Re: possible slab deadlock while doing ifenslave

[David Rientjes <rientjes@google.com>]

On Thu, 13 Oct 2011, Hans Schillstrom wrote:

> > > Hello,
> > > I got this when I was testing a VLAN patch i.e. using Dave Millers net-next from today.
> > > When doing this on a single core i686 I got the warning every time,
> > > however ifenslave is not hanging it's just a warning
> > > Have not been testing this on a multicore jet.
> > > 
> > > There is no warnings with a 3.0.4 kernel.
> > > 
> > > Is this a known warning ?
> > > 
> > > ~ # ifenslave bond0 eth1 eth2
> > > 
> > > =============================================
> > > [ INFO: possible recursive locking detected ]
> > > 3.1.0-rc9+ #3
> > > ---------------------------------------------
> > > ifenslave/749 is trying to acquire lock:
> > >  (&(&parent->list_lock)->rlock){-.-...}, at: [<c14234a0>] cache_flusharray+0x41/0xdb
> > > 
> > > but task is already holding lock:
> > >  (&(&parent->list_lock)->rlock){-.-...}, at: [<c14234a0>] cache_flusharray+0x41/0xdb
> > > 
> > 
> > Hmm, the only candidate that I can see that may have caused this is 
> > 83835b3d9aec ("slab, lockdep: Annotate slab -> rcu -> debug_object -> 
> > slab").  Could you try reverting that patch in your local tree and seeing 
> > if it helps?
> > 
> 
> That was not our candidate ...
> i.e. same results
> 

Ok, I think this may be related to what Sitsofe reported in the "lockdep 
recursive locking detected" thread on LKML (see 
http://marc.info/?l=linux-kernel&m=131805699106560).

Peter and Christoph hypothesized that 056c62418cc6 ("slab: fix lockdep 
warnings") may not have had full coverage when setting lockdep classes for 
kmem_list3 locks that may be called inside of each other because of 
off-slab metadata.

I think it's safe to say there is no deadlock possibility here or we would 
have seen it since 2006 and this is just a matter of lockdep annotation 
that needs to be done.  So don't worry too much about the warning even 
though I know it's annoying and it suppresses future lockdep output (even 
more annoying!).

I'm not sure if there's a patch to address that yet, I think one was in 
the works.  If not, I'll take a look at rewriting that lockdep annotation.

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