netdev - Re: [ 2375.793397] WARNING: CPU: 0 PID: 1149 at net/netlink/genetlink.c:1037 genl

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Date:	Wed, 14 Jan 2015 21:20:39 -0500
From:	Jeff Layton <jeff.layton@...marydata.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Jeff Layton <jeff.layton@...marydata.com>, netdev@...r.kernel.org
Subject: Re: [ 2375.793397] WARNING: CPU: 0 PID: 1149 at
 net/netlink/genetlink.c:1037 genl_unbind+0xc0/0xd0()

On Thu, 15 Jan 2015 00:25:46 +0100
Johannes Berg <johannes@...solutions.net> wrote:

> On Wed, 2015-01-14 at 23:48 +0100, Johannes Berg wrote:
> 
> > > [ 2375.793396] ------------[ cut here ]------------
> > > [ 2375.793397] WARNING: CPU: 0 PID: 1149 at net/netlink/genetlink.c:1037 genl_unbind+0xc0/0xd0()
> > 
> > This warning is supposed to happen only when you somehow manage to
> > unsubscribe from a generic netlink group that doesn't actually exist, or
> > so.
> 
> Ok - after long deliberation I found a way to trigger it. It requires
> that you leave a multicast group (likely by destroying a socket) at the
> same time as the kernel unregisters the generic netlink group. I have no
> idea what generic netlink group you might be using here, but I could
> reproduce it with a strategically placed delay in the netlink code and
> the nl80211 genl group by opening a socket, closing the socket, and
> removing the cfg80211 module (to unregister the nl80211 genl group)
> while the socket was still being closed.
> 
> I'll think about a fix tomorrow - it doesn't seem trivial due to
> possible locking concerns.
> 
> On the bright side, I cannot see a way to reproduce this without
> removing the genl family at the same time - which is good because it
> means that I've just again audited the case I was worried about (the
> bind/unbind not being symmetric) - it is asymmetric but only in the case
> of genl family removal which seems reasonable (but I should document
> it.)
> 
> johannes
> 

Cool.

FWIW, it popped around a dozen times or so?

Unfortunately, I didn't save the logs from the run. I'll try to
reproduce it again tomorrow (and save the logs this time), but I don't
see it every time.

-- 
Jeff Layton <jlayton@...marydata.com>
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