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Date: Thu, 13 Jul 2006 16:16:20 -0700
From: Andrew Morton <akpm@osdl.org>
To: Christoph Lameter <clameter@sgi.com>
Cc: arjan@infradead.org, torvalds@osdl.org, penberg@cs.helsinki.fi,
       mingo@elte.hu, sekharan@us.ibm.com, linux-kernel@vger.kernel.org,
       nagar@watson.ibm.com, balbir@in.ibm.com
Subject: Re: [patch] lockdep: annotate mm/slab.c
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On Thu, 13 Jul 2006 15:51:09 -0700 (PDT)
Christoph Lameter <clameter@sgi.com> wrote:

> On Thu, 13 Jul 2006, Arjan van de Ven wrote:
> 
> > [*] Note Note Note
> > there is a corner case in the slab code that I personally don't trust at
> > all. In the NUMA case, if the memory is not originally from your own
> > node, the cache_free_alien() function takes, while having your own local
> > lock, the lock of the remote node as well. (at least on my reading of
> > the code) to free the memory to that node. I have yet to see where in
> > the code it safeguards against that remote node doing the exact same
> > thing in the opposite direction concurrently, and causing a basic ABBA
> > deadlock.
> 
> Second look: I cannot find where we take our own local nodes list_lock.
> We only take the lock from the remote node. Or is this related to the 
> OFF_SLAB kfree issue?
> 
> 
> We either have a alien cache structure established then:
> 
> We take a lock on the alien structure for node x from our own node 
> (without holding our local list_lock!) and then we need take the remote 
> list_lock for node x if the alien structure overflows and we then free
> to the remote nodes list.
> 
> Or we do not have a alien cache structure established yet. Then:
> 
> We simply take the remote list_lock on node x and free directly to the 
> foreign nodes list.
> 
> 
> 
> In an OFF_SLAB situation this may differ because then we call 
> kmem_cache_free from slab_destroy. Ughhh... This looks extremely bad. 

uh-oh.

> Whew! We drop the list lock before calling slab_destroy.

Well we did, up until about ten minutes ago.

free_block()'s droppage of l3->list_lock around the slab_destroy() call was
just reverted, due to Shailabh confirming that it caused corruption.