team: protect features update by RCU to avoid deadlock

Function __team_compute_features() is protected by team->lock
mutex when it is called from team_compute_features() used when
features of an underlying device is changed. This causes
a deadlock when NETDEV_FEAT_CHANGE notifier for underlying device
is fired due to change propagated from team driver (e.g. MTU
change). It's because callbacks like team_change_mtu() or
team_vlan_rx_{add,del}_vid() protect their port list traversal
by team->lock mutex.

Example (r8169 case where this driver disables TSO for certain MTU
values):
...
[ 6391.348202]  __mutex_lock.isra.6+0x2d0/0x4a0
[ 6391.358602]  team_device_event+0x9d/0x160 [team]
[ 6391.363756]  notifier_call_chain+0x47/0x70
[ 6391.368329]  netdev_update_features+0x56/0x60
[ 6391.373207]  rtl8169_change_mtu+0x14/0x50 [r8169]
[ 6391.378457]  dev_set_mtu_ext+0xe1/0x1d0
[ 6391.387022]  dev_set_mtu+0x52/0x90
[ 6391.390820]  team_change_mtu+0x64/0xf0 [team]
[ 6391.395683]  dev_set_mtu_ext+0xe1/0x1d0
[ 6391.399963]  do_setlink+0x231/0xf50
...

In fact team_compute_features() called from team_device_event()
does not need to be protected by team->lock mutex and rcu_read_lock()
is sufficient there for port list traversal.

Fixes: 3d249d4c ("net: introduce ethernet teaming device")
Cc: Saeed Mahameed <saeed@kernel.org>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Reviewed-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20210125074416.4056484-1-ivecera@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/team/team.c

@@ -992,7 +992,8 @@ static void __team_compute_features(struct team *team)
 	unsigned int dst_release_flag = IFF_XMIT_DST_RELEASE |
 					IFF_XMIT_DST_RELEASE_PERM;
 
-	list_for_each_entry(port, &team->port_list, list) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(port, &team->port_list, list) {
 		vlan_features = netdev_increment_features(vlan_features,
 					port->dev->vlan_features,
 					TEAM_VLAN_FEATURES);
@@ -1006,6 +1007,7 @@ static void __team_compute_features(struct team *team)
 		if (port->dev->hard_header_len > max_hard_header_len)
 			max_hard_header_len = port->dev->hard_header_len;
 	}
+	rcu_read_unlock();
 
 	team->dev->vlan_features = vlan_features;
 	team->dev->hw_enc_features = enc_features | NETIF_F_GSO_ENCAP_ALL |
@@ -1020,9 +1022,7 @@ static void __team_compute_features(struct team *team)
 
 static void team_compute_features(struct team *team)
 {
-	mutex_lock(&team->lock);
 	__team_compute_features(team);
-	mutex_unlock(&team->lock);
 	netdev_change_features(team->dev);
 }