Re: [PATCH] block: Fix bug in runtime-resume handling

Martin Kepplinger
2020-08-29T07:25:14Z

On 27.08.20 22:29, Alan Stern wrote:
> On Thu, Aug 27, 2020 at 07:42:43PM +0200, Martin Kepplinger wrote:
>> On 26.08.20 09:48, Martin Kepplinger wrote:
>>> On 24.08.20 22:13, Alan Stern wrote:
>
>>>> Martin:
>>>>
>>>> (I forgot to ask this question several weeks ago, while you were running
>>>> your tests. Better ask it now before I forget again...)
>>>>
>>>> I suspect the old runtime-PM code in the block layer would have worked
>>>> okay in your SD cardreader test if the BLK_MQ_REQ_PREEMPT flag had not
>>>> been set. Do you know why the flag was set, or what line of code caused
>>>> it to be set?
>>>
>>> Correct. if not set, I could handle all I need in the scsi error path.
>>
>> this thread becomes a bit confusing. I thought about REQ_FAILFAST_DEV
>> but you're talking about something different.
>>
>> the only place I see BLK_MQ_REQ_PREEMPT getting passed on is in
>> __scsi_execute() which is the case when mounting/unmounting. At least
>> that about the only place I can find.
>
> Ah yes, I see what you mean.
>
>> I remember *only* your block pm fix would let me mount/unmount, but not
>> use files yet (REQ_FAILFAST_DEV and so on).
>>
>> When I revert your fix and remove BLK_MQ_REQ_PREEMPT from being passed
>> on to blk_get_request() in __scsi_execute(), that line gets executed
>> exactly once during startup and I'm missing the /dev/sda device from the
>> cardreader then.
>>
>> Is this what you're asking?
>
> Not quite sure, but it doesn't matter. Removing BLK_MQ_REQ_PREEMPT in
> __scsi_execute() is probably not a safe thing to do.
>
> Instead, look at sd_resume(). That routine calls __scsi_execute()
> indirectly through sd_start_stop_device(), and the only reason it does
> this is because the sdkp->device->manage_start_stop flag is set. You
> ought to be able to clear this flag in sysfs, by writing to
> /sys/block/sda/device/scsi_disk/*/manage_start_stop. If you do this
> before allowing the card reader to go into runtime suspend, does it then
> resume okay?

manage_start_stop in sysfs is 0 here.

>
> (Yes, I know you still won't be able to read it because of the FAILFAST
> flag. I just want to know if the runtime resume actually takes place.)
>
> Alan Stern
>