[PATCH v6 net-next 13/15] net: net_failover: ensure .ndo_get_stats64 can sleep

[Vladimir Oltean <olteanv@gmail.com>]

From: Vladimir Oltean <vladimir.oltean@nxp.com>

The failover framework sets up a virtio_net interface [ when it has the
VIRTIO_NET_F_STANDBY feature ] and a VF interface, having the same MAC
address, in a standby/active relationship. When the active VF is
unplugged, the standby virtio_net temporarily kicks in.

The failover framework registers a common upper for the active and the
standby interface, which is what the application layer uses. This is
similar to bonding/team. The statistics of the upper interface are the
sum of the statistics of the active and of the standby interface.

There is an effort to convert .ndo_get_stats64 to sleepable context, and
for that to work, we need to prevent callers of dev_get_stats from using
atomic locking. The failover driver needs protection via an RCU
read-side critical section to access the standby and the active
interface. This is ok because it is reentrant, and generally speaking,
dev_get_stats is recursive. But it is also not ok, because holding
rcu_read_lock() while calling dev_get_stats will incur atomic context
upon it, and that needs to change.

The existing logic can be rehashed just a little bit such that the
recursive dev_get_stats call will not be under any lock. We can achieve
that by "cheating" a little bit and use dev_hold() to take a reference
on the active and backup interfaces, and netdev_wait_allrefs() will just
have to wait until dev_get_stats() finishes.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
Changes in v6:
Slightly touch up the commit message.

Changes in v5:
Use rcu_read_lock() and do not change the locking architecture of the
driver.

Changes in v4:
Now there is code to propagate errors.

Changes in v3:
None.

Changes in v2:
Switched to the new scheme of holding just a refcnt to the slave
interfaces while recursing with dev_get_stats.

 drivers/net/net_failover.c | 64 ++++++++++++++++++++++++++++----------
 1 file changed, 47 insertions(+), 17 deletions(-)

diff --git a/drivers/net/net_failover.c b/drivers/net/net_failover.c
index 2815228a34d5..8b7a6d1eab30 100644
--- a/drivers/net/net_failover.c
+++ b/drivers/net/net_failover.c
@@ -183,38 +183,64 @@ static int net_failover_get_stats(struct net_device *dev,
 				  struct rtnl_link_stats64 *stats)
 {
 	struct net_failover_info *nfo_info = netdev_priv(dev);
-	struct rtnl_link_stats64 temp;
-	struct net_device *slave_dev;
+	struct rtnl_link_stats64 primary_stats;
+	struct rtnl_link_stats64 standby_stats;
+	struct net_device *primary_dev;
+	struct net_device *standby_dev;
 	int err = 0;
 
-	spin_lock(&nfo_info->stats_lock);
-	memcpy(stats, &nfo_info->failover_stats, sizeof(*stats));
-
 	rcu_read_lock();
 
-	slave_dev = rcu_dereference(nfo_info->primary_dev);
-	if (slave_dev) {
-		err = dev_get_stats(slave_dev, &temp);
+	primary_dev = rcu_dereference(nfo_info->primary_dev);
+	if (primary_dev)
+		dev_hold(primary_dev);
+
+	standby_dev = rcu_dereference(nfo_info->standby_dev);
+	if (standby_dev)
+		dev_hold(standby_dev);
+
+	rcu_read_unlock();
+
+	/* Don't hold rcu_read_lock while calling dev_get_stats, just a
+	 * reference to ensure they won't get unregistered.
+	 */
+	if (primary_dev) {
+		err = dev_get_stats(primary_dev, &primary_stats);
 		if (err)
 			goto out;
-		net_failover_fold_stats(stats, &temp, &nfo_info->primary_stats);
-		memcpy(&nfo_info->primary_stats, &temp, sizeof(temp));
 	}
 
-	slave_dev = rcu_dereference(nfo_info->standby_dev);
-	if (slave_dev) {
-		err = dev_get_stats(slave_dev, &temp);
+	if (standby_dev) {
+		err = dev_get_stats(standby_dev, &standby_stats);
 		if (err)
 			goto out;
-		net_failover_fold_stats(stats, &temp, &nfo_info->standby_stats);
-		memcpy(&nfo_info->standby_stats, &temp, sizeof(temp));
 	}
 
-out:
-	rcu_read_unlock();
+	spin_lock(&nfo_info->stats_lock);
+
+	memcpy(stats, &nfo_info->failover_stats, sizeof(*stats));
+
+	if (primary_dev) {
+		net_failover_fold_stats(stats, &primary_stats,
+					&nfo_info->primary_stats);
+		memcpy(&nfo_info->primary_stats, &primary_stats,
+		       sizeof(primary_stats));
+	}
+	if (standby_dev) {
+		net_failover_fold_stats(stats, &standby_stats,
+					&nfo_info->standby_stats);
+		memcpy(&nfo_info->standby_stats, &standby_stats,
+		       sizeof(standby_stats));
+	}
 
 	memcpy(&nfo_info->failover_stats, stats, sizeof(*stats));
+
 	spin_unlock(&nfo_info->stats_lock);
+out:
+	if (primary_dev)
+		dev_put(primary_dev);
+	if (standby_dev)
+		dev_put(standby_dev);
 
 	return err;
 }
@@ -728,6 +754,7 @@ static struct failover_ops net_failover_ops = {
 struct failover *net_failover_create(struct net_device *standby_dev)
 {
 	struct device *dev = standby_dev->dev.parent;
+	struct net_failover_info *nfo_info;
 	struct net_device *failover_dev;
 	struct failover *failover;
 	int err;
@@ -772,6 +799,9 @@ struct failover *net_failover_create(struct net_device *standby_dev)
 	failover_dev->min_mtu = standby_dev->min_mtu;
 	failover_dev->max_mtu = standby_dev->max_mtu;
 
+	nfo_info = netdev_priv(failover_dev);
+	spin_lock_init(&nfo_info->stats_lock);
+
 	err = register_netdev(failover_dev);
 	if (err) {
 		dev_err(dev, "Unable to register failover_dev!\n");
-- 
2.25.1