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>

diff --git a/fs/char_dev.c b/fs/char_dev.c
index 3f152b92a94ad2824876fd99c9fbfdd05098d0cf..afc2bb6917806e85ee916d8b2c25cb2183890055 100644 (file)
--- 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 = {