linux-kernel - Re: lockdep complaints in slab allocator

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Message-ID: <1258729748.4104.223.camel@laptop>
Date:	Fri, 20 Nov 2009 16:09:08 +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 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@...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?

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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