Subject: Re: netlink circular locking dependency

From: Marcel Holtmann <[email-masked]>
Date: Tue, 17 Jun 2008 03:45:54 +0200

Hi Patrick,

> >> =======================================================
> >> [ INFO: possible circular locking dependency detected ]
> >> 2.6.26-rc2 #5
> >> -------------------------------------------------------
> >> hcid/4136 is trying to acquire lock:
> >>  (genl_mutex){--..}, at: [<c0000000002ace4c>] .ctrl_dumpfamily+0x74/0x174
> >>
> >> but task is already holding lock:
> >>  (nlk->cb_mutex){--..}, at: [<c0000000002a766c>] .netlink_dump+0x58/0x27c
> >>
> >> which lock already depends on the new lock.
> >>     
> > ...
> >
> > Hi,
> >
> > IMHO it looks like a real lockup threat. Probably it needs something
> > better, but for now here is my simplistic patch proposal for testing.
> >   
> So we have:
> 
> genl_rcv()            : take genl_mutex
> genl_rcv_msg()        : call netlink_dump_start() while holding genl_mutex
> netlink_dump_start(),
> netlink_dump()        : take nlk->cb_mutex
> ctrl_dumpfamily()     : try to detect this case and not take genl_mutex a
>                         second time
> 
> netlink_rcv()         : call netlink_dump
> netlink_dump          : take nlk->cb_mutex
> ctrl_dumpfamily()     : take genl_mutex
> 
> which is a real bug.
> 
> It seems the best fix is to use genl_mutex for the netlink cb_mutex,
> drop genl_mutex before calling netlink_dump_start and don't take it
> in ctrl_dumpfamily, relying completely on af_netlink.c for dump
> locking. Unfortunately this creates a race since the ops passed to
> netlink_dump_start are also protect by the mutex, so this patch
> is just for testing whether it fixes the warning.

I updated my test kernel to 2.6.26-rc6 and then applied your patch and
the lockdep warning goes away.

Regards

Marcel