linux-kernel - Re: [PATCH RFC] fix RCU-callback-after-kmem_cache

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Date:	Tue, 30 Jun 2009 09:58:22 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Matt Mackall <mpm@...enic.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, jdb@...x.dk
Subject: Re: [PATCH RFC] fix RCU-callback-after-kmem_cache_destroy problem in 
	sl[aou]b

On Tue, Jun 30, 2009 at 9:00 AM, Paul E.
McKenney<paulmck@...ux.vnet.ibm.com> wrote:
> On Mon, Jun 29, 2009 at 07:06:34PM -0500, Matt Mackall wrote:
>> On Mon, 2009-06-29 at 19:19 -0400, Christoph Lameter wrote:
>> > On Mon, 29 Jun 2009, Matt Mackall wrote:
>> >
>> > > This is a reasonable point, and in keeping with the design principle
>> > > 'callers should handle their own special cases'. However, I think it
>> > > would be more than a little surprising for kmem_cache_free() to do the
>> > > right thing, but not kmem_cache_destroy().
>> >
>> > kmem_cache_free() must be used carefully when using SLAB_DESTROY_BY_RCU.
>> > The freed object can be accessed after free until the rcu interval
>> > expires (well sortof, it may even be reallocated within the interval).
>> >
>> > There are special RCU considerations coming already with the use of
>> > kmem_cache_free().
>> >
>> > Adding RCU operations to the kmem_cache_destroy() logic may result in
>> > unnecessary RCU actions for slabs where the coder is ensuring that the
>> > RCU interval has passed by other means.
>>
>> Do we care? Cache destruction shouldn't be in anyone's fast path.
>> Correctness is more important and users are more liable to be correct
>> with this patch.
>
> I am with Matt on this one -- if we are going to hand the users of
> SLAB_DESTROY_BY_RCU a hand grenade, let's at least leave the pin in.

I don't even claim to understand all the RCU details here but I don't
see why we should care about _kmem_cache_destroy()_ performance at
this level. Christoph, hmmm?

                              Pekka
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