linux-kernel - Re: lockdep complaints in slab allocator

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Date:	Fri, 20 Nov 2009 06:48:55 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Peter Zijlstra <peterz@...radead.org>, 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, Nov 20, 2009 at 01:05:58PM +0200, Pekka Enberg wrote:
> 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?

Hmmm...  If the nc->lock spinlocks are always from different slabs
(as alloc_slabmgmt()'s block comment claims), why not just give each
array_cache structure's lock its own struct lock_class_key?  They
are zero size unless you have lockdep enabled.

							Thanx, Paul
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