Driver core: driver_find() drops reference before returning

As part of the removal of get_driver()/put_driver(), this patch
(as1510) changes driver_find(); it now drops the reference it acquires
before returning. The patch also adjusts all the callers of
driver_find() to remove the now unnecessary calls to put_driver().

In addition, the patch adds a warning to driver_find(): Callers must
make sure the driver they are searching for does not get unloaded
while they are using it. This has always been the case; driver_find()
has never prevented a driver from being unregistered or unloaded.
Hence the patch will not introduce any new bugs. The existing callers
all seem to be okay in this respect, however I don't understand the
video drivers well enough to be certain about them.

Signed-off-by: Alan Stern <[e-mail]>
CC: Dmitry Torokhov <[e-mail]>
CC: Kyungmin Park <[e-mail]>
CC: Andy Walls <[e-mail]>
CC: Martin Schwidefsky <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/driver.c

···
 
 	other = driver_find(drv->name, drv->bus);
 	if (other) {
-		put_driver(other);
 		printk(KERN_ERR "Error: Driver '%s' is already registered, "
 			"aborting...\n", drv->name);
 		return -EBUSY;
···
  * Call kset_find_obj() to iterate over list of drivers on
  * a bus to find driver by name. Return driver if found.
  *
- * Note that kset_find_obj increments driver's reference count.
+ * This routine provides no locking to prevent the driver it returns
+ * from being unregistered or unloaded while the caller is using it.
+ * The caller is responsible for preventing this.
  */
 struct device_driver *driver_find(const char *name, struct bus_type *bus)
 {
···
 	struct driver_private *priv;
 
 	if (k) {
+		/* Drop reference added by kset_find_obj() */
+		kobject_put(k);
 		priv = to_driver(k);
 		return priv->driver;
 	}

drivers/input/gameport/gameport.c

···
 	} else if ((drv = driver_find(buf, &gameport_bus)) != NULL) {
 		gameport_disconnect_port(gameport);
 		error = gameport_bind_driver(gameport, to_gameport_driver(drv));
-		put_driver(drv);
 	} else {
 		error = -EINVAL;
 	}

drivers/input/serio/serio.c

···
 	} else if ((drv = driver_find(buf, &serio_bus)) != NULL) {
 		serio_disconnect_port(serio);
 		error = serio_bind_driver(serio, to_serio_driver(drv));
-		put_driver(drv);
 		serio_remove_duplicate_events(serio, SERIO_RESCAN_PORT);
 	} else {
 		error = -EINVAL;

drivers/media/video/cx18/cx18-alsa-main.c

···
 
 	drv = driver_find("cx18", &pci_bus_type);
 	ret = driver_for_each_device(drv, NULL, NULL, cx18_alsa_exit_callback);
-	put_driver(drv);
 
 	cx18_ext_init = NULL;
 	printk(KERN_INFO "cx18-alsa: module unload complete\n");

drivers/media/video/ivtv/ivtvfb.c

···
 
 	drv = driver_find("ivtv", &pci_bus_type);
 	err = driver_for_each_device(drv, NULL, &registered, ivtvfb_callback_init);
-	put_driver(drv);
 	if (!registered) {
 		printk(KERN_ERR "ivtvfb:  no cards found\n");
 		return -ENODEV;
···
 
 	drv = driver_find("ivtv", &pci_bus_type);
 	err = driver_for_each_device(drv, NULL, NULL, ivtvfb_callback_cleanup);
-	put_driver(drv);
 }
 
 module_init(ivtvfb_init);

drivers/media/video/s5p-fimc/fimc-mdevice.c

···
 		return -ENODEV;
 	ret = driver_for_each_device(driver, NULL, fmd,
 				     fimc_register_callback);
-	put_driver(driver);
 	if (ret)
 		return ret;
 
 	driver = driver_find(CSIS_DRIVER_NAME, &platform_bus_type);
-	if (driver) {
+	if (driver)
 		ret = driver_for_each_device(driver, NULL, fmd,
 					     csis_register_callback);
-		put_driver(driver);
-	}
 	return ret;
 }
 

drivers/media/video/s5p-tv/mixer_video.c

···
 	}
 
 done:
-	put_driver(drv);
 	return sd;
 }
 

drivers/s390/net/smsgiucv_app.c

···
 	rc = dev_set_name(smsg_app_dev, KMSG_COMPONENT);
 	if (rc) {
 		kfree(smsg_app_dev);
-		goto fail_put_driver;
+		goto fail;
 	}
 	smsg_app_dev->bus = &iucv_bus;
 	smsg_app_dev->parent = iucv_root;
···
 	rc = device_register(smsg_app_dev);
 	if (rc) {
 		put_device(smsg_app_dev);
-		goto fail_put_driver;
+		goto fail;
 	}
 
 	/* convert sender to uppercase characters */
···
 	rc = smsg_register_callback(SMSG_PREFIX, smsg_app_callback);
 	if (rc) {
 		device_unregister(smsg_app_dev);
-		goto fail_put_driver;
+		goto fail;
 	}
 
 	rc = 0;
-fail_put_driver:
-	put_driver(smsgiucv_drv);
+fail:
 	return rc;
 }
 module_init(smsgiucv_app_init);