Subject: Re: [PATCH v2 10/10] nf_conntrack: Use rcu_barrier() and fix kmem_cache_create flags

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 25 Jun 2009 12:32:24 -0700

On Thu, Jun 25, 2009 at 11:29:13AM +0200, Jesper Dangaard Brouer wrote:
> 
> On Wed, 2009-06-24 at 15:58 +0200, Patrick McHardy wrote:
> > Jesper Dangaard Brouer wrote:
> > > Adjusting SLAB_DESTROY_BY_RCU flags.
> > > 
> > >  kmem_cache_create("nf_conntrack", ...) does not need the
> > >  SLAB_DESTROY_BY_RCU flag.
> > 
> > It does need it. We're using it instead of call_rcu() for conntracks.
> > 
> > >  But the
> > >  kmem_cache_create("nf_conntrack_expect", ...) should use the
> > >  SLAB_DESTROY_BY_RCU flag, because it uses a call_rcu() callback to
> > >  invoke kmem_cache_free().
> > 
> > No, using call_rcu() means we don't need SLAB_DESTROY_BY_RCU.
> > Please see the note in include/linux/slab.h.
> 
> Oh, I see.  The description is some what cryptic, but I think I got it,
> after reading through the code.
> 
> BUT this still means that we need to do rcu_barrier() if the
> SLAB_DESTROY_BY_RCU is NOT set and we do call_rcu() our self.
> 
> Look at: slab.c kmem_cache_destroy()
> 
> void kmem_cache_destroy(struct kmem_cache *cachep)
> {
> 	...<cut>...
> 	if (__cache_shrink(cachep)) {
> 		slab_error(cachep, "Can't free all objects");
> 		...<cut>...
> 		return;
> 	}
> 
> 	if (unlikely(cachep->flags & SLAB_DESTROY_BY_RCU))
> 		synchronize_rcu();
> 
> 	__kmem_cache_destroy(cachep);
> 	...<cut>...
> }
> 
> My understanding for the code is (please feel free to correct me): that
> if SLAB_DESTROY_BY_RCU _is_ set, then the __cache_shrink() call will
> call drain_freelist(), which calls slab_destroy().
> 
> If SLAB_DESTROY_BY_RCU _is_ set, then slab_destroy() will then start a
> call_rcu() callback to kmem_rcu_free() which calls kmem_cache_free().
> Given that the callback code kmem_rcu_free() is not removed, we are not
> worried about unloading the module at this point.
> 
> I'm a bit worried about what happens if __kmem_cache_destroy() is
> invoked and there is still callbacks for kmem_rcu_free() in flight?
> The synchronize_rcu() between __cache_shrink() and
> __kmem_cache_destroy() should perhaps be changed to rcu_barrier()?

It looks to me like it should, good catch!!!  I sent a proposed patch
to the maintainers.

							Thanx, Paul

> But I'm sure that the SLAB/MM guys will tell me that this case is
> handled (and something about its unlinked from the appropiate
> lists)??? ;-)
> 
> 
> > > RCU barriers, rcu_barrier(), is inserted two places.
> > > 
> > >  In nf_conntrack_expect.c nf_conntrack_expect_fini() before the
> > >  kmem_cache_destroy(), even though the use of the SLAB_DESTROY_BY_RCU
> > >  flag, because slub does not (currently) handle rcu sync correctly.
> > 
> > I think that should be fixed in slub then.
> 
> I don't think so, we/I'm are talking about "nf_conntrack_expect" and not
> "nf_conntrack" slab.  Clearly the slab "nf_conntrack" is handled
> correcly (according to description above). 
> 
> We still need to make sure the callbacks for "nf_conntrack_expect", are
> done before unloading/removing the code they are about to call.
> 
> 
> > >  And in nf_conntrack_extend.c nf_ct_extend_unregister(), inorder to
> > >  wait for completion of callbacks to __nf_ct_ext_free_rcu(), which is
> > >  invoked by __nf_ct_ext_add().  It might be more efficient to call
> > >  rcu_barrier() in nf_conntrack_core.c nf_conntrack_cleanup_net(), but
> > >  thats make it more difficult to read the code (as the callback code
> > >  in located in nf_conntrack_extend.c).
> > 
> > This one looks fine.
> 
> Should I make two different patchs?
> 
> 
> > > diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> > > index 5f72b94..438ce84 100644
> > > --- a/net/netfilter/nf_conntrack_core.c
> > > +++ b/net/netfilter/nf_conntrack_core.c
> > > @@ -1242,7 +1242,7 @@ static int nf_conntrack_init_init_net(void)
> > >  
> > >  	nf_conntrack_cachep = kmem_cache_create("nf_conntrack",
> > >  						sizeof(struct nf_conn),
> > > -						0, SLAB_DESTROY_BY_RCU, NULL);
> > > +						0, 0, NULL);
> > >  	if (!nf_conntrack_cachep) {
> > >  		printk(KERN_ERR "Unable to create nf_conn slab cache\n");
> > >  		ret = -ENOMEM;
> > > diff --git a/net/netfilter/nf_conntrack_expect.c b/net/netfilter/nf_conntrack_expect.c
> > > index afde8f9..56227c2 100644
> > > --- a/net/netfilter/nf_conntrack_expect.c
> > > +++ b/net/netfilter/nf_conntrack_expect.c
> > > @@ -593,7 +593,7 @@ int nf_conntrack_expect_init(struct net *net)
> > >  	if (net_eq(net, &init_net)) {
> > >  		nf_ct_expect_cachep = kmem_cache_create("nf_conntrack_expect",
> > >  					sizeof(struct nf_conntrack_expect),
> > > -					0, 0, NULL);
> > > +					0, SLAB_DESTROY_BY_RCU, NULL);
> > >  		if (!nf_ct_expect_cachep)
> > >  			goto err2;
> > >  	}
> > > @@ -617,8 +617,15 @@ err1:
> > >  void nf_conntrack_expect_fini(struct net *net)
> > >  {
> > >  	exp_proc_remove(net);
> > > -	if (net_eq(net, &init_net))
> > > +	if (net_eq(net, &init_net)) {
> > > +		/* [email-masked] 2009-06-24: The rcu_barrier() can be
> > > +		 * removed once the sl*b allocators has been fixed
> > > +		 * regarding handling the SLAB_DESTROY_BY_RCU flag
> > > +		 * correctly.
> > > +		 */
> > > +		rcu_barrier(); /* Wait for call_rcu() before destroy */
> > >  		kmem_cache_destroy(nf_ct_expect_cachep);
> > > +	}
> > >  	nf_ct_free_hashtable(net->ct.expect_hash, net->ct.expect_vmalloc,
> > >  			     nf_ct_expect_hsize);
> > >  }
> > > diff --git a/net/netfilter/nf_conntrack_extend.c b/net/netfilter/nf_conntrack_extend.c
> > > index 4b2c769..fef95be 100644
> > > --- a/net/netfilter/nf_conntrack_extend.c
> > > +++ b/net/netfilter/nf_conntrack_extend.c
> > > @@ -186,6 +186,6 @@ void nf_ct_extend_unregister(struct nf_ct_ext_type *type)
> > >  	rcu_assign_pointer(nf_ct_ext_types[type->id], NULL);
> > >  	update_alloc_size(type);
> > >  	mutex_unlock(&nf_ct_ext_type_mutex);
> > > -	synchronize_rcu();
> > > +	rcu_barrier(); /* Wait for completion of call_rcu()'s */
> > >  }
> > >  EXPORT_SYMBOL_GPL(nf_ct_extend_unregister);
> > > 
> > 
> -- 
> Med venlig hilsen / Best regards
>   Jesper Brouer
>   ComX Networks A/S
>   Linux Network developer
>   Cand. Scient Datalog / MSc.
>   Author of http://adsl-optimizer.dk
>   LinkedIn: http://www.linkedin.com/in/brouer

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