net: sched: refactor tc_ctl_chain() to use block->lock

In order to remove dependency on rtnl lock, modify chain API to use
block->lock to protect chain from concurrent modification. Rearrange
tc_ctl_chain() code to call tcf_chain_hold() while holding block->lock to
prevent concurrent chain removal.
Signed-off-by: NVlad Buslov <vladbu@mellanox.com>
Acked-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -2255,6 +2255,8 @@ static int tc_ctl_chain(struct sk_buff *skb, struct nlmsghdr *n,
 		err = -EINVAL;
 		goto errout_block;
 	}
+
+	mutex_lock(&block->lock);
 	chain = tcf_chain_lookup(block, chain_index);
 	if (n->nlmsg_type == RTM_NEWCHAIN) {
 		if (chain) {
@@ -2266,41 +2268,49 @@ static int tc_ctl_chain(struct sk_buff *skb, struct nlmsghdr *n,
 			} else {
 				NL_SET_ERR_MSG(extack, "Filter chain already exists");
 				err = -EEXIST;
-				goto errout_block;
+				goto errout_block_locked;
 			}
 		} 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");
 				err = -ENOENT;
-				goto errout_block;
+				goto errout_block_locked;
 			}
 			chain = tcf_chain_create(block, chain_index);
 			if (!chain) {
 				NL_SET_ERR_MSG(extack, "Failed to create filter chain");
 				err = -ENOMEM;
-				goto errout_block;
+				goto errout_block_locked;
 			}
 		}
 	} else {
 		if (!chain || tcf_chain_held_by_acts_only(chain)) {
 			NL_SET_ERR_MSG(extack, "Cannot find specified filter chain");
 			err = -EINVAL;
-			goto errout_block;
+			goto errout_block_locked;
 		}
 		tcf_chain_hold(chain);
 	}
 
+	if (n->nlmsg_type == RTM_NEWCHAIN) {
+		/* Modifying chain requires holding parent block lock. In case
+		 * the chain was successfully added, take a reference to the
+		 * chain. This ensures that an empty chain does not disappear at
+		 * the end of this function.
+		 */
+		tcf_chain_hold(chain);
+		chain->explicitly_created = true;
+	}
+	mutex_unlock(&block->lock);
+
 	switch (n->nlmsg_type) {
 	case RTM_NEWCHAIN:
 		err = tc_chain_tmplt_add(chain, net, tca, extack);
-		if (err)
+		if (err) {
+			tcf_chain_put_explicitly_created(chain);
 			goto errout;
-		/* In case the chain was successfully added, take a reference
-		 * to the chain. This ensures that an empty chain
-		 * does not disappear at the end of this function.
-		 */
-		tcf_chain_hold(chain);
-		chain->explicitly_created = true;
+		}
+
 		tc_chain_notify(chain, NULL, 0, NLM_F_CREATE | NLM_F_EXCL,
 				RTM_NEWCHAIN, false);
 		break;
@@ -2334,6 +2344,10 @@ static int tc_ctl_chain(struct sk_buff *skb, struct nlmsghdr *n,
 		/* Replay the request. */
 		goto replay;
 	return err;
+
+errout_block_locked:
+	mutex_unlock(&block->lock);
+	goto errout_block;
 }
 
 /* called with RTNL */