linux-kernel - Re: [PATCH] slub: fix slab_pad

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Date:	Thu, 3 Sep 2009 17:43:12 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Zdenek Kabelac <zdenek.kabelac@...il.com>,
	Patrick McHardy <kaber@...sh.net>, Robin Holt <holt@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jesper Dangaard Brouer <hawk@...x.dk>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH] slub: fix slab_pad_check()

On Thu, 3 Sep 2009, Paul E. McKenney wrote:

> 2.	CPU 0 discovers that the slab cache can now be destroyed.
>
> 	It determines that there are no users, and has guaranteed
> 	that there will be no future users.  So it knows that it
> 	can safely do kmem_cache_destroy().
>
> 3.	In absence of rcu_barrier(), kmem_cache_destroy() would
> 	immediately tear down the slab data structures.

Of course. This has been discussed before.

You need to ensure that no objects are in use before destroying a slab. In
case of DESTROY_BY_RCU you must ensure that there are no potential
readers. So use a suitable rcu barrier or something else like a
synchronize_rcu...

> > But going through the RCU period is pointless since no user of the cache
> > remains.
>
> Which is irrelevant.  The outstanding RCU callback was posted by the
> slab cache itself, -not- by the user of the slab cache.

There will be no rcu callbacks generated at kmem_cache_destroy with the
patch I posted.

> > The dismantling does not need RCU since there are no operations on the
> > objects in progress. So simply switch DESTROY_BY_RCU off for close.
>
> Unless I am missing something, this patch re-introduces the bug that
> the rcu_barrier() was added to prevent.  So, in absence of a better
> explanation of what I am missing:

The "fix" was ill advised. Slab users must ensure that no objects are in
use before destroying a slab. Only the slab users know how the objects
are being used. The slab allocator itself cannot know how to ensure that
there are no pending references. Putting a rcu_barrier in there creates an
inconsistency in the operation of kmem_cache_destroy() and an expectation
of functionality that the function cannot provide.

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