Re: [PATCH rcu/dev 2/3] net: Use call_rcu_flush() for in_dev_rcu_put

[Joel Fernandes]

Hi Eric,

On Thu, Nov 17, 2022 at 01:58:18PM -0800, Eric Dumazet wrote:
> On Wed, Nov 16, 2022 at 7:16 PM Joel Fernandes (Google)
> <joel@xxxxxxxxxxxxxxxxx> wrote:
> >
> > In a networking test on ChromeOS, we find that using the new CONFIG_RCU_LAZY
> > causes a networking test to fail in the teardown phase.
> >
> > The failure happens during: ip netns del <name>
> >
> > Using ftrace, I found the callbacks it was queuing which this series fixes. Use
> > call_rcu_flush() to revert to the old behavior. With that, the test passes.
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> >  net/ipv4/devinet.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
> > index e8b9a9202fec..98b20f333e00 100644
> > --- a/net/ipv4/devinet.c
> > +++ b/net/ipv4/devinet.c
> > @@ -328,7 +328,7 @@ static void inetdev_destroy(struct in_device *in_dev)
> >         neigh_parms_release(&arp_tbl, in_dev->arp_parms);
> >         arp_ifdown(dev);
> >
> > -       call_rcu(&in_dev->rcu_head, in_dev_rcu_put);
> > +       call_rcu_flush(&in_dev->rcu_head, in_dev_rcu_put);
> >  }
> 
> For this one, I suspect the issue is about device refcount lingering ?
> 
> I think we should release refcounts earlier (and only delegate the
> freeing part after RCU grace period, which can be 'lazy' just fine)
> 
> Something like:

The below diff where you reduce refcount before RCU grace period, also makes the
test pass.

If you are Ok with it, I can roll it into a patch with your Author tag and my
Tested-by. Let me know what you prefer?

Also, looking through the patch, I don't see any issue. One thing is
netdev_put() now happens before a grace period, instead of after. But I don't
think that's an issue.

thanks!

 - Joel


> 
> diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
> index e8b9a9202fecd913137f169f161dfdccc16f7edf..e0258aef4211ec6a72d062963470a32776e6d010
> 100644
> --- a/net/ipv4/devinet.c
> +++ b/net/ipv4/devinet.c
> @@ -234,13 +234,21 @@ static void inet_free_ifa(struct in_ifaddr *ifa)
>         call_rcu(&ifa->rcu_head, inet_rcu_free_ifa);
>  }
> 
> +static void in_dev_free_rcu(struct rcu_head *head)
> +{
> +       struct in_device *idev = container_of(head, struct in_device, rcu_head);
> +
> +       kfree(rcu_dereference_protected(idev->mc_hash, 1));
> +       kfree(idev);
> +}
> +
>  void in_dev_finish_destroy(struct in_device *idev)
>  {
>         struct net_device *dev = idev->dev;
> 
>         WARN_ON(idev->ifa_list);
>         WARN_ON(idev->mc_list);
> -       kfree(rcu_dereference_protected(idev->mc_hash, 1));
> +
>  #ifdef NET_REFCNT_DEBUG
>         pr_debug("%s: %p=%s\n", __func__, idev, dev ? dev->name : "NIL");
>  #endif
> @@ -248,7 +256,7 @@ void in_dev_finish_destroy(struct in_device *idev)
>         if (!idev->dead)
>                 pr_err("Freeing alive in_device %p\n", idev);
>         else
> -               kfree(idev);
> +               call_rcu(&idev->rcu_head, in_dev_free_rcu);
>  }
>  EXPORT_SYMBOL(in_dev_finish_destroy);
> 
> @@ -298,12 +306,6 @@ static struct in_device *inetdev_init(struct
> net_device *dev)
>         goto out;
>  }
> 
> -static void in_dev_rcu_put(struct rcu_head *head)
> -{
> -       struct in_device *idev = container_of(head, struct in_device, rcu_head);
> -       in_dev_put(idev);
> -}
> -
>  static void inetdev_destroy(struct in_device *in_dev)
>  {
>         struct net_device *dev;
> @@ -328,7 +330,7 @@ static void inetdev_destroy(struct in_device *in_dev)
>         neigh_parms_release(&arp_tbl, in_dev->arp_parms);
>         arp_ifdown(dev);
> 
> -       call_rcu(&in_dev->rcu_head, in_dev_rcu_put);
> +       in_dev_put(in_dev);
>  }
> 
>  int inet_addr_onlink(struct in_device *in_dev, __be32 a, __be32 b)