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

Jesper Dangaard Brouer
2009-06-25T09:28:23Z

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()?

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