tc/act: remove unneeded RCU lock in action callback

Each lockless action currently does its own RCU locking in ->act().
This allows using plain RCU accessor, even if the context
is really RCU BH.

This change drops the per action RCU lock, replace the accessors
with the _bh variant, cleans up a bit the surrounding code and
documents the RCU status in the relevant header.
No functional nor performance change is intended.

The goal of this patch is clarifying that the RCU critical section
used by the tc actions extends up to the classifier's caller.

v1 -> v2:
 - preserve rcu lock in act_bpf: it's needed by eBPF helpers,
   as pointed out by Daniel

v3 -> v4:
 - fixed some typos in the commit message (JiriP)

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/act_api.h

@@ -85,7 +85,7 @@ struct tc_action_ops {
 	size_t	size;
 	struct module		*owner;
 	int     (*act)(struct sk_buff *, const struct tc_action *,
-		       struct tcf_result *);
+		       struct tcf_result *); /* called under RCU BH lock*/
 	int     (*dump)(struct sk_buff *, struct tc_action *, int, int);
 	void	(*cleanup)(struct tc_action *);
 	int     (*lookup)(struct net *net, struct tc_action **a, u32 index,

include/net/sch_generic.h

@@ -285,6 +285,8 @@ struct tcf_proto {
 	/* Fast access part */
 	struct tcf_proto __rcu	*next;
 	void __rcu		*root;
+
+	/* called under RCU BH lock*/
 	int			(*classify)(struct sk_buff *,
 					    const struct tcf_proto *,
 					    struct tcf_result *);

net/sched/act_csum.c

@@ -561,15 +561,14 @@ static int tcf_csum(struct sk_buff *skb, const struct tc_action *a,
 	u32 update_flags;
 	int action;
 
-	rcu_read_lock();
-	params = rcu_dereference(p->params);
+	params = rcu_dereference_bh(p->params);
 
 	tcf_lastuse_update(&p->tcf_tm);
 	bstats_cpu_update(this_cpu_ptr(p->common.cpu_bstats), skb);
 
 	action = READ_ONCE(p->tcf_action);
 	if (unlikely(action == TC_ACT_SHOT))
-		goto drop_stats;
+		goto drop;
 
 	update_flags = params->update_flags;
 	switch (tc_skb_protocol(skb)) {
@@ -583,16 +582,11 @@ static int tcf_csum(struct sk_buff *skb, const struct tc_action *a,
 		break;
 	}
 
-unlock:
-	rcu_read_unlock();
 	return action;
 
 drop:
-	action = TC_ACT_SHOT;
-
-drop_stats:
 	qstats_drop_inc(this_cpu_ptr(p->common.cpu_qstats));
-	goto unlock;
+	return TC_ACT_SHOT;
 }
 
 static int tcf_csum_dump(struct sk_buff *skb, struct tc_action *a, int bind,

net/sched/act_ife.c

@@ -820,14 +820,11 @@ static int tcf_ife_act(struct sk_buff *skb, const struct tc_action *a,
 	struct tcf_ife_params *p;
 	int ret;
 
-	rcu_read_lock();
-	p = rcu_dereference(ife->params);
+	p = rcu_dereference_bh(ife->params);
 	if (p->flags & IFE_ENCODE) {
 		ret = tcf_ife_encode(skb, a, res, p);
-		rcu_read_unlock();
 		return ret;
 	}
-	rcu_read_unlock();
 
 	return tcf_ife_decode(skb, a, res);
 }

net/sched/act_mirred.c

@@ -181,11 +181,10 @@ static int tcf_mirred(struct sk_buff *skb, const struct tc_action *a,
 	tcf_lastuse_update(&m->tcf_tm);
 	bstats_cpu_update(this_cpu_ptr(m->common.cpu_bstats), skb);
 
-	rcu_read_lock();
 	m_mac_header_xmit = READ_ONCE(m->tcfm_mac_header_xmit);
 	m_eaction = READ_ONCE(m->tcfm_eaction);
 	retval = READ_ONCE(m->tcf_action);
-	dev = rcu_dereference(m->tcfm_dev);
+	dev = rcu_dereference_bh(m->tcfm_dev);
 	if (unlikely(!dev)) {
 		pr_notice_once("tc mirred: target device is gone\n");
 		goto out;
@@ -236,7 +235,6 @@ out:
 		if (tcf_mirred_is_act_redirect(m_eaction))
 			retval = TC_ACT_SHOT;
 	}
-	rcu_read_unlock();
 
 	return retval;
 }

net/sched/act_sample.c

@@ -140,8 +140,7 @@ static int tcf_sample_act(struct sk_buff *skb, const struct tc_action *a,
 	bstats_cpu_update(this_cpu_ptr(s->common.cpu_bstats), skb);
 	retval = READ_ONCE(s->tcf_action);
 
-	rcu_read_lock();
-	psample_group = rcu_dereference(s->psample_group);
+	psample_group = rcu_dereference_bh(s->psample_group);
 
 	/* randomly sample packets according to rate */
 	if (psample_group && (prandom_u32() % s->rate == 0)) {
@@ -165,7 +164,6 @@ static int tcf_sample_act(struct sk_buff *skb, const struct tc_action *a,
 			skb_pull(skb, skb->mac_len);
 	}
 
-	rcu_read_unlock();
 	return retval;
 }
 

net/sched/act_skbedit.c

@@ -43,8 +43,7 @@ static int tcf_skbedit(struct sk_buff *skb, const struct tc_action *a,
 	tcf_lastuse_update(&d->tcf_tm);
 	bstats_cpu_update(this_cpu_ptr(d->common.cpu_bstats), skb);
 
-	rcu_read_lock();
-	params = rcu_dereference(d->params);
+	params = rcu_dereference_bh(d->params);
 	action = READ_ONCE(d->tcf_action);
 
 	if (params->flags & SKBEDIT_F_PRIORITY)
@@ -77,14 +76,11 @@ static int tcf_skbedit(struct sk_buff *skb, const struct tc_action *a,
 	}
 	if (params->flags & SKBEDIT_F_PTYPE)
 		skb->pkt_type = params->ptype;
-
-unlock:
-	rcu_read_unlock();
 	return action;
+
 err:
 	qstats_drop_inc(this_cpu_ptr(d->common.cpu_qstats));
-	action = TC_ACT_SHOT;
-	goto unlock;
+	return TC_ACT_SHOT;
 }
 
 static const struct nla_policy skbedit_policy[TCA_SKBEDIT_MAX + 1] = {

net/sched/act_skbmod.c

@@ -41,20 +41,14 @@ static int tcf_skbmod_run(struct sk_buff *skb, const struct tc_action *a,
 	 * then MAX_EDIT_LEN needs to change appropriately
 	*/
 	err = skb_ensure_writable(skb, MAX_EDIT_LEN);
-	if (unlikely(err)) { /* best policy is to drop on the floor */
-		qstats_overlimit_inc(this_cpu_ptr(d->common.cpu_qstats));
-		return TC_ACT_SHOT;
-	}
+	if (unlikely(err)) /* best policy is to drop on the floor */
+		goto drop;
 
-	rcu_read_lock();
 	action = READ_ONCE(d->tcf_action);
-	if (unlikely(action == TC_ACT_SHOT)) {
-		qstats_overlimit_inc(this_cpu_ptr(d->common.cpu_qstats));
-		rcu_read_unlock();
-		return action;
-	}
+	if (unlikely(action == TC_ACT_SHOT))
+		goto drop;
 
-	p = rcu_dereference(d->skbmod_p);
+	p = rcu_dereference_bh(d->skbmod_p);
 	flags = p->flags;
 	if (flags & SKBMOD_F_DMAC)
 		ether_addr_copy(eth_hdr(skb)->h_dest, p->eth_dst);
@@ -62,7 +56,6 @@ static int tcf_skbmod_run(struct sk_buff *skb, const struct tc_action *a,
 		ether_addr_copy(eth_hdr(skb)->h_source, p->eth_src);
 	if (flags & SKBMOD_F_ETYPE)
 		eth_hdr(skb)->h_proto = p->eth_type;
-	rcu_read_unlock();
 
 	if (flags & SKBMOD_F_SWAPMAC) {
 		u16 tmpaddr[ETH_ALEN / 2]; /* ether_addr_copy() requirement */
@@ -73,6 +66,10 @@ static int tcf_skbmod_run(struct sk_buff *skb, const struct tc_action *a,
 	}
 
 	return action;
+
+drop:
+	qstats_overlimit_inc(this_cpu_ptr(d->common.cpu_qstats));
+	return TC_ACT_SHOT;
 }
 
 static const struct nla_policy skbmod_policy[TCA_SKBMOD_MAX + 1] = {

net/sched/act_tunnel_key.c

@@ -31,9 +31,7 @@ static int tunnel_key_act(struct sk_buff *skb, const struct tc_action *a,
 	struct tcf_tunnel_key_params *params;
 	int action;
 
-	rcu_read_lock();
-
-	params = rcu_dereference(t->params);
+	params = rcu_dereference_bh(t->params);
 
 	tcf_lastuse_update(&t->tcf_tm);
 	bstats_cpu_update(this_cpu_ptr(t->common.cpu_bstats), skb);
@@ -53,8 +51,6 @@ static int tunnel_key_act(struct sk_buff *skb, const struct tc_action *a,
 		break;
 	}
 
-	rcu_read_unlock();
-
 	return action;
 }
 

net/sched/act_vlan.c

@@ -40,11 +40,9 @@ static int tcf_vlan(struct sk_buff *skb, const struct tc_action *a,
 	if (skb_at_tc_ingress(skb))
 		skb_push_rcsum(skb, skb->mac_len);
 
-	rcu_read_lock();
-
 	action = READ_ONCE(v->tcf_action);
 
-	p = rcu_dereference(v->vlan_p);
+	p = rcu_dereference_bh(v->vlan_p);
 
 	switch (p->tcfv_action) {
 	case TCA_VLAN_ACT_POP:
@@ -61,7 +59,7 @@ static int tcf_vlan(struct sk_buff *skb, const struct tc_action *a,
 	case TCA_VLAN_ACT_MODIFY:
 		/* No-op if no vlan tag (either hw-accel or in-payload) */
 		if (!skb_vlan_tagged(skb))
-			goto unlock;
+			goto out;
 		/* extract existing tag (and guarantee no hw-accel tag) */
 		if (skb_vlan_tag_present(skb)) {
 			tci = skb_vlan_tag_get(skb);
@@ -86,18 +84,15 @@ static int tcf_vlan(struct sk_buff *skb, const struct tc_action *a,
 		BUG();
 	}
 
-	goto unlock;
-
-drop:
-	action = TC_ACT_SHOT;
-	qstats_drop_inc(this_cpu_ptr(v->common.cpu_qstats));
-
-unlock:
-	rcu_read_unlock();
+out:
 	if (skb_at_tc_ingress(skb))
 		skb_pull_rcsum(skb, skb->mac_len);
 
 	return action;
+
+drop:
+	qstats_drop_inc(this_cpu_ptr(v->common.cpu_qstats));
+	return TC_ACT_SHOT;
 }
 
 static const struct nla_policy vlan_policy[TCA_VLAN_MAX + 1] = {