devlink: add by-instance dump infra

Most dumpit implementations walk the devlink instances.
This requires careful lock taking and reference dropping.
Factor the loop out and provide just a callback to handle
a single instance dump.

Convert one user as an example, other users converted
in the next change.

Slightly inspired by ethtool netlink code.
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/devlink/devl_internal.h

@@ -122,6 +122,11 @@ struct devlink_nl_dump_state {
 	};
 };
 
+struct devlink_gen_cmd {
+	int (*dump_one)(struct sk_buff *msg, struct devlink *devlink,
+			struct netlink_callback *cb);
+};
+
 /* Iterate over registered devlink instances for devlink dump.
  * devlink_put() needs to be called for each iterated devlink pointer
  * in loop body in order to release the reference.
@@ -140,6 +145,9 @@ struct devlink *devlink_get_from_attrs(struct net *net, struct nlattr **attrs);
 void devlink_notify_unregister(struct devlink *devlink);
 void devlink_notify_register(struct devlink *devlink);
 
+int devlink_nl_instance_iter_dump(struct sk_buff *msg,
+				  struct netlink_callback *cb);
+
 static inline struct devlink_nl_dump_state *
 devlink_dump_state(struct netlink_callback *cb)
 {
@@ -175,6 +183,8 @@ devlink_linecard_get_from_info(struct devlink *devlink, struct genl_info *info);
 void devlink_linecard_put(struct devlink_linecard *linecard);
 
 /* Rates */
+extern const struct devlink_gen_cmd devl_gen_rate_get;
+
 struct devlink_rate *
 devlink_rate_get_from_info(struct devlink *devlink, struct genl_info *info);
 struct devlink_rate *

net/devlink/leftover.c

@@ -1219,18 +1219,15 @@ static void devlink_rate_notify(struct devlink_rate *devlink_rate,
 				0, DEVLINK_MCGRP_CONFIG, GFP_KERNEL);
 }
 
-static int devlink_nl_cmd_rate_get_dumpit(struct sk_buff *msg,
+static int
+devlink_nl_cmd_rate_get_dump_one(struct sk_buff *msg, struct devlink *devlink,
 				 struct netlink_callback *cb)
 {
 	struct devlink_nl_dump_state *state = devlink_dump_state(cb);
-	struct devlink *devlink;
-	int err = 0;
-
-	devlink_dump_for_each_instance_get(msg, state, devlink) {
 	struct devlink_rate *devlink_rate;
 	int idx = 0;
+	int err = 0;
 
-		devl_lock(devlink);
 	list_for_each_entry(devlink_rate, &devlink->rate_list, list) {
 		enum devlink_command cmd = DEVLINK_CMD_RATE_NEW;
 		u32 id = NETLINK_CB(cb->skb).portid;
@@ -1243,23 +1240,19 @@ static int devlink_nl_cmd_rate_get_dumpit(struct sk_buff *msg,
 					   cb->nlh->nlmsg_seq,
 					   NLM_F_MULTI, NULL);
 		if (err) {
-				devl_unlock(devlink);
-				devlink_put(devlink);
 			state->idx = idx;
-				goto out;
+			break;
 		}
 		idx++;
 	}
-		devl_unlock(devlink);
-		devlink_put(devlink);
-	}
-out:
-	if (err != -EMSGSIZE)
-		return err;
 
-	return msg->len;
+	return err;
 }
 
+const struct devlink_gen_cmd devl_gen_rate_get = {
+	.dump_one		= devlink_nl_cmd_rate_get_dump_one,
+};
+
 static int devlink_nl_cmd_rate_get_doit(struct sk_buff *skb,
 					struct genl_info *info)
 {
@@ -9130,7 +9123,7 @@ const struct genl_small_ops devlink_nl_ops[56] = {
 	{
 		.cmd = DEVLINK_CMD_RATE_GET,
 		.doit = devlink_nl_cmd_rate_get_doit,
-		.dumpit = devlink_nl_cmd_rate_get_dumpit,
+		.dumpit = devlink_nl_instance_iter_dump,
 		.internal_flags = DEVLINK_NL_FLAG_NEED_RATE,
 		/* can be retrieved by unprivileged users */
 	},

net/devlink/netlink.c

@@ -5,6 +5,7 @@
  */
 
 #include <net/genetlink.h>
+#include <net/sock.h>
 
 #include "devl_internal.h"
 
@@ -177,6 +178,39 @@ static void devlink_nl_post_doit(const struct genl_split_ops *ops,
 	devlink_put(devlink);
 }
 
+static const struct devlink_gen_cmd *devl_gen_cmds[] = {
+	[DEVLINK_CMD_RATE_GET]		= &devl_gen_rate_get,
+};
+
+int devlink_nl_instance_iter_dump(struct sk_buff *msg,
+				  struct netlink_callback *cb)
+{
+	const struct genl_dumpit_info *info = genl_dumpit_info(cb);
+	struct devlink_nl_dump_state *state = devlink_dump_state(cb);
+	const struct devlink_gen_cmd *cmd;
+	struct devlink *devlink;
+	int err = 0;
+
+	cmd = devl_gen_cmds[info->op.cmd];
+
+	devlink_dump_for_each_instance_get(msg, state, devlink) {
+		devl_lock(devlink);
+		err = cmd->dump_one(msg, devlink, cb);
+		devl_unlock(devlink);
+		devlink_put(devlink);
+
+		if (err)
+			break;
+
+		/* restart sub-object walk for the next instance */
+		state->idx = 0;
+	}
+
+	if (err != -EMSGSIZE)
+		return err;
+	return msg->len;
+}
+
 struct genl_family devlink_nl_family __ro_after_init = {
 	.name		= DEVLINK_GENL_NAME,
 	.version	= DEVLINK_GENL_VERSION,