sysctl: Initial support for auto-unregistering sysctl tables.

Add nreg to ctl_table_header.  When nreg drops to 0 the ctl_table_header
will be unregistered.

Factor out drop_sysctl_table from unregister_sysctl_table, and add
the logic for decrementing nreg.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

2 files changed, 20 insertions, 9 deletions

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 15444850b3e8..13faa48c467e 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -29,8 +29,9 @@ static struct ctl_table root_table[1];
 static struct ctl_table_root sysctl_table_root;
 static struct ctl_table_header root_table_header = {
         {{.count = 1,
-        .ctl_table = root_table,
-        .ctl_entry = LIST_HEAD_INIT(sysctl_table_root.default_set.list),}},
+          .nreg = 1,
+          .ctl_table = root_table,
+          .ctl_entry = LIST_HEAD_INIT(sysctl_table_root.default_set.list),}},
         .root = &sysctl_table_root,
         .set = &sysctl_table_root.default_set,
 };
@@ -938,6 +939,7 @@ struct ctl_table_header *__register_sysctl_table(
         header->unregistering = NULL;
         header->root = root;
         header->count = 1;
+        header->nreg = 1;
         if (sysctl_check_table(path, table))
                 goto fail;
 
@@ -1192,6 +1194,20 @@ struct ctl_table_header *register_sysctl_table(struct ctl_table *table)
 }
 EXPORT_SYMBOL(register_sysctl_table);
 
+static void drop_sysctl_table(struct ctl_table_header *header)
+{
+        if (--header->nreg)
+                return;
+
+        start_unregistering(header);
+        if (!--header->parent->count) {
+                WARN_ON(1);
+                kfree_rcu(header->parent, rcu);
+        }
+        if (!--header->count)
+                kfree_rcu(header, rcu);
+}
+
 /**
  * unregister_sysctl_table - unregister a sysctl table hierarchy
  * @header: the header returned from register_sysctl_table
@@ -1224,13 +1240,7 @@ void unregister_sysctl_table(struct ctl_table_header * header)
         }
 
         spin_lock(&sysctl_lock);
-        start_unregistering(header);
-        if (!--header->parent->count) {
-                WARN_ON(1);
-                kfree_rcu(header->parent, rcu);
-        }
-        if (!--header->count)
-                kfree_rcu(header, rcu);
+        drop_sysctl_table(header);
         spin_unlock(&sysctl_lock);
 }
 EXPORT_SYMBOL(unregister_sysctl_table);
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 094bc5ccf1e2..e40b8f6e5d0e 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -1032,6 +1032,7 @@ struct ctl_table_header
                         struct list_head ctl_entry;
                         int used;
                         int count;
+                        int nreg;
                 };
                 struct rcu_head rcu;
         };