proc_sysctl: enhance documentation

Expand documentation to clarify:

  o that paths don't need to exist for the new API callers
  o clarify that we *require* callers to keep the memory of
    the table around during the lifetime of the sysctls
  o annotate routines we are trying to deprecate and later remove

Cc: stable@vger.kernel.org # v5.17
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

fs/proc/proc_sysctl.c

@@ -1316,7 +1316,10 @@ static struct ctl_dir *sysctl_mkdir_p(struct ctl_dir *dir, const char *path)
  * __register_sysctl_table - register a leaf sysctl table
  * @set: Sysctl tree to register on
  * @path: The path to the directory the sysctl table is in.
- * @table: the top-level table structure without any child
+ * @table: the top-level table structure without any child. This table
+ * 	 should not be free'd after registration. So it should not be
+ * 	 used on stack. It can either be a global or dynamically allocated
+ * 	 by the caller and free'd later after sysctl unregistration.
  *
  * Register a sysctl table hierarchy. @table should be a filled in ctl_table
  * array. A completely 0 filled entry terminates the table.
@@ -1410,8 +1413,15 @@ struct ctl_table_header *__register_sysctl_table(
 
 /**
  * register_sysctl - register a sysctl table
- * @path: The path to the directory the sysctl table is in.
- * @table: the table structure
+ * @path: The path to the directory the sysctl table is in. If the path
+ * 	doesn't exist we will create it for you.
+ * @table: the table structure. The calller must ensure the life of the @table
+ * 	will be kept during the lifetime use of the syctl. It must not be freed
+ * 	until unregister_sysctl_table() is called with the given returned table
+ * 	with this registration. If your code is non modular then you don't need
+ * 	to call unregister_sysctl_table() and can instead use something like
+ * 	register_sysctl_init() which does not care for the result of the syctl
+ * 	registration.
  *
  * Register a sysctl table. @table should be a filled in ctl_table
  * array. A completely 0 filled entry terminates the table.
@@ -1427,8 +1437,11 @@ EXPORT_SYMBOL(register_sysctl);
 
 /**
  * __register_sysctl_init() - register sysctl table to path
- * @path: path name for sysctl base
- * @table: This is the sysctl table that needs to be registered to the path
+ * @path: path name for sysctl base. If that path doesn't exist we will create
+ * 	it for you.
+ * @table: This is the sysctl table that needs to be registered to the path.
+ * 	The caller must ensure the life of the @table will be kept during the
+ * 	lifetime use of the sysctl.
  * @table_name: The name of sysctl table, only used for log printing when
  *              registration fails
  *
@@ -1570,6 +1583,7 @@ static int register_leaf_sysctl_tables(const char *path, char *pos,
  *
  * Register a sysctl table hierarchy. @table should be a filled in ctl_table
  * array. A completely 0 filled entry terminates the table.
+ * We are slowly deprecating this call so avoid its use.
  *
  * See __register_sysctl_table for more details.
  */
@@ -1641,6 +1655,7 @@ struct ctl_table_header *__register_sysctl_paths(
  *
  * Register a sysctl table hierarchy. @table should be a filled in ctl_table
  * array. A completely 0 filled entry terminates the table.
+ * We are slowly deprecating this caller so avoid future uses of it.
  *
  * See __register_sysctl_paths for more details.
  */