xen/blkback: don't use xen_blkif_get() in xen-blkback kthread

There is no need to use xen_blkif_get()/xen_blkif_put() in the kthread
of xen-blkback. Thread stopping is synchronous and using the blkif
reference counting in the kthread will avoid to ever let the reference
count drop to zero at the end of an I/O running concurrent to
disconnecting and multiple rings.

Setting ring->xenblkd to NULL after stopping the kthread isn't needed
as the kthread does this already.

Signed-off-by: Juergen Gross <jgross@suse.com>
Tested-by: Steven Haigh <netwiz@crc.id.au>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/blkback.c

@@ -609,8 +609,6 @@ int xen_blkif_schedule(void *arg)
 	unsigned long timeout;
 	int ret;
 
-	xen_blkif_get(blkif);
-
 	set_freezable();
 	while (!kthread_should_stop()) {
 		if (try_to_freeze())
@@ -665,7 +663,6 @@ purge_gnt_list:
 		print_stats(ring);
 
 	ring->xenblkd = NULL;
-	xen_blkif_put(blkif);
 
 	return 0;
 }

drivers/block/xen-blkback/xenbus.c

@@ -255,7 +255,6 @@ static int xen_blkif_disconnect(struct xen_blkif *blkif)
 		if (ring->xenblkd) {
 			kthread_stop(ring->xenblkd);
 			wake_up(&ring->shutdown_wq);
-			ring->xenblkd = NULL;
 		}
 
 		/* The above kthread_stop() guarantees that at this point we