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

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Date:	Fri, 04 Sep 2009 00:17:34 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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()

Eric Dumazet a écrit :
> 
> 
> 
> Problem is not _objects_ Christoph, but _slabs_, and your patch is not working.
> 
> Its true that when User calls kmem_cache_destroy(), all _objects_ were previously freed.
> This is mandatory, with or withou SLAB_DESTROY_BY_RCU thing
> 
> Problem is that slub has some internal state, including some to-be-freed _slabs_,
> that User have no control at all on it.
> 
> User cannot "know" slabs are freed, inuse, or whatever state in cache or call_rcu queues.
> 
> Face it, SLAB_DESTROY_BY_RCU is internal affair (to slub/slab/... allocators)
> 
> We absolutely need a rcu_barrier() somewhere, believe it or not. You can argue that it should
> be done *before*, but it gives no speedup, only potential bugs.
> 
> Only case User should do its rcu_barrier() itself is if it knows some call_rcu() are pending
> and are delaying _objects_ freeing (typical !SLAB_DESTROY_RCU usage in RCU algos).
> 
> I dont even understand why you care so much about kmem_cache_destroy(SLAB_DESTROY_BY_RCU),
> given that almost nobody use it. We took almost one month to find out what the bug was in first
> place...


So maybe the safest thing would be to include the rcu_barrier() to insure all objects where freed

And another one for SLAB_DESTROY_BY_RCU to make sure slabs where freed

void kmem_cache_destroy(struct kmem_cache *s)
{
	/*
	 * Make sure no objects are waiting in call_rcu queues to be freed
	 */
	rcu_barrier();

	down_write(&slub_lock);
	s->refcount--;
	if (!s->refcount) {
                list_del(&s->list);
                up_write(&slub_lock);
                if (kmem_cache_close(s)) {
                        printk(KERN_ERR "SLUB %s: %s called for cache that "
                                "still has objects.\n", s->name, __func__);
                        dump_stack();
                }
		/*
		 * Make sure no slabs are waiting in call_rcu queues to be freed
		 */
                if (s->flags & SLAB_DESTROY_BY_RCU)
                        rcu_barrier();
                sysfs_slab_remove(s);
        } else
                up_write(&slub_lock);
}
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