netdev - [PATCH v2 net-next 1/7] ipv6: Remove rcu_read_lock() in fib6_get

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Message-ID: <20250516022759.44392-2-kuniyu@amazon.com>
Date: Thu, 15 May 2025 19:27:17 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, "Paolo
 Abeni" <pabeni@...hat.com>
CC: Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuniyu@...zon.com>,
	Kuniyuki Iwashima <kuni1840@...il.com>, <netdev@...r.kernel.org>
Subject: [PATCH v2 net-next 1/7] ipv6: Remove rcu_read_lock() in fib6_get_table().

Once allocated, the IPv6 routing table is not freed until
netns is dismantled.

fib6_get_table() uses rcu_read_lock() while iterating
net->ipv6.fib_table_hash[], but it's not needed and
rather confusing.

Because some callers have this pattern,

  table = fib6_get_table();

  rcu_read_lock();
  /* ... use table here ... */
  rcu_read_unlock();

  [ See: addrconf_get_prefix_route(), ip6_route_del(),
         rt6_get_route_info(), rt6_get_dflt_router() ]

and this looks illegal but is actually safe.

Let's remove rcu_read_lock() in fib6_get_table() and pass true
to the last argument of hlist_for_each_entry_rcu() to bypass
the RCU check.

Note that protection is not needed but RCU helper is used to
avoid data-race.

Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
---
 net/ipv6/ip6_fib.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 1f860340690c..88770ecd2da1 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -281,22 +281,20 @@ EXPORT_SYMBOL_GPL(fib6_new_table);
 
 struct fib6_table *fib6_get_table(struct net *net, u32 id)
 {
-	struct fib6_table *tb;
 	struct hlist_head *head;
-	unsigned int h;
+	struct fib6_table *tb;
 
-	if (id == 0)
+	if (!id)
 		id = RT6_TABLE_MAIN;
-	h = id & (FIB6_TABLE_HASHSZ - 1);
-	rcu_read_lock();
-	head = &net->ipv6.fib_table_hash[h];
-	hlist_for_each_entry_rcu(tb, head, tb6_hlist) {
-		if (tb->tb6_id == id) {
-			rcu_read_unlock();
+
+	head = &net->ipv6.fib_table_hash[id & (FIB6_TABLE_HASHSZ - 1)];
+
+	/* See comment in fib6_link_table().  RCU is not required,
+	 * but rcu_dereference_raw() is used to avoid data-race.
+	 */
+	hlist_for_each_entry_rcu(tb, head, tb6_hlist, true)
+		if (tb->tb6_id == id)
 			return tb;
-		}
-	}
-	rcu_read_unlock();
 
 	return NULL;
 }
-- 
2.49.0