8021q: use RCU for egress QoS mappings

The TX fast path and reporting paths walk egress QoS mappings without
RTNL. Convert the mapping lists to RCU-protected pointers, use RCU
reader annotations in readers, and defer freeing mapping nodes with an
embedded rcu_head.

This prepares the egress QoS mapping code for safe removal of mapping
nodes in a follow-up change while preserving the current behavior.

Co-developed-by: Yuan Tan <yuantan098@gmail.com>
Signed-off-by: Yuan Tan <yuantan098@gmail.com>
Signed-off-by: Longxuan Yu <ylong030@ucr.edu>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Link: https://patch.msgid.link/9136768189f8c6d3f824f476c62d2fa1111688e8.1776647968.git.yuantan098@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: fengsxy

include/linux/if_vlan.h

@@ -147,11 +147,13 @@ extern __be16 vlan_dev_vlan_proto(const struct net_device *dev);
  *	@priority: skb priority
  *	@vlan_qos: vlan priority: (skb->priority << 13) & 0xE000
  *	@next: pointer to next struct
+ *	@rcu: used for deferred freeing of mapping nodes
  */
 struct vlan_priority_tci_mapping {
 	u32					priority;
 	u16					vlan_qos;
-	struct vlan_priority_tci_mapping	*next;
+	struct vlan_priority_tci_mapping __rcu	*next;
+	struct rcu_head			rcu;
 };
 
 struct proc_dir_entry;
@@ -177,7 +179,7 @@ struct vlan_dev_priv {
 	unsigned int				nr_ingress_mappings;
 	u32					ingress_priority_map[8];
 	unsigned int				nr_egress_mappings;
-	struct vlan_priority_tci_mapping	*egress_priority_map[16];
+	struct vlan_priority_tci_mapping __rcu	*egress_priority_map[16];
 
 	__be16					vlan_proto;
 	u16					vlan_id;
@@ -209,19 +211,24 @@ static inline u16
 vlan_dev_get_egress_qos_mask(struct net_device *dev, u32 skprio)
 {
 	struct vlan_priority_tci_mapping *mp;
+	u16 vlan_qos = 0;
 
-	smp_rmb(); /* coupled with smp_wmb() in vlan_dev_set_egress_priority() */
+	rcu_read_lock();
 
-	mp = vlan_dev_priv(dev)->egress_priority_map[(skprio & 0xF)];
+	mp = rcu_dereference(vlan_dev_priv(dev)->egress_priority_map[skprio & 0xF]);
 	while (mp) {
 		if (mp->priority == skprio) {
-			return mp->vlan_qos; /* This should already be shifted
-					      * to mask correctly with the
-					      * VLAN's TCI */
+			vlan_qos = READ_ONCE(mp->vlan_qos);
+			break;
 		}
-		mp = mp->next;
+		mp = rcu_dereference(mp->next);
 	}
-	return 0;
+	rcu_read_unlock();
+
+	/* This should already be shifted to mask correctly with
+	 * the VLAN's TCI.
+	 */
+	return vlan_qos;
 }
 
 extern bool vlan_do_receive(struct sk_buff **skb);

net/8021q/vlan_dev.c

@@ -172,39 +172,34 @@ int vlan_dev_set_egress_priority(const struct net_device *dev,
 				 u32 skb_prio, u16 vlan_prio)
 {
 	struct vlan_dev_priv *vlan = vlan_dev_priv(dev);
-	struct vlan_priority_tci_mapping *mp = NULL;
+	struct vlan_priority_tci_mapping *mp;
 	struct vlan_priority_tci_mapping *np;
+	u32 bucket = skb_prio & 0xF;
 	u32 vlan_qos = (vlan_prio << VLAN_PRIO_SHIFT) & VLAN_PRIO_MASK;
 
 	/* See if a priority mapping exists.. */
-	mp = vlan->egress_priority_map[skb_prio & 0xF];
+	mp = rtnl_dereference(vlan->egress_priority_map[bucket]);
 	while (mp) {
 		if (mp->priority == skb_prio) {
 			if (mp->vlan_qos && !vlan_qos)
 				vlan->nr_egress_mappings--;
 			else if (!mp->vlan_qos && vlan_qos)
 				vlan->nr_egress_mappings++;
-			mp->vlan_qos = vlan_qos;
+			WRITE_ONCE(mp->vlan_qos, vlan_qos);
 			return 0;
 		}
-		mp = mp->next;
+		mp = rtnl_dereference(mp->next);
 	}
 
 	/* Create a new mapping then. */
-	mp = vlan->egress_priority_map[skb_prio & 0xF];
 	np = kmalloc_obj(struct vlan_priority_tci_mapping);
 	if (!np)
 		return -ENOBUFS;
 
-	np->next = mp;
 	np->priority = skb_prio;
 	np->vlan_qos = vlan_qos;
-	/* Before inserting this element in hash table, make sure all its fields
-	 * are committed to memory.
-	 * coupled with smp_rmb() in vlan_dev_get_egress_qos_mask()
-	 */
-	smp_wmb();
-	vlan->egress_priority_map[skb_prio & 0xF] = np;
+	RCU_INIT_POINTER(np->next, rtnl_dereference(vlan->egress_priority_map[bucket]));
+	rcu_assign_pointer(vlan->egress_priority_map[bucket], np);
 	if (vlan_qos)
 		vlan->nr_egress_mappings++;
 	return 0;
@@ -604,11 +599,17 @@ void vlan_dev_free_egress_priority(const struct net_device *dev)
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(vlan->egress_priority_map); i++) {
-		while ((pm = vlan->egress_priority_map[i]) != NULL) {
-			vlan->egress_priority_map[i] = pm->next;
-			kfree(pm);
+		pm = rtnl_dereference(vlan->egress_priority_map[i]);
+		RCU_INIT_POINTER(vlan->egress_priority_map[i], NULL);
+		while (pm) {
+			struct vlan_priority_tci_mapping *next;
+
+			next = rtnl_dereference(pm->next);
+			kfree_rcu(pm, rcu);
+			pm = next;
 		}
 	}
+	vlan->nr_egress_mappings = 0;
 }
 
 static void vlan_dev_uninit(struct net_device *dev)

net/8021q/vlan_netlink.c

@@ -260,13 +260,15 @@ static int vlan_fill_info(struct sk_buff *skb, const struct net_device *dev)
 			goto nla_put_failure;
 
 		for (i = 0; i < ARRAY_SIZE(vlan->egress_priority_map); i++) {
-			for (pm = vlan->egress_priority_map[i]; pm;
-			     pm = pm->next) {
-				if (!pm->vlan_qos)
+			for (pm = rcu_dereference_rtnl(vlan->egress_priority_map[i]); pm;
+			     pm = rcu_dereference_rtnl(pm->next)) {
+				u16 vlan_qos = READ_ONCE(pm->vlan_qos);
+
+				if (!vlan_qos)
 					continue;
 
 				m.from = pm->priority;
-				m.to   = (pm->vlan_qos >> 13) & 0x7;
+				m.to   = (vlan_qos >> 13) & 0x7;
 				if (nla_put(skb, IFLA_VLAN_QOS_MAPPING,
 					    sizeof(m), &m))
 					goto nla_put_failure;

net/8021q/vlanproc.c

@@ -262,15 +262,19 @@ static int vlandev_seq_show(struct seq_file *seq, void *offset)
 		   vlan->ingress_priority_map[7]);
 
 	seq_printf(seq, " EGRESS priority mappings: ");
+	rcu_read_lock();
 	for (i = 0; i < 16; i++) {
-		const struct vlan_priority_tci_mapping *mp
-			= vlan->egress_priority_map[i];
+		const struct vlan_priority_tci_mapping *mp =
+			rcu_dereference(vlan->egress_priority_map[i]);
 		while (mp) {
+			u16 vlan_qos = READ_ONCE(mp->vlan_qos);
+
 			seq_printf(seq, "%u:%d ",
-				   mp->priority, ((mp->vlan_qos >> 13) & 0x7));
-			mp = mp->next;
+				   mp->priority, ((vlan_qos >> 13) & 0x7));
+			mp = rcu_dereference(mp->next);
 		}
 	}
+	rcu_read_unlock();
 	seq_puts(seq, "\n");
 
 	return 0;