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

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Message-ID: <20090904204335.GG6751@linux.vnet.ibm.com>
Date:	Fri, 4 Sep 2009 13:43:35 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
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 Fri, Sep 04, 2009 at 11:42:17AM -0400, Christoph Lameter wrote:
> On Thu, 3 Sep 2009, Paul E. McKenney wrote:
> 
> > If it were the user of the slab who was invoking some variant of
> > call_rcu(), then I would agree with you.
> 
> The user already has to deal with it as explained by Eric.

I didn't read his email that way.  Perhaps I misinterpreted it.

> > However, call_rcu() is instead being invoked by the slab itself in the
> > case of SLAB_DESTROY_BY_RCU, so that there is no variation in usage.
> > Requiring that the user call rcu_barrier() is asking for subtle bugs.
> > Therefore, the best approach is to have kmem_cache_destroy() handle
> > the RCU cleanup, given that this cleanup is for actions taken by
> > kmem_cache_free(), not by the user.
> 
> The user already has to properly handle barriers and rcu logic in order to
> use objects handled with RCU properly. Moreover the user even has to check
> that the object is not suddenly checked under it. Its already complex.

mm/slab.c has had RCU calls since 2.6.9, so this is not new.

> Guess we are doing this ... Sigh. Are you going to add support other rcu
> versions to slab as well as time permits and as the need arises? Should
> we add you as a maintainer? ;-)

I don't see any point in adding anything resembling SLAB_DESTROY_BY_RCU_BH,
SLAB_DESTROY_BY_RCU_SCHED, or SLAB_DESTROY_BY_SRCU unless and until
someone needs it.  And I most especially don't see the point of adding
(say) SLAB_DESTROY_BY_RCU_BH_SCHED until someone convinces me of the
need for it.  I would prefer to put the energy into further streamlining
the underlying RCU implementation, maybe someday collapsing RCU-BH back
into RCU.  ;-)

We have gotten along fine with only SLAB_DESTROY_BY_RCU for almost
five years, so I think we are plenty fine with what we have.  So, as
you say, "as the need arises".

I don't see any more need to add me as maintainer of slab and friends
than of btrfs, netfilter, selinux, decnet, afs, wireless, or any of a
number of other subsystems that use RCU.

							Thanx, Paul
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