linux-kernel - Re: lockdep recursive locking detected (rcu_kthread / __cache

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Message-ID: <20111003214739.GK2403@linux.vnet.ibm.com>
Date:	Mon, 3 Oct 2011 14:47:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...two.org>
Cc:	Sitsofe Wheeler <sitsofe@...oo.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	penberg@...nel.org, mpm@...enic.com, linux-mm@...ck.org
Subject: Re: lockdep recursive locking detected (rcu_kthread / __cache_free)

On Mon, Oct 03, 2011 at 03:46:11PM -0500, Christoph Lameter wrote:
> On Mon, 3 Oct 2011, Paul E. McKenney wrote:
> 
> > The first lock was acquired here in an RCU callback.  The later lock that
> > lockdep complained about appears to have been acquired from a recursive
> > call to __cache_free(), with no help from RCU.  This looks to me like
> > one of the issues that arise from the slab allocator using itself to
> > allocate slab metadata.
> 
> Right. However, this is a false positive since the slab cache with
> the metadata is different from the slab caches with the slab data. The slab
> cache with the metadata does not use itself any metadata slab caches.

Wouldn't it be possible to pass a new flag to the metadata slab caches
upon creation so that their locks could be placed in a separate lock
class?  Just allocate a separate lock_class_key structure for each such
lock in that case, and then use lockdep_set_class_and_name to associate
that structure with the corresponding lock.  I do this in kernel/rcutree.c
in order to allow the rcu_node tree's locks to nest properly.

							Thanx, Paul
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