From: Jesper Dangaard Brouer Subject: Re: [PATCH v2 10/10] nf_conntrack: Use rcu_barrier() and fix kmem_cache_create flags Date: Thu, 25 Jun 2009 11:29:13 +0200 Message-ID: <1245922153.24921.56.camel@localhost.localdomain> References: <20090623150330.22490.87327.stgit@localhost> <20090623150444.22490.27931.stgit@localhost> <4A410185.3090706@trash.net> <1245834139.6695.31.camel@localhost.localdomain> <1245836409.6695.35.camel@localhost.localdomain> <4A423108.60109@trash.net> Reply-To: jdb@comx.dk Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: "David S. Miller" , "Paul E. McKenney" , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, dougthompson@xmission.com, bluesmoke-devel@lists.sourceforge.net, axboe@kernel.dk, christine.caulfield@googlemail.com, Trond.Myklebust@netapp.com, linux-wireless@vger.kernel.org, johannes@sipsolutions.net, yoshfuji@linux-ipv6.org, shemminger@linux-foundation.org, linux-nfs@vger.kernel.org, bfields@fieldses.org, neilb@suse.de, linux-ext4@vger.kernel.org, tytso@mit.edu, adilger@sun.com, netfilter-devel@vger.kernel.org To: Patrick McHardy , Christoph Lameter , linux-mm@kvack.org Return-path: Received: from lanfw001a.cxnet.dk ([87.72.215.196]:55496 "EHLO lanfw001a.cxnet.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751751AbZFYJ3O (ORCPT ); Thu, 25 Jun 2009 05:29:14 -0400 In-Reply-To: <4A423108.60109@trash.net> Sender: linux-ext4-owner@vger.kernel.org List-ID: 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) { ...... if (__cache_shrink(cachep)) { slab_error(cachep, "Can't free all objects"); ...... return; } if (unlikely(cachep->flags & SLAB_DESTROY_BY_RCU)) synchronize_rcu(); __kmem_cache_destroy(cachep); ...... } 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()? 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)) { > > + /* hawk@comx.dk 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