rbd: don't drop the rbd_id too early

Currently an rbd device's id is released when it is removed, but it
is done before the code is run to clean up sysfs-related files (such
as /sys/bus/rbd/devices/1).

It's possible that an rbd is still in use after the rbd_remove()
call has been made.  It's essentially the same as an active inode
that stays around after it has been removed--until its final close
operation.  This means that the id shows up as free for reuse at a
time it should not be.

The effect of this was seen by Jens Rehpoehler, who:
    - had a filesystem mounted on an rbd device
    - unmapped that filesystem (without unmounting)
    - found that the mount still worked properly
    - but hit a panic when he attempted to re-map a new rbd device

This re-map attempt found the previously-unmapped id available.
The subsequent attempt to reuse it was met with a panic while
attempting to (re-)install the sysfs entry for the new mapped
device.

Fix this by holding off "putting" the rbd id, until the rbd_device
release function is called--when the last reference is finally
dropped.

Note: This fixes: http://tracker.newdream.net/issues/1907

Signed-off-by: Alex Elder <elder@dreamhost.com>
Signed-off-by: Sage Weil <sage@newdream.net>

drivers/block/rbd.c

@@ -2421,7 +2421,12 @@ static ssize_t rbd_add(struct bus_type *bus,
 	if (rc)
 		goto err_out_blkdev;
 
-	/* set up and announce blkdev mapping */
+	/*
+	 * At this point cleanup in the event of an error is the job
+	 * of the sysfs code (initiated by rbd_bus_del_dev()).
+	 *
+	 * Set up and announce blkdev mapping.
+	 */
 	rc = rbd_init_disk(rbd_dev);
 	if (rc)
 		goto err_out_bus;
@@ -2433,8 +2438,6 @@ static ssize_t rbd_add(struct bus_type *bus,
 	return count;
 
 err_out_bus:
-	rbd_id_put(rbd_dev);
-
 	/* this will also clean up rest of rbd_dev stuff */
 
 	rbd_bus_del_dev(rbd_dev);
@@ -2492,6 +2495,9 @@ static void rbd_dev_release(struct device *dev)
 	/* clean up and free blkdev */
 	rbd_free_disk(rbd_dev);
 	unregister_blkdev(rbd_dev->major, rbd_dev->name);
+
+	/* done with the id, and with the rbd_dev */
+	rbd_id_put(rbd_dev);
 	kfree(rbd_dev);
 
 	/* release module ref */
@@ -2524,8 +2530,6 @@ static ssize_t rbd_remove(struct bus_type *bus,
 		goto done;
 	}
 
-	rbd_id_put(rbd_dev);
-
 	__rbd_remove_all_snaps(rbd_dev);
 	rbd_bus_del_dev(rbd_dev);