[PATCH resubmit] xen-blkfront: Don't send closing notification to backend in blkfront_closing()

[Joe Jin <joe.jin@oracle.com>]

When we do block device attach/detach test with below steps, umount hang and the
guest unable to shutdown:

1. start guest with the latest kernel.
2. attach new block device by xm block-attach in Dom0
3. mount new disk in guest
4. execute xm block-detach to detach the block device in dom0 until timeout
5. try to unmount the disk in guest, umount hung. at here, any IOs to the 
   device will hang.

Checking the code found when 'xm block-detach' set backend device's state to
'XenbusStateClosing', frontend received the notification and blkfront_closing()
be called, at the moment, the disk still using by guest, so frontend refused
to close. In the blkfront_closing(), frontend send a notification to backend
said that the its state switched to 'Closing', when backend got the
event, it will disconnect from real device, at here any IO request will
be stuck, even tried to release the disk by umount.

Per our test, below patch fix this issue.

Signed-off-by: Joe Jin <joe.jin@oracle.com>
Signed-off-by: Annie Li <annie.li@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jens Axboe <jaxboe@fusionio.com>
Cc: stable@kernel.org

---
 xen-blkfront.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index b536a9c..f6d8ac2 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1088,7 +1088,7 @@ blkfront_closing(struct blkfront_info *info)
 	if (bdev->bd_openers) {
 		xenbus_dev_error(xbdev, -EBUSY,
 				 "Device in use; refusing to close");
-		xenbus_switch_state(xbdev, XenbusStateClosing);
+		xbdev->state = XenbusStateClosing;
 	} else {
 		xlvbd_release_gendisk(info);
 		xenbus_frontend_closed(xbdev);