[systemd-devel] [PATCH 1/2] md: Inform udev about device removal when stopping

[NeilBrown]

On Wed, Feb 17 2016, Shaohua Li wrote:

> On Tue, Feb 16, 2016 at 03:44:36PM +0100, Sebastian Parschauer wrote:
>> 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.
>> 
>> A change is likely also required in mdadm because of the support
>> for kernels prior to 2.6.28.
>
> I didn't follow why we need the change. Shouldn't the KOBJ_REMOVE event be sent
> automatically when gendisk is deleted?
> mddev_put()->mddev_delayed_delete()->md_free()->del_gendisk().
>
> Thanks,
> Shaohua

For a bit of context: this KOBJ_CHANGE event was added in Oct 2008

Commit: 934d9c23b4c7 ("md: destroy partitions and notify udev when md array is stopped.")

At the time, md devices weren't getting removed at all.
Now they are (I figured out the locking), though they can still come
back.

There are still two stages.  The array is stopped, and then the block
device is destroyed.  It is theoretically possible to stop the array
without destroying the block device, though I don't think that happens
in practice.

So this KOBJ_CHANGE is, I think, technically correct (change from
"active" to "inactive")  but probably isn't needed any more - not to the
extent it was at the time.

There are some annoying races with caused by udev responding (belatedly)
to events by running programs that open s/dev/mdXX and so automatically
re-creates the md device.
The real problem here is not the event or the delays in udev.  It is the
fact that opening /dev/mdXX transparently creates a device.

The only way (I know of) to really avoid these races is to use named
arrays.
Put
   CREATE names=yes

in mdadm.conf.  Then md arrays will be created by writing a name to a
magic file in /sys.  The arrays have a minor number >=512 and are not
auto-re-created if the device node is re-opened before udev unlinks it.

So: the patch might be safe, and might solve a particular problem, but
it is really just a bandaid.  The best fix is "CREATE named=yes" (and
use named like "md_home", not "md4".

NeilBrown
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