend/resume issue.
>>>>>
>>>>> The optical drive stays usable after resume, even suspending/resuming
>>>>> during playback of CDDA content works flawlessly and playback resumes
>>>>> seamlessly after system resume.
>>>>>
>>>>> So, from my perspective: Good one!
>>>>
>>>> In place of Bart's fix, could you please try this patch ?
>>>>
>>>> diff --git a/drivers/ata/libata-eh.c b/drivers/ata/libata-eh.c
>>>> index b80e68000dd3..a81eb4f882ab 100644
>>>> --- a/drivers/ata/libata-eh.c
>>>> +++ b/drivers/ata/libata-eh.c
>>>> @@ -4006,9 +4006,32 @@ static void ata_eh_handle_port_resume(struct
>>>> ata_port *ap)
>>>>          /* tell ACPI that we're resuming */
>>>>          ata_acpi_on_resume(ap);
>>>>
>>>> -       /* update the flags */
>>>>          spin_lock_irqsave(ap->lock, flags);
>>>> +
>>>> +       /* Update the flags */
>>>>          ap->pflags &= ~(ATA_PFLAG_PM_PENDING | ATA_PFLAG_SUSPENDED);
>>>> +
>>>> +       /*
>>>> +        * Resuming the port will trigger a rescan of the ATA device(s)
>>>> +        * connected to it. Before scheduling the rescan, make sure that
>>>> +        * the associated scsi device(s) are fully resumed as well.
>>>> +        */
>>>> +       ata_for_each_link(link, ap, HOST_FIRST) {
>>>> +               ata_for_each_dev(dev, link, ENABLED) {
>>>> +                       struct scsi_device *sdev = dev->sdev;
>>>> +
>>>> +                       if (!sdev)
>>>> +                               continue;
>>>> +                       if (scsi_device_get(sdev))
>>>> +                               continue;
>>>> +
>>>> +                       spin_unlock_irqrestore(ap->lock, flags);
>>>> +                       device_pm_wait_for_dev(&ap->tdev,
>>>> +                                              &sdev->sdev_gendev);
>>>> +                       scsi_device_put(sdev);
>>>> +                       spin_lock_irqsave(ap->lock, flags);
>>>> +               }
>>>> +       }
>>>>          spin_unlock_irqrestore(ap->lock, flags);
>>>>   }
>>>>   #endif /* CONFIG_PM */
>>>>
>>>> Thanks !
>>>>
>>> Well; not sure if that'll work out.
>>> The whole reason why we initial a rescan is that we need to check if the
>>> ports are still connected, and whether the devices react.
>>> So we can't iterate the ports here as this is the very thing which gets
>>> checked during EH.
>>
>> Hmmm... Right. So we need to move that loop into ata_scsi_dev_rescan(),
>> which itself already loops over the port devices anyway.
>>
>>> We really should claim resume to be finished as soon as we can talk with
>>> the HBA, and kick off EH asynchronously to let it finish the job after
>>> resume has completed.
>>
>> That is what's done already:
>>
>> static int ata_port_pm_resume(struct device *dev)
>> {
>> 	ata_port_resume_async(to_ata_port(dev), PMSG_RESUME);
>> 	pm_runtime_disable(dev);
>> 	pm_runtime_set_active(dev);
>> 	pm_runtime_enable(dev);
>> 	return 0;
>> }
>>
>> EH is kicked by ata_port_resume_async() -> ata_port_request_pm() and it
>> is async. There is no synchronization in EH with the PM side though. We
>> probably should have EH check that the port resume is done first, which
>> can be done in ata_eh_handle_port_resume() since that is the first thing
>> done when entering EH.
>>
>> The problem remains though that we *must* wait for the scsi device
>> resume to be done before calling scsi_rescan_device(), which is done
>> asynchronously from EH, as a different work. So that one needs to wait
>> for the scsi side resume to be done.
>>
>> I also thought of trigerring the rescan from the scsi side, but since
>> the resume may be asynchronous, we could endup trigerring it with the
>> ata side not yet resumed... That would only turn the problem around
>> instead of solving it.
> 
> The order in which devices get resumed isn't arbitrary.  If the system 
> is set up not to use async suspends/resumes then the order is always the 
> same as the order in which the devices were originally registered (for 
> resume, that is -- suspend obviously takes place in the reverse order).
> 
> So if you're trying to perform an action that requires two devices to be 
> active, you must not do it in the resume handler for the device that was 
> registered first.  I don't know how the ATA and SCSI pieces interact 
> here, but regardless, this is a pretty strict requirement.
> 
> It should be okay to perform the action in the resume handler for the 
> device that was registered second.  But if the two devices aren't in an 
> ancestor-descendant relationship then you also have to call 
> device_pm_wait_for_dev() (or use device links as Rafael mentioned) to 
> handle the async case properly.
> 
>> Or... Why the heck scsi_rescan_device() is calling device_lock() ? This
>> is the only place in scsi code I can see that takes this lock. I suspect
>> this is to serialize either rescans, or serialize with resume, or both.
>> For serializing rescans, we can use another lock. For serializing with
>> PM, we should wait for PM transitions...
>> Something is not right here.
> 
> Here's what commit e27829dc92e5 ("scsi: serialize ->rescan against 
> ->remove", written by Christoph Hellwig) says:
> 
>     Lock the device embedded in the scsi_device to protect against
>     concurrent calls to ->remove.
> 
> That's the commit which added the device_lock() call.

Thanks for the information.

+Christoph

Why is adding the device_lock() needed ? We could just do a
scsi_device_get()+scsi_device_put() to serialize against remove. No ?

> 
> Alan Stern

-- 
Damien Le Moal
Western Digital Research