[PATCH net-next v2 04/15] net: sched: act_ife: remove dependency on rtnl lock

[Vlad Buslov <vladbu@mellanox.com>]

Use tcf spinlock and rcu to protect params pointer from concurrent
modification during dump and init. Use rcu swap operation to reassign
params pointer under protection of tcf lock. (old params value is not used
by init, so there is no need of standalone rcu dereference step)

Ife action has meta-actions that are compiled as standalone modules. Rtnl
mutex must be released while loading a kernel module. In order to support
execution without rtnl mutex, propagate 'rtnl_held' argument to meta action
loading functions. When requesting meta action module, conditionally
release rtnl lock depending on 'rtnl_held' argument.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
---
 net/sched/act_ife.c | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/net/sched/act_ife.c b/net/sched/act_ife.c
index df4060e32d43..5d200495e467 100644
--- a/net/sched/act_ife.c
+++ b/net/sched/act_ife.c
@@ -268,7 +268,8 @@ static const char *ife_meta_id2name(u32 metaid)
  * under ife->tcf_lock for existing action
 */
 static int load_metaops_and_vet(struct tcf_ife_info *ife, u32 metaid,
-				void *val, int len, bool exists)
+				void *val, int len, bool exists,
+				bool rtnl_held)
 {
 	struct tcf_meta_ops *ops = find_ife_oplist(metaid);
 	int ret = 0;
@@ -278,9 +279,11 @@ static int load_metaops_and_vet(struct tcf_ife_info *ife, u32 metaid,
 #ifdef CONFIG_MODULES
 		if (exists)
 			spin_unlock_bh(&ife->tcf_lock);
-		rtnl_unlock();
+		if (rtnl_held)
+			rtnl_unlock();
 		request_module("ife-meta-%s", ife_meta_id2name(metaid));
-		rtnl_lock();
+		if (rtnl_held)
+			rtnl_lock();
 		if (exists)
 			spin_lock_bh(&ife->tcf_lock);
 		ops = find_ife_oplist(metaid);
@@ -421,7 +424,7 @@ static void tcf_ife_cleanup(struct tc_action *a)
 
 /* under ife->tcf_lock for existing action */
 static int populate_metalist(struct tcf_ife_info *ife, struct nlattr **tb,
-			     bool exists)
+			     bool exists, bool rtnl_held)
 {
 	int len = 0;
 	int rc = 0;
@@ -433,7 +436,8 @@ static int populate_metalist(struct tcf_ife_info *ife, struct nlattr **tb,
 			val = nla_data(tb[i]);
 			len = nla_len(tb[i]);
 
-			rc = load_metaops_and_vet(ife, i, val, len, exists);
+			rc = load_metaops_and_vet(ife, i, val, len, exists,
+						  rtnl_held);
 			if (rc != 0)
 				return rc;
 
@@ -454,7 +458,7 @@ static int tcf_ife_init(struct net *net, struct nlattr *nla,
 	struct tc_action_net *tn = net_generic(net, ife_net_id);
 	struct nlattr *tb[TCA_IFE_MAX + 1];
 	struct nlattr *tb2[IFE_META_MAX + 1];
-	struct tcf_ife_params *p, *p_old;
+	struct tcf_ife_params *p;
 	struct tcf_ife_info *ife;
 	u16 ife_type = ETH_P_IFE;
 	struct tc_ife *parm;
@@ -558,7 +562,7 @@ static int tcf_ife_init(struct net *net, struct nlattr *nla,
 			return err;
 		}
 
-		err = populate_metalist(ife, tb2, exists);
+		err = populate_metalist(ife, tb2, exists, rtnl_held);
 		if (err)
 			goto metadata_parse_err;
 
@@ -581,13 +585,13 @@ static int tcf_ife_init(struct net *net, struct nlattr *nla,
 	}
 
 	ife->tcf_action = parm->action;
+	/* protected by tcf_lock when modifying existing action */
+	rcu_swap_protected(ife->params, p, 1);
+
 	if (exists)
 		spin_unlock_bh(&ife->tcf_lock);
-
-	p_old = rtnl_dereference(ife->params);
-	rcu_assign_pointer(ife->params, p);
-	if (p_old)
-		kfree_rcu(p_old, rcu);
+	if (p)
+		kfree_rcu(p, rcu);
 
 	if (ret == ACT_P_CREATED)
 		tcf_idr_insert(tn, *a);
@@ -600,16 +604,20 @@ static int tcf_ife_dump(struct sk_buff *skb, struct tc_action *a, int bind,
 {
 	unsigned char *b = skb_tail_pointer(skb);
 	struct tcf_ife_info *ife = to_ife(a);
-	struct tcf_ife_params *p = rtnl_dereference(ife->params);
+	struct tcf_ife_params *p;
 	struct tc_ife opt = {
 		.index = ife->tcf_index,
 		.refcnt = refcount_read(&ife->tcf_refcnt) - ref,
 		.bindcnt = atomic_read(&ife->tcf_bindcnt) - bind,
-		.action = ife->tcf_action,
-		.flags = p->flags,
 	};
 	struct tcf_t t;
 
+	spin_lock_bh(&ife->tcf_lock);
+	opt.action = ife->tcf_action;
+	p = rcu_dereference_protected(ife->params,
+				      lockdep_is_held(&ife->tcf_lock));
+	opt.flags = p->flags;
+
 	if (nla_put(skb, TCA_IFE_PARMS, sizeof(opt), &opt))
 		goto nla_put_failure;
 
@@ -635,9 +643,11 @@ static int tcf_ife_dump(struct sk_buff *skb, struct tc_action *a, int bind,
 		pr_info("Failed to dump metalist\n");
 	}
 
+	spin_unlock_bh(&ife->tcf_lock);
 	return skb->len;
 
 nla_put_failure:
+	spin_unlock_bh(&ife->tcf_lock);
 	nlmsg_trim(skb, b);
 	return -1;
 }
-- 
2.7.5