Re: MD devnode still present after 'remove' udev event, and mdadm reports 'does not appear to be active'

[NeilBrown <neilb@suse.de>]

On Tue, 30 Aug 2011 18:18:11 +0300 Alexander Lyakas <alex.bolshoy@gmail.com>
wrote:

> Thanks, Neil.
> 
> Although according to udev documentation: "the udev events are sent
> out after udev has finished its event processing, all rules have been
> processed, and needed device nodes are created."
> 
> Also looking at udev-worker code of udevd, the
> udev_monitor_send_device() call is done after all the rules have been
> processed.
> 
> Nevertheless, I looked at udevadm_settle.c and did some equivalent of
> that in my code, and it looks like the issue is resolved. Perhaps
> there is something md-specific here?

I cannot see how it would be md-specific.  mdadm doesn't create or remove
devices when udev is active - it leaves all that to udev.
If you are curious I suggest you ask the udev developers.

> 
> Another thing, since you are reading this thread, I wanted to ask
> whether you have any advice on the "RAID5: failing an active component
> during spare rebuild - arrays hangs" thread I opened some time ago.
> Since you were not answering, I assume there is nothing additional you
> can advise about, correct? I apologize if this off-topic was
> inappropriate.

It could mean that I had nothing extra to say, but it could also mean that I
got distracted, forgot, and never got back to it.  I live in a world of
distractions :-(

But a reminder never hurts - it shows that it is important to you, so that
makes it at least a little bit important to me.  I'll go back and have a look
and see if I have anything useful to add.

NeilBrown


> 
> Thanks for the help,
>   Alex.
> 
> 
> 
> On Tue, Aug 30, 2011 at 12:25 AM, NeilBrown <neilb@suse.de> wrote:
> > On Mon, 29 Aug 2011 20:17:34 +0300 Alexander Lyakas <alex.bolshoy@gmail.com>
> > wrote:
> >
> >> Greetings everybody,
> >>
> >> I issue
> >> mdadm --stop /dev/md0
> >> and I want to reliably determine that the MD devnode (/dev/md0) is gone.
> >> So I look for the udev 'remove' event for that devnode.
> >> However, in some cases even after I see the udev event, I issue
> >> mdadm --detail /dev/md0
> >> and I get:
> >> mdadm: md device /dev/md0 does not appear to be active
> >>
> >> According to Detail.c, this means that mdadm can successfully do
> >> open("/dev/md0") and receive a valid fd.
> >> But later, when issuing ioctl(fd, GET_ARRAY_INFO) it receives ENODEV
> >> from the kernel.
> >>
> >> Can somebody suggest an explanation for this behavior? Is there a
> >> reliable way to know when a MD devnode is gone?
> >
> > run "udevadm settle" after stopping /dev/md0  is most likely to work.
> >
> > I suspect that udev removes the node *after* you see the 'remove' event.
> > Sometimes so soon after that you don't see the lag - sometimes a bit later.
> >
> > NeilBrown
> >
> >>
> >> Thanks,
> >>   Alex.
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