netdev - Re: [PATCH net-next 09/11] can: gw: switch cangw_pernet

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Date:   Mon, 7 Feb 2022 10:50:56 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Oliver Hartkopp <socketcan@...tkopp.net>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        David Ahern <dsahern@...nel.org>,
        Marc Kleine-Budde <mkl@...gutronix.de>
Subject: Re: [PATCH net-next 09/11] can: gw: switch cangw_pernet_exit() to
 batch mode

On Mon, Feb 7, 2022 at 10:40 AM Oliver Hartkopp <socketcan@...tkopp.net> wrote:
>
>
>
> On 07.02.22 18:54, Eric Dumazet wrote:
> > On Mon, Feb 7, 2022 at 9:41 AM Oliver Hartkopp <socketcan@...tkopp.net> wrote:
> (..)
> >>> -static void __net_exit cangw_pernet_exit(struct net *net)
> >>> +static void __net_exit cangw_pernet_exit_batch(struct list_head *net_list)
> >>>    {
> >>> +     struct net *net;
> >>> +
> >>>        rtnl_lock();
> >>> -     cgw_remove_all_jobs(net);
> >>> +     list_for_each_entry(net, net_list, exit_list)
> >>> +             cgw_remove_all_jobs(net);
> >>
> >> Instead of removing the jobs for ONE net namespace it seems you are
> >> remove removing the jobs for ALL net namespaces?
> >>
> >> Looks wrong to me.
> >
> > I see nothing wrong in my patch.
> >
> > I think you have to look more closely at ops_exit_list() in
> > net/core/net_namespace.c
>
> Ok, thanks. Your patch just moved the list_for_each_entry() to gw.c.
> So there is no functional difference.
>
> Acked-by: Oliver Hartkopp <socketcan@...tkopp.net>
>
> > BTW, the sychronize_rcu() call in cgw_remove_all_jobs is definitely
> > bad, you should absolutely replace it by call_rcu() or kfree_rcu()
>
> Advise is welcome!
>
> The synchronize_rcu() has been introduced in fb8696ab14ad ("can: gw:
> synchronize rcu operations before removing gw job entry") as
> can_can_gw_rcv() is called under RCU protection (NET_RX softirq).
>
> That patch was a follow-up to d5f9023fa61e ("can: bcm: delay release of
> struct bcm_op after synchronize_rcu()") where Thadeu Lima de Souza
> Cascardo detected a race in the BCM code.
>
> When call_rcu() is enough to make sure we do not get a race in
> can_can_gw_rcv() while receiving skbs and removing filters with
> cgw_unregister_filter() I would be happy this rcu thing being fixed up.
>

Can you test this straightforward patch, thanks !
diff --git a/net/can/gw.c b/net/can/gw.c
index d8861e862f157aec36c417b71eb7e8f59bd064b9..20e74fe7d0906dccc65732b8f9e7e14e2d1192c3
100644
--- a/net/can/gw.c
+++ b/net/can/gw.c
@@ -577,6 +577,13 @@ static inline void cgw_unregister_filter(struct
net *net, struct cgw_job *gwj)
                          gwj->ccgw.filter.can_mask, can_can_gw_rcv, gwj);
 }

+static void cgw_job_free_rcu(struct rcu_head *rcu_head)
+{
+       struct cgw_job *gwj = container_of(rcu_head, struct cgw_job, rcu);
+
+       kmem_cache_free(cgw_cache, gwj);
+}
+
 static int cgw_notifier(struct notifier_block *nb,
                        unsigned long msg, void *ptr)
 {
@@ -596,8 +603,7 @@ static int cgw_notifier(struct notifier_block *nb,
                        if (gwj->src.dev == dev || gwj->dst.dev == dev) {
                                hlist_del(&gwj->list);
                                cgw_unregister_filter(net, gwj);
-                               synchronize_rcu();
-                               kmem_cache_free(cgw_cache, gwj);
+                               call_rcu(&gwj->rcu, cgw_job_free_rcu);
                        }
                }
        }
@@ -1155,8 +1161,7 @@ static void cgw_remove_all_jobs(struct net *net)
        hlist_for_each_entry_safe(gwj, nx, &net->can.cgw_list, list) {
                hlist_del(&gwj->list);
                cgw_unregister_filter(net, gwj);
-               synchronize_rcu();
-               kmem_cache_free(cgw_cache, gwj);
+               call_rcu(&gwj->rcu, cgw_job_free_rcu);
        }
 }

@@ -1224,8 +1229,7 @@ static int cgw_remove_job(struct sk_buff *skb,
struct nlmsghdr *nlh,

                hlist_del(&gwj->list);
                cgw_unregister_filter(net, gwj);
-               synchronize_rcu();
-               kmem_cache_free(cgw_cache, gwj);
+               call_rcu(&gwj->rcu, cgw_job_free_rcu);
                err = 0;
                break;
        }