linux-kernel - Re: lockdep complaints in slab allocator

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Date:	Fri, 20 Nov 2009 13:05:58 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Peter Zijlstra <peterz@...radead.org>
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

Peter Zijlstra kirjoitti:
> 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.

Uh, ok, so apparently I was right after all. There's a comment in 
free_block() above the slab_destroy() call that refers to the comment 
above alloc_slabmgmt() function definition which explains it all.

Long story short: ->slab_cachep never points to the same kmalloc cache 
we're allocating or freeing from. Where do we need to put the 
spin_lock_nested() annotation? Would it be enough to just use it in 
cache_free_alien() for alien->lock or do we need it in 
cache_flusharray() as well?

			Pekka
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