Subject: Re: [ 2375.793397] WARNING: CPU: 0 PID: 1149 at net/netlink/genetlink.c:1037 genl_unbind+0xc0/0xd0()

From: Johannes Berg <[email-masked]>
Date: Thu, 15 Jan 2015 00:25:46 +0100

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