IPv4 local and main route tables are merged by default to avoid
unnecessary rule lookups.

When the first IPv4 rule is created, fib_unmerge() splits the
two tables.

However, fib4_rule_configure() currently always calls fib_unmerge(),
and even fetching a table via fib_get_table() requires RTNL (or RCU).

We will drop RTNL from fib_newrule() if not needed.

Let's call fib_unmerge() only once for the first rule.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv4/fib_rules.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/fib_rules.c b/net/ipv4/fib_rules.c
index 51d0ab423ed4..16d202246a36 100644
--- a/net/ipv4/fib_rules.c
+++ b/net/ipv4/fib_rules.c

@@ -301,10 +301,12 @@ static int fib4_rule_configure(struct fib_rule *rule, struct sk_buff *skb,
 	    fib4_nl2rule_dscp_mask(tb[FRA_DSCP_MASK], rule4, extack) < 0)
 		goto errout;
 
-	/* split local/main if they are not already split */
-	err = fib_unmerge(net);
-	if (err)
-		goto errout;
+	if (!net->ipv4.fib_has_custom_rules) {
+		/* split local/main if they are not already split */
+		err = fib_unmerge(net);
+		if (err)
+			goto errout;
+	}
 
 	if (rule->table == RT_TABLE_UNSPEC && !rule->l3mdev) {
 		if (rule->action == FR_ACT_TO_TBL) {

-- 
2.55.0.rc0.799.gd6f94ed593-goog