[PATCH v2] net: ipmr: fix suspicious RCU warning

[Anders Roxell <anders.roxell@linaro.org>]

When booting an arm64 allmodconfig kernel on linux-next next-20191115
The following "suspicious RCU usage" warning shows up.  This bug seems
to have been introduced by commit f0ad0860d01e ("ipv4: ipmr: support
multiple tables") in 2010, but the warning was added only in this past
year by commit 28875945ba98 ("rcu: Add support for consolidated-RCU
reader checking").

[   32.496021][    T1] =============================
[   32.497616][    T1] WARNING: suspicious RCU usage
[   32.499614][    T1] 5.4.0-rc6-next-20191108-00003-gf74bac957b5c-dirty #2 Not tainted
[   32.502018][    T1] -----------------------------
[   32.503976][    T1] net/ipv4/ipmr.c:136 RCU-list traversed in non-reader section!!
[   32.506746][    T1]
[   32.506746][    T1] other info that might help us debug this:
[   32.506746][    T1]
[   32.509794][    T1]
[   32.509794][    T1] rcu_scheduler_active = 2, debug_locks = 1
[   32.512661][    T1] 1 lock held by swapper/0/1:
[   32.514169][    T1]  #0: ffffa000150dd678 (pernet_ops_rwsem){+.+.}, at: register_pernet_subsys+0x24/0x50
[   32.517621][    T1]
[   32.517621][    T1] stack backtrace:
[   32.519930][    T1] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.4.0-rc6-next-20191108-00003-gf74bac957b5c-dirty #2
[   32.523063][    T1] Hardware name: linux,dummy-virt (DT)
[   32.524787][    T1] Call trace:
[   32.525946][    T1]  dump_backtrace+0x0/0x2d0
[   32.527433][    T1]  show_stack+0x20/0x30
[   32.528811][    T1]  dump_stack+0x204/0x2ac
[   32.530258][    T1]  lockdep_rcu_suspicious+0xf4/0x108
[   32.531993][    T1]  ipmr_get_table+0xc8/0x170
[   32.533496][    T1]  ipmr_new_table+0x48/0xa0
[   32.535002][    T1]  ipmr_net_init+0xe8/0x258
[   32.536465][    T1]  ops_init+0x280/0x2d8
[   32.537876][    T1]  register_pernet_operations+0x210/0x420
[   32.539707][    T1]  register_pernet_subsys+0x30/0x50
[   32.541372][    T1]  ip_mr_init+0x54/0x180
[   32.542785][    T1]  inet_init+0x25c/0x3e8
[   32.544186][    T1]  do_one_initcall+0x4c0/0xad8
[   32.545757][    T1]  kernel_init_freeable+0x3e0/0x500
[   32.547443][    T1]  kernel_init+0x14/0x1f0
[   32.548875][    T1]  ret_from_fork+0x10/0x18

This commit therefore holds RTNL mutex around the problematic code path,
which is function ipmr_rules_init() in ipmr_net_init().  This commit
also adds a lockdep_rtnl_is_held() check to the ipmr_for_each_table()
macro.

Suggested-by: David Miller <davem@davemloft.net>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
---
 net/ipv4/ipmr.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index 6e68def66822..53dff9a0e60a 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -110,7 +110,8 @@ static void ipmr_expire_process(struct timer_list *t);
 
 #ifdef CONFIG_IP_MROUTE_MULTIPLE_TABLES
 #define ipmr_for_each_table(mrt, net) \
-	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list)
+	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list, \
+				lockdep_rtnl_is_held())
 
 static struct mr_table *ipmr_mr_table_iter(struct net *net,
 					   struct mr_table *mrt)
@@ -3086,7 +3087,9 @@ static int __net_init ipmr_net_init(struct net *net)
 	if (err)
 		goto ipmr_notifier_fail;
 
+	rtnl_lock();
 	err = ipmr_rules_init(net);
+	rtnl_unlock();
 	if (err < 0)
 		goto ipmr_rules_fail;
 
-- 
2.20.1