Re: [PATCH 2/2] Manage: Inform udev about device removal when stopping

[Hannes Reinecke <hare@suse.de>]

On 02/16/2016 07:03 PM, Sebastian Parschauer wrote:
> On 16.02.2016 18:41, Jes Sorensen wrote:
>> Sebastian Parschauer <sebastian.riemer@profitbricks.com> writes:
>>> When stopping an MD device, then its device node /dev/mdX may still
>>> exist afterwards or it is recreated by udev. The next open() call
>>> can lead to creation of an inoperable MD device. The reason for
>>> this is that a change event (KOBJ_CHANGE) is announced to udev.
>>> So announce a removal event (KOBJ_REMOVE) to udev instead.
>>>
>>> This also overrides the change event sent by the kernel.
>>>
>>> Signed-off-by: Sebastian Parschauer <sebastian.riemer@profitbricks.com>
>>> ---
>>>   Manage.c |    6 +++---
>>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/Manage.c b/Manage.c
>>> index 7e1b94b..bc89764 100644
>>> --- a/Manage.c
>>> +++ b/Manage.c
>>> @@ -494,13 +494,13 @@ done:
>>>   		goto out;
>>>   	}
>>>   	/* prior to 2.6.28, KOBJ_CHANGE was not sent when an md array
>>> -	 * was stopped, so We'll do it here just to be sure.  Drop any
>>> -	 * partitions as well...
>>> +	 * was stopped, it should be KOBJ_REMOVE instead, so we set the
>>> +	 * remove event here just to be sure. Drop any partitions as well...
>>>   	 */
>>>   	if (fd >= 0)
>>>   		ioctl(fd, BLKRRPART, 0);
>>>   	if (mdi)
>>> -		sysfs_uevent(mdi, "change");
>>> +		sysfs_uevent(mdi, "remove");
>>
>> I am a little concerned about this change. You assume the kernel and
>> mdadm will be updated in sync, which is unlikely to happen. I believe
>> you need to match the kernel version and send the corresponding event
>> currectly for this to work correctly?
>
> The worst thing that can happen is that the kernel sends the change
> event after the remove event. Then it is the current situation again.
>  From my tests mdadm does enough other stuff in between. Udev is able to
> handle receiving two remove events from my testing. Multiple mdadm
> instances can't run in parallel any ways. So userspace around it needs
> some serialization for it any ways. So also stopping an MD device and
> assembling a new one with the same minor number shouldn't race.
>
> I still prefer this solution here. But if you decide to drop the udev
> event sending in mdadm, then I'm also fine with that.
>
I strongly prefer removing the udev event generation altogether.
As this appears to be a carry-over from older kernels, it looks to me as 
being an incomplete conversion:
At one point udev introduced 'ONLINE' and 'OFFLINE' events, which were 
supposed to be used for this kind of scenario.
(ONLINE being a companion to 'ADD', and 'OFFLINE' being the companion to 
'DELETE'). However, later the 'ONLINE' got modified to 'CHANGE', and the 
'OFFLINE' got dropped completely.
Or that was the plan.
So it looks as if the conversion to 'CHANGE' got applied to the 
'OFFLINE' event, too.
Hence I strongly recommend to drop it completely, and let the kernel or 
the MD module decide if and when a uevent should be send.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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