[PATCH net-next v2 2/10] bonding: rebuild the lock use for bond_mii_monitor()

[Ding Tianhong <dingtianhong@huawei.com>]

The bond_mii_monitor() still use bond lock to protect bond slave list,
it is no effect, I have 2 way to fix the problem, move the RTNL to the
top of the function, or add RCU to protect the bond_has_slaves() and
bond_miimon_inspect(), according to the Jay Vosburgh's opinion, 10 times
one second is a truely big performance loss if use RTNL to protect the
whole function, so I would take the advice and use RCU to protect the
two functions, of course it need to add more modify, the
bond_has_slave_rcu() is add for RCU use, and the bond_for_each_slave
need to replace with bond_for_each_slave_rcu in bond_miimon_inspect.

I move the peer notify before the queue_delayed_work(), and obviously
it is no need to lock the RTNL twice if call bond_miimon_commit() and
peer notify together, other path is no logic change, I think the
performance is better than before.

Suggested-by: Jay Vosburgh <fubar@us.ibm.com>
Suggested-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
---
 drivers/net/bonding/bond_main.c | 40 ++++++++++++++++++++--------------------
 drivers/net/bonding/bonding.h   |  6 ++++++
 2 files changed, 26 insertions(+), 20 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index ba18719..def489d 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -1913,7 +1913,7 @@ static int bond_miimon_inspect(struct bonding *bond)
 
 	ignore_updelay = !bond->curr_active_slave ? true : false;
 
-	bond_for_each_slave(bond, slave, iter) {
+	bond_for_each_slave_rcu(bond, slave, iter) {
 		slave->new_link = BOND_LINK_NOCHANGE;
 
 		link_state = bond_check_dev_link(bond, slave->dev, 0);
@@ -2111,47 +2111,47 @@ void bond_mii_monitor(struct work_struct *work)
 	bool should_notify_peers = false;
 	unsigned long delay;
 
-	read_lock(&bond->lock);
-
 	delay = msecs_to_jiffies(bond->params.miimon);
 
-	if (!bond_has_slaves(bond))
+	rcu_read_lock();
+
+	if (!bond_has_slaves_rcu(bond)) {
+		rcu_read_unlock();
 		goto re_arm;
+	}
 
 	should_notify_peers = bond_should_notify_peers(bond);
 
 	if (bond_miimon_inspect(bond)) {
-		read_unlock(&bond->lock);
+		rcu_read_unlock();
 
 		/* Race avoidance with bond_close cancel of workqueue */
 		if (!rtnl_trylock()) {
-			read_lock(&bond->lock);
 			delay = 1;
-			should_notify_peers = false;
 			goto re_arm;
 		}
 
-		read_lock(&bond->lock);
-
 		bond_miimon_commit(bond);
 
-		read_unlock(&bond->lock);
+		if (should_notify_peers)
+			call_netdevice_notifiers(NETDEV_NOTIFY_PEERS,
+					bond->dev);
+
 		rtnl_unlock();	/* might sleep, hold no other locks */
-		read_lock(&bond->lock);
+	} else {
+		rcu_read_unlock();
+		if (should_notify_peers) {
+			if (!rtnl_trylock())
+				goto re_arm;
+			call_netdevice_notifiers(NETDEV_NOTIFY_PEERS,
+					bond->dev);
+			rtnl_unlock();
+		}
 	}
 
 re_arm:
 	if (bond->params.miimon)
 		queue_delayed_work(bond->wq, &bond->mii_work, delay);
-
-	read_unlock(&bond->lock);
-
-	if (should_notify_peers) {
-		if (!rtnl_trylock())
-			return;
-		call_netdevice_notifiers(NETDEV_NOTIFY_PEERS, bond->dev);
-		rtnl_unlock();
-	}
 }
 
 static bool bond_has_this_ip(struct bonding *bond, __be32 ip)
diff --git a/drivers/net/bonding/bonding.h b/drivers/net/bonding/bonding.h
index 046a605..deb9738 100644
--- a/drivers/net/bonding/bonding.h
+++ b/drivers/net/bonding/bonding.h
@@ -81,6 +81,12 @@
 
 #define bond_has_slaves(bond) !list_empty(bond_slave_list(bond))
 
+#define bond_has_slaves_rcu(bond)	\
+	({struct list_head *__ptr = (bond_slave_list(bond)); \
+	 struct list_head *__next = ACCESS_ONCE(__ptr->next); \
+	 __ptr != __next; \
+	 })
+
 /* IMPORTANT: bond_first/last_slave can return NULL in case of an empty list */
 #define bond_first_slave(bond) \
 	(bond_has_slaves(bond) ? \
-- 
1.8.2.1