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

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Date:	Thu, 3 Sep 2009 14:24:17 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	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>,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] slub: fix slab_pad_check()

On Thu, 3 Sep 2009, Eric Dumazet wrote:

> Point is we cannot deal with RCU quietness before disposing the slab cache,
> (if SLAB_DESTROY_BY_RCU was set on the cache) since this disposing *will*
> make call_rcu() calls when a full slab is freed/purged.

There is no need to do call_rcu calls for frees at that point since
objects are no longer in use. We could simply disable SLAB_DESTROY_BY_RCU
for the final clearing of caches.

> And when RCU grace period is elapsed, the callback *will* need access to
> the cache we want to dismantle. Better to not have kfreed()/poisoned it...

But going through the RCU period is pointless since no user of the cache
remains.

> I believe you mix two RCU uses here.
>
> 1) The one we all know, is use normal caches (!SLAB_DESTROY_BY_RCU)
> (or kmalloc()), and use call_rcu(... kfree_something)
>
>    In this case, you are 100% right that the subsystem itself has
>    to call rcu_barrier() (or respect whatever self-synchro) itself,
>    before calling kmem_cache_destroy()
>
> 2) The SLAB_DESTROY_BY_RCU one.
>
>    Part of cache dismantle needs to call rcu_barrier() itself.
>    Caller doesnt have to use rcu_barrier(). It would be a waste of time,
>    as kmem_cache_destroy() will refill rcu wait queues with its own stuff.

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.


---
 mm/slub.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c	2009-09-03 10:14:51.000000000 -0500
+++ linux-2.6/mm/slub.c	2009-09-03 10:18:32.000000000 -0500
@@ -2594,9 +2594,9 @@ static inline int kmem_cache_close(struc
  */
 void kmem_cache_destroy(struct kmem_cache *s)
 {
-	if (s->flags & SLAB_DESTROY_BY_RCU)
-		rcu_barrier();
 	down_write(&slub_lock);
+	/* Stop deferring frees so that we can immediately free structures */
+	s->flags &= ~SLAB_DESTROY_BY_RCU;
 	s->refcount--;
 	if (!s->refcount) {
 		list_del(&s->list);
--
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