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

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Message-ID: <20090630060031.GL7070@linux.vnet.ibm.com>
Date:	Mon, 29 Jun 2009 23:00:31 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	penberg@...helsinki.fi, jdb@...x.dk
Subject: Re: [PATCH RFC] fix RCU-callback-after-kmem_cache_destroy problem
	in sl[aou]b

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.

							Thanx, Paul
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