Subject: Re: lockdep complaints in slab allocator
From: Peter Zijlstra <peterz@infradead.org>
To: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: paulmck@linux.vnet.ibm.com, linux-mm@kvack.org, cl@linux-foundation.org,
       mpm@selenic.com, LKML <linux-kernel@vger.kernel.org>,
       Nick Piggin <npiggin@suse.de>
In-Reply-To: <4B067816.6070304@cs.helsinki.fi>
References: <20091118181202.GA12180@linux.vnet.ibm.com>
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	 <84144f020911200238w3d3ecb38k92ca595beee31de5@mail.gmail.com>
	 <1258714328.11284.522.camel@laptop>  <4B067816.6070304@cs.helsinki.fi>
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Date: Fri, 20 Nov 2009 16:09:08 +0100
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On Fri, 2009-11-20 at 13:05 +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@infradead.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?

You'd have to somehow push the nested state down from the
kmem_cache_free() call in slab_destroy() to all nc->lock sites below.


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