[PATCH net-next 05/13] net: sched: use Qdisc rcu API instead of relying on rtnl lock

[Vlad Buslov <vladbu@mellanox.com>]

As a preparation from removing rtnl lock dependency from rules update path,
use Qdisc rcu and reference counting capabilities instead of relying on
rtnl lock while working with Qdiscs. Create new tcf_block_release()
function, and use it to free resources taken by tcf_block_find().
Currently, this function only releases Qdisc and it is extended in next
patches in this series.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
---
 net/sched/cls_api.c | 88 ++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 73 insertions(+), 15 deletions(-)

diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index 1a67af8a6e8c..cfa4a02a6a1a 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -527,6 +527,17 @@ static struct tcf_block *tcf_block_lookup(struct net *net, u32 block_index)
 	return idr_find(&tn->idr, block_index);
 }
 
+static void tcf_qdisc_put(struct Qdisc *q, bool rtnl_held)
+{
+	if (!q)
+		return;
+
+	if (rtnl_held)
+		qdisc_put(q);
+	else
+		qdisc_put_unlocked(q);
+}
+
 /* Find tcf block.
  * Set q, parent, cl when appropriate.
  */
@@ -537,6 +548,7 @@ static struct tcf_block *tcf_block_find(struct net *net, struct Qdisc **q,
 					struct netlink_ext_ack *extack)
 {
 	struct tcf_block *block;
+	int err = 0;
 
 	if (ifindex == TCM_IFINDEX_MAGIC_BLOCK) {
 		block = tcf_block_lookup(net, block_index);
@@ -548,55 +560,91 @@ static struct tcf_block *tcf_block_find(struct net *net, struct Qdisc **q,
 		const struct Qdisc_class_ops *cops;
 		struct net_device *dev;
 
+		rcu_read_lock();
+
 		/* Find link */
-		dev = __dev_get_by_index(net, ifindex);
-		if (!dev)
+		dev = dev_get_by_index_rcu(net, ifindex);
+		if (!dev) {
+			rcu_read_unlock();
 			return ERR_PTR(-ENODEV);
+		}
 
 		/* Find qdisc */
 		if (!*parent) {
 			*q = dev->qdisc;
 			*parent = (*q)->handle;
 		} else {
-			*q = qdisc_lookup(dev, TC_H_MAJ(*parent));
+			*q = qdisc_lookup_rcu(dev, TC_H_MAJ(*parent));
 			if (!*q) {
 				NL_SET_ERR_MSG(extack, "Parent Qdisc doesn't exists");
-				return ERR_PTR(-EINVAL);
+				err = -EINVAL;
+				goto errout_rcu;
 			}
 		}
 
+		*q = qdisc_refcount_inc_nz(*q);
+		if (!*q) {
+			NL_SET_ERR_MSG(extack, "Parent Qdisc doesn't exists");
+			err = -EINVAL;
+			goto errout_rcu;
+		}
+
 		/* Is it classful? */
 		cops = (*q)->ops->cl_ops;
 		if (!cops) {
 			NL_SET_ERR_MSG(extack, "Qdisc not classful");
-			return ERR_PTR(-EINVAL);
+			err = -EINVAL;
+			goto errout_rcu;
 		}
 
 		if (!cops->tcf_block) {
 			NL_SET_ERR_MSG(extack, "Class doesn't support blocks");
-			return ERR_PTR(-EOPNOTSUPP);
+			err = -EOPNOTSUPP;
+			goto errout_rcu;
 		}
 
+		/* At this point we know that qdisc is not noop_qdisc,
+		 * which means that qdisc holds a reference to net_device
+		 * and we hold a reference to qdisc, so it is safe to release
+		 * rcu read lock.
+		 */
+		rcu_read_unlock();
+
 		/* Do we search for filter, attached to class? */
 		if (TC_H_MIN(*parent)) {
 			*cl = cops->find(*q, *parent);
 			if (*cl == 0) {
 				NL_SET_ERR_MSG(extack, "Specified class doesn't exist");
-				return ERR_PTR(-ENOENT);
+				err = -ENOENT;
+				goto errout_qdisc;
 			}
 		}
 
 		/* And the last stroke */
 		block = cops->tcf_block(*q, *cl, extack);
-		if (!block)
-			return ERR_PTR(-EINVAL);
+		if (!block) {
+			err = -EINVAL;
+			goto errout_qdisc;
+		}
 		if (tcf_block_shared(block)) {
 			NL_SET_ERR_MSG(extack, "This filter block is shared. Please use the block index to manipulate the filters");
-			return ERR_PTR(-EOPNOTSUPP);
+			err = -EOPNOTSUPP;
+			goto errout_qdisc;
 		}
 	}
 
 	return block;
+
+errout_rcu:
+	rcu_read_unlock();
+errout_qdisc:
+	tcf_qdisc_put(*q, true);
+	return ERR_PTR(err);
+}
+
+static void tcf_block_release(struct Qdisc *q, struct tcf_block *block)
+{
+	tcf_qdisc_put(q, true);
 }
 
 struct tcf_block_owner_item {
@@ -1332,6 +1380,7 @@ static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 errout:
 	if (chain)
 		tcf_chain_put(chain);
+	tcf_block_release(q, block);
 	if (err == -EAGAIN)
 		/* Replay the request. */
 		goto replay;
@@ -1453,6 +1502,7 @@ static int tc_del_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 errout:
 	if (chain)
 		tcf_chain_put(chain);
+	tcf_block_release(q, block);
 	return err;
 }
 
@@ -1538,6 +1588,7 @@ static int tc_get_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 errout:
 	if (chain)
 		tcf_chain_put(chain);
+	tcf_block_release(q, block);
 	return err;
 }
 
@@ -1854,7 +1905,8 @@ static int tc_ctl_chain(struct sk_buff *skb, struct nlmsghdr *n,
 	chain_index = tca[TCA_CHAIN] ? nla_get_u32(tca[TCA_CHAIN]) : 0;
 	if (chain_index > TC_ACT_EXT_VAL_MASK) {
 		NL_SET_ERR_MSG(extack, "Specified chain index exceeds upper limit");
-		return -EINVAL;
+		err = -EINVAL;
+		goto errout_block;
 	}
 	chain = tcf_chain_lookup(block, chain_index);
 	if (n->nlmsg_type == RTM_NEWCHAIN) {
@@ -1866,23 +1918,27 @@ static int tc_ctl_chain(struct sk_buff *skb, struct nlmsghdr *n,
 				tcf_chain_hold(chain);
 			} else {
 				NL_SET_ERR_MSG(extack, "Filter chain already exists");
-				return -EEXIST;
+				err = -EEXIST;
+				goto errout_block;
 			}
 		} else {
 			if (!(n->nlmsg_flags & NLM_F_CREATE)) {
 				NL_SET_ERR_MSG(extack, "Need both RTM_NEWCHAIN and NLM_F_CREATE to create a new chain");
-				return -ENOENT;
+				err = -ENOENT;
+				goto errout_block;
 			}
 			chain = tcf_chain_create(block, chain_index);
 			if (!chain) {
 				NL_SET_ERR_MSG(extack, "Failed to create filter chain");
-				return -ENOMEM;
+				err = -ENOMEM;
+				goto errout_block;
 			}
 		}
 	} else {
 		if (!chain || tcf_chain_held_by_acts_only(chain)) {
 			NL_SET_ERR_MSG(extack, "Cannot find specified filter chain");
-			return -EINVAL;
+			err = -EINVAL;
+			goto errout_block;
 		}
 		tcf_chain_hold(chain);
 	}
@@ -1924,6 +1980,8 @@ static int tc_ctl_chain(struct sk_buff *skb, struct nlmsghdr *n,
 
 errout:
 	tcf_chain_put(chain);
+errout_block:
+	tcf_block_release(q, block);
 	if (err == -EAGAIN)
 		/* Replay the request. */
 		goto replay;
-- 
2.7.5