dm: wait until embedded kobject is released before destroying a device

There may be other parts of the kernel holding a reference on the dm
kobject.  We must wait until all references are dropped before
deallocating the mapped_device structure.

The dm_kobject_release method signals that all references are dropped
via completion.  But dm_kobject_release doesn't free the kobject (which
is embedded in the mapped_device structure).

This is the sequence of operations:
* when destroying a DM device, call kobject_put from dm_sysfs_exit
* wait until all users stop using the kobject, when it happens the
  release method is called
* the release method signals the completion and should return without
  delay
* the dm device removal code that waits on the completion continues
* the dm device removal code drops the dm_mod reference the device had
* the dm device removal code frees the mapped_device structure that
  contains the kobject

Using kobject this way should avoid the module unload race that was
mentioned at the beginning of this thread:
https://lkml.org/lkml/2014/1/4/83

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

drivers/md/dm-sysfs.c

@@ -79,13 +79,19 @@ static const struct sysfs_ops dm_sysfs_ops = {
 	.show	= dm_attr_show,
 };
 
+static void dm_kobject_release(struct kobject *kobj)
+{
+	complete(dm_get_completion_from_kobject(kobj));
+}
+
 /*
  * dm kobject is embedded in mapped_device structure
  * no need to define release function here
  */
 static struct kobj_type dm_ktype = {
 	.sysfs_ops	= &dm_sysfs_ops,
 	.default_attrs	= dm_attrs,
+	.release	= dm_kobject_release,
 };
 
 /*
@@ -104,5 +110,7 @@ int dm_sysfs_init(struct mapped_device *md)
  */
 void dm_sysfs_exit(struct mapped_device *md)
 {
-	kobject_put(dm_kobject(md));
+	struct kobject *kobj = dm_kobject(md);
+	kobject_put(kobj);
+	wait_for_completion(dm_get_completion_from_kobject(kobj));
 }

drivers/md/dm.c

@@ -203,6 +203,9 @@ struct mapped_device {
 	/* sysfs handle */
 	struct kobject kobj;
 
+	/* wait until the kobject is released */
+	struct completion kobj_completion;
+
 	/* zero-length flush that will be cloned and submitted to targets */
 	struct bio flush_bio;
 
@@ -2041,6 +2044,7 @@ static struct mapped_device *alloc_dev(int minor)
 	init_waitqueue_head(&md->wait);
 	INIT_WORK(&md->work, dm_wq_work);
 	init_waitqueue_head(&md->eventq);
+	init_completion(&md->kobj_completion);
 
 	md->disk->major = _major;
 	md->disk->first_minor = minor;
@@ -2919,6 +2923,13 @@ struct mapped_device *dm_get_from_kobject(struct kobject *kobj)
 	return md;
 }
 
+struct completion *dm_get_completion_from_kobject(struct kobject *kobj)
+{
+	struct mapped_device *md = container_of(kobj, struct mapped_device, kobj);
+
+	return &md->kobj_completion;
+}
+
 int dm_suspended_md(struct mapped_device *md)
 {
 	return test_bit(DMF_SUSPENDED, &md->flags);

drivers/md/dm.h

@@ -15,6 +15,7 @@
 #include <linux/list.h>
 #include <linux/blkdev.h>
 #include <linux/hdreg.h>
+#include <linux/completion.h>
 
 #include "dm-stats.h"
 
@@ -152,6 +153,7 @@ int dm_sysfs_init(struct mapped_device *md);
 void dm_sysfs_exit(struct mapped_device *md);
 struct kobject *dm_kobject(struct mapped_device *md);
 struct mapped_device *dm_get_from_kobject(struct kobject *kobj);
+struct completion *dm_get_completion_from_kobject(struct kobject *kobj);
 
 /*
  * Targets for linear and striped mappings