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From:   Qian Cai <cai@lca.pw>
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Subject: Re: [PATCH] ipv4: fix a RCU-list lock in fib_triestat_seq_show
Date:   Wed, 25 Mar 2020 13:34:46 -0400
Message-Id: <92C7474D-4592-44BF-B0ED-26253196511E@lca.pw>
References: <5e2ed86a-23bc-d3e5-05ad-4e7ed147539c@gmail.com>
Cc:     davem@davemloft.net, kuznet@ms2.inr.ac.ru, yoshfuji@linux-ipv6.org,
        kuba@kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
In-Reply-To: <5e2ed86a-23bc-d3e5-05ad-4e7ed147539c@gmail.com>
To:     Eric Dumazet <eric.dumazet@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Mar 25, 2020, at 12:13 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:=

>=20
> I would prefer :
>=20
> diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
> index ff0c24371e3309b3068980f46d1ed743337d2a3e..4b98ffb27136d3b43f179d6b1b=
42fe84586acc06 100644
> --- a/net/ipv4/fib_trie.c
> +++ b/net/ipv4/fib_trie.c
> @@ -2581,6 +2581,7 @@ static int fib_triestat_seq_show(struct seq_file *se=
q, void *v)
>                struct hlist_head *head =3D &net->ipv4.fib_table_hash[h];
>                struct fib_table *tb;
>=20
> +               rcu_read_lock();
>                hlist_for_each_entry_rcu(tb, head, tb_hlist) {
>                        struct trie *t =3D (struct trie *) tb->tb_data;
>                        struct trie_stat stat;
> @@ -2596,6 +2597,7 @@ static int fib_triestat_seq_show(struct seq_file *se=
q, void *v)
>                        trie_show_usage(seq, t->stats);
> #endif
>                }
> +               rcu_read_unlock();
>        }
>=20
>        return 0;

I have no strong opinion either way. My initial thought was to save 255 extr=
a lock/unlock with a single lock/unlock, but I am not sure how time-consumin=
g for each iteration of the outer loop could be. If it could take a bit too l=
ong, it does make a lot of sense to reduce the critical section.=