[systemd-devel] udev and probing of eMMC partition devices

[Alan Perry]

On 9/22/20 7:44 AM, Lennart Poettering wrote:
> On Mo, 21.09.20 19:03, Alan Perry (alanp at snowmoose.com) wrote:
>
>> Hi,
>>
>> I am trying to understand behavior that I am seeing with udev and eMMC
>> partition devices and was hoping that someone here could help.
>>
>> The system that I am running has an eMMC device with something like 7-8
>> partitions. The kernel does add block events for the device and each of it
>> partitions and the logs show them being queued up by udevd. Then things get
>> interesting. Sometimes processing of the add event for the device itself
>> gets to a probe step and, in the time frame of the logs that I have, never
>> gets further in that processing and the add block device events for the
>> partitions are never processed. However, usually, it will process the add
>> block device event for the entire device and after that (as per the
>> implementation of is_devpath_busy()) start processing the add block events
>> for the partition devices. Often times, the processing of the partition
>> device add events will get stuck at the probe step.
> "Get stuck"? What does that mean? What is it actually doing? What does
> a stack trace say? Anything in the logs?

When this happens, the last thing seen in the log for those devices is 
the probe ("probe /dev/mmcblk0<part> raid offset=0").

I have been given a bunch of logs and have yet to be able to reproduce 
the problem myself. I got additional hints for reproducing the problem 
last night and am putting together a systemd with additional diagnostic 
output in the code that does the probe.
>
>> The partition devices
>> stuck at the probe are then "waiting" and the services that depend on them
>> blocked.
>>
>> Can anyone here give me some insight into what is going on, in particular,
>> how there is such difference in behavior between test runs on the same
>> system?
> Have you checked the logs?
Yes. I have been trying to debug the problem by log output and code 
analysis.

I had initially thought that the problem was somewhere else, but now the 
logs are pointing me towards the probe.
>
> Please always start with saying which systemd version this is, and
> which distro, otherwise it's very hard to grok what your setup is
> like?

Good point. Sorry. I think it is v239 systemd plus a lot of patches. The 
distro is an internal Microsoft one based on the 5.4 kernel.

>
> Maybe the eMMC driver has issues and simply hangs? dmesg might show
> something in that case.

Thanks. I don't have dmesg output associated with most of the logs. But 
I will be sure to look there once I am able to reproduce it myself.

alan

>
> Lennart
>
> --
> Lennart Poettering, Berlin