char_dev: pin parent kobject

In certain cases (for example when a cdev structure is embedded into
another object whose lifetime is controlled by a separate kobject) it is
beneficial to tie lifetime of another object to the lifetime of
character device so that related object is not freed until after
char_dev object is freed.

To achieve this let's pin kobject's parent when doing cdev_add() and
unpin when last reference to cdev structure is being released.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 1 deletions

diff --git a/fs/char_dev.c b/fs/char_dev.c
index 3f152b92a94a..afc2bb691780 100644
--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -471,9 +471,19 @@ static int exact_lock(dev_t dev, void *data)
  */
 int cdev_add(struct cdev *p, dev_t dev, unsigned count)
 {
+        int error;
+
         p->dev = dev;
         p->count = count;
-        return kobj_map(cdev_map, dev, count, NULL, exact_match, exact_lock, p);
+
+        error = kobj_map(cdev_map, dev, count, NULL,
+                         exact_match, exact_lock, p);
+        if (error)
+                return error;
+
+        kobject_get(p->kobj.parent);
+
+        return 0;
 }
 
 static void cdev_unmap(dev_t dev, unsigned count)
@@ -498,14 +508,20 @@ void cdev_del(struct cdev *p)
 static void cdev_default_release(struct kobject *kobj)
 {
         struct cdev *p = container_of(kobj, struct cdev, kobj);
+        struct kobject *parent = kobj->parent;
+
         cdev_purge(p);
+        kobject_put(parent);
 }
 
 static void cdev_dynamic_release(struct kobject *kobj)
 {
         struct cdev *p = container_of(kobj, struct cdev, kobj);
+        struct kobject *parent = kobj->parent;
+
         cdev_purge(p);
         kfree(p);
+        kobject_put(parent);
 }
 
 static struct kobj_type ktype_cdev_default = {