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

From: Martin Kepplinger <martin.kepplinger@puri.sm>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Bart Van Assche <bvanassche@acm.org>,
	Can Guo <cang@codeaurora.org>,
	linux-scsi@vger.kernel.org, linux-block@vger.kernel.org,
	kernel@puri.sm
Subject: Re: [PATCH] block: Fix bug in runtime-resume handling
Date: Sat, 29 Aug 2020 09:24:30 +0200	[thread overview]
Message-ID: <478fdc57-f51e-f480-6fde-f34596394624@puri.sm> (raw)
In-Reply-To: <20200827202952.GA449067@rowland.harvard.edu>

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
> 