[systemd-devel] [PATCH 2/2] Manage: Inform udev about device removal when stopping

[Hannes Reinecke]

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On 02/16/2016 09:46 PM, NeilBrown wrote:
> On Wed, Feb 17 2016, Jes Sorensen wrote:
> 
>> Hannes Reinecke <hare at suse.de> writes:
>>> On 02/16/2016 07:03 PM, Sebastian Parschauer wrote:
>>>> On 16.02.2016 18:41, Jes Sorensen wrote:
>>>>> Sebastian Parschauer
>>>>> <sebastian.riemer at 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 at 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.
>> 
>> I am totally fine with this, however we should make mdadm
>> fail if run against a pre-2.6.28 kernel then.
>> 
>> Cheers, Jes
> 
> I would suggest protecting the
> 
> if (fd >= 0) ioctl(fd, BLKRRPART, 0); if (mdi) 
> sysfs_uevent(mdi, "change");
> 
> code with
> 
> if (get_linux_version() < 2006028)
> 
> That should be completely safe - 2.6.28 and later do this (if
> needed).
> 
+1.

Yes, this is the best solution.

Cheers,

Hannes
- -- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare at suse.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
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