linux-kernel - Re: lockdep complaints in slab allocator

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Message-ID: <1258714328.11284.522.camel@laptop>
Date:	Fri, 20 Nov 2009 11:52:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	paulmck@...ux.vnet.ibm.com, linux-mm@...ck.org,
	cl@...ux-foundation.org, mpm@...enic.com,
	LKML <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: lockdep complaints in slab allocator

On Fri, 2009-11-20 at 12:38 +0200, Pekka Enberg wrote:
> 
> 
> On Fri, Nov 20, 2009 at 11:25 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >  2) propagate the nesting information and user spin_lock_nested(), given
> > that slab is already a rat's nest, this won't make it any less obvious.
> 
> spin_lock_nested() doesn't really help us here because there's a
> _real_ possibility of a recursive spin lock here, right? 

Well, I was working under the assumption that your analysis of it being
a false positive was right ;-)

I briefly tried to verify that, but got lost and gave up, at which point
I started looking for ways to annotate.

If you're now saying its a real deadlock waiting to happen, then the
quick fix is to always do the call_rcu() thing, or a slightly longer fix
might be to take that slab object and propagate it out up the callchain
and free it once we drop the nc->lock for the current __cache_free() or
something.



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