Re: [PATCH net-next 1/2] netconsole: publish the userdata payload with RCU

update_userdata() takes target_list_lock to swap nt->userdata and
nt->userdata_length, then frees the old buffer. Since commit
7eab73b18630 ("netconsole: convert to NBCON console infrastructure")
that lock is also the console's device_lock, so writing a userdata value
from configfs serialises against the printk core emitting messages.

The buffer is immutable once published, which is what RCU is for. Move
the string and its length into a single netconsole_userdata object and
publish it with rcu_replace_pointer(), freeing the old one with
kfree_rcu().

New userdata design:

0) Unify the userdata fields into a struct netcons_userdata
1) update_userdata() no longer needs target_list_lock.
2) writers stay serialised by dynamic_netconsole_mutex.
3) reading userdata needs an RCU read lock.

No functional change intended.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/net/netconsole.c | 88 +++++++++++++++++++++++++++++-------------------
 1 file changed, 54 insertions(+), 34 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 862001d09aa84..c88c82f2136cb 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c

@@ -135,13 +135,27 @@ enum target_state {
        STATE_DEACTIVATED,
 };

+/**
+ * struct netcons_userdata - Formatted userdata payload of a target.
+ * @rcu:       Used to free the payload after a grace period.
+ * @length:    Length of @data, excluding the NUL terminator.
+ * @data:      Formatted " key=value\n" entries, NUL terminated.
+ *
+ * Immutable once published, so the transmit path never observes @data and
+ * @length disagreeing.
+ */
+struct netcons_userdata {
+       struct rcu_head         rcu;
+       size_t                  length;
+       char                    data[];
+};
+
 /**
  * struct netconsole_target - Represents a configured netconsole target.
  * @list:      Links this target into the target_list.
  * @group:     Links us into the configfs subsystem hierarchy.
  * @userdata_group:    Links to the userdata configfs hierarchy
- * @userdata:          Cached, formatted string of append
- * @userdata_length:   String length of userdata.
+ * @userdata:          Cached, formatted userdata payload. RCU protected.
  * @sysdata:           Cached, formatted string of append
  * @sysdata_fields:    Sysdata features enabled.
  * @msgcounter:        Message sent counter.

@@ -176,8 +190,7 @@ struct netconsole_target {
 #ifdef CONFIG_NETCONSOLE_DYNAMIC
        struct config_group     group;
        struct config_group     userdata_group;
-       char                    *userdata;
-       size_t                  userdata_length;
+       struct netcons_userdata __rcu *userdata;