[PATCH net-next v3 07/16] net: sched: introduce reference counting for tcf_proto

[Vlad Buslov <vladbu@mellanox.com>]

In order to remove dependency on rtnl lock and allow concurrent tcf_proto
modification, extend tcf_proto with reference counter. Implement helper
get/put functions for tcf proto and use them to modify cls API to always
take reference to tcf_proto while using it. Only release reference to
parent chain after releasing last reference to tp.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
---

Changes from V2 to V3:
  - Add local tp_next var to tcf_chain_flush() and use it to store
    tp->next pointer dereferenced with rcu_dereference_protected() to
    satisfy kbuild test robot.
  - Reset tp pointer to NULL at the beginning of tc_new_tfilter() to
    prevent its uninitialized usage in error handling code. This code
    was already implemented in patch 10, but must be in patch 8 to
    preserve code bisectability.
  - Put parent chain in tcf_proto_destroy(). In previous version this
    code was implemented in patch 1 which was removed in V3.

 include/net/sch_generic.h |  1 +
 net/sched/cls_api.c       | 53 ++++++++++++++++++++++++++++++++++++++---------
 2 files changed, 44 insertions(+), 10 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 85993d7efee6..4372c08fc4d9 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -322,6 +322,7 @@ struct tcf_proto {
 	void			*data;
 	const struct tcf_proto_ops	*ops;
 	struct tcf_chain	*chain;
+	refcount_t		refcnt;
 	struct rcu_head		rcu;
 };
 
diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index d3fa1043b7fd..90a59b5966fe 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -172,6 +172,7 @@ static struct tcf_proto *tcf_proto_create(const char *kind, u32 protocol,
 	tp->protocol = protocol;
 	tp->prio = prio;
 	tp->chain = chain;
+	refcount_set(&tp->refcnt, 1);
 
 	err = tp->ops->init(tp);
 	if (err) {
@@ -185,14 +186,29 @@ static struct tcf_proto *tcf_proto_create(const char *kind, u32 protocol,
 	return ERR_PTR(err);
 }
 
+static void tcf_proto_get(struct tcf_proto *tp)
+{
+	refcount_inc(&tp->refcnt);
+}
+
+static void tcf_chain_put(struct tcf_chain *chain);
+
 static void tcf_proto_destroy(struct tcf_proto *tp,
 			      struct netlink_ext_ack *extack)
 {
 	tp->ops->destroy(tp, extack);
+	tcf_chain_put(tp->chain);
 	module_put(tp->ops->owner);
 	kfree_rcu(tp, rcu);
 }
 
+static void tcf_proto_put(struct tcf_proto *tp,
+			  struct netlink_ext_ack *extack)
+{
+	if (refcount_dec_and_test(&tp->refcnt))
+		tcf_proto_destroy(tp, extack);
+}
+
 #define ASSERT_BLOCK_LOCKED(block)					\
 	lockdep_assert_held(&(block)->lock)
 
@@ -437,18 +453,18 @@ static void tcf_chain_put_explicitly_created(struct tcf_chain *chain)
 
 static void tcf_chain_flush(struct tcf_chain *chain)
 {
-	struct tcf_proto *tp;
+	struct tcf_proto *tp, *tp_next;
 
 	mutex_lock(&chain->filter_chain_lock);
 	tp = tcf_chain_dereference(chain->filter_chain, chain);
+	RCU_INIT_POINTER(chain->filter_chain, NULL);
 	tcf_chain0_head_change(chain, NULL);
 	mutex_unlock(&chain->filter_chain_lock);
 
 	while (tp) {
-		RCU_INIT_POINTER(chain->filter_chain, tp->next);
-		tcf_proto_destroy(tp, NULL);
-		tp = rtnl_dereference(chain->filter_chain);
-		tcf_chain_put(chain);
+		tp_next = rcu_dereference_protected(tp->next, 1);
+		tcf_proto_put(tp, NULL);
+		tp = tp_next;
 	}
 }
 
@@ -1492,9 +1508,9 @@ static void tcf_chain_tp_insert(struct tcf_chain *chain,
 {
 	if (*chain_info->pprev == chain->filter_chain)
 		tcf_chain0_head_change(chain, tp);
+	tcf_proto_get(tp);
 	RCU_INIT_POINTER(tp->next, tcf_chain_tp_prev(chain, chain_info));
 	rcu_assign_pointer(*chain_info->pprev, tp);
-	tcf_chain_hold(chain);
 }
 
 static void tcf_chain_tp_remove(struct tcf_chain *chain,
@@ -1506,7 +1522,6 @@ static void tcf_chain_tp_remove(struct tcf_chain *chain,
 	if (tp == chain->filter_chain)
 		tcf_chain0_head_change(chain, next);
 	RCU_INIT_POINTER(*chain_info->pprev, next);
-	tcf_chain_put(chain);
 }
 
 static struct tcf_proto *tcf_chain_tp_find(struct tcf_chain *chain,
@@ -1533,7 +1548,12 @@ static struct tcf_proto *tcf_chain_tp_find(struct tcf_chain *chain,
 		}
 	}
 	chain_info->pprev = pprev;
-	chain_info->next = tp ? tp->next : NULL;
+	if (tp) {
+		chain_info->next = tp->next;
+		tcf_proto_get(tp);
+	} else {
+		chain_info->next = NULL;
+	}
 	return tp;
 }
 
@@ -1691,6 +1711,7 @@ static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 	prio = TC_H_MAJ(t->tcm_info);
 	prio_allocate = false;
 	parent = t->tcm_parent;
+	tp = NULL;
 	cl = 0;
 
 	if (prio == 0) {
@@ -1808,6 +1829,12 @@ static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 errout:
 	if (chain)
 		tcf_chain_put(chain);
+	if (chain) {
+		if (tp && !IS_ERR(tp))
+			tcf_proto_put(tp, NULL);
+		if (!tp_created)
+			tcf_chain_put(chain);
+	}
 	tcf_block_release(q, block);
 	if (err == -EAGAIN)
 		/* Replay the request. */
@@ -1938,8 +1965,11 @@ static int tc_del_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 	}
 
 errout:
-	if (chain)
+	if (chain) {
+		if (tp && !IS_ERR(tp))
+			tcf_proto_put(tp, NULL);
 		tcf_chain_put(chain);
+	}
 	tcf_block_release(q, block);
 	return err;
 
@@ -2030,8 +2060,11 @@ static int tc_get_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 	}
 
 errout:
-	if (chain)
+	if (chain) {
+		if (tp && !IS_ERR(tp))
+			tcf_proto_put(tp, NULL);
 		tcf_chain_put(chain);
+	}
 	tcf_block_release(q, block);
 	return err;
 }
-- 
2.13.6