virtio-mem: fix sleeping in RCU read side section in virtio_mem_online_page_cb()

virtio_mem_set_fake_offline() might sleep now, and we call it under
rcu_read_lock(). To fix it, simply move the rcu_read_unlock() further
up, as we're done with the device.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 6cc26d77: "virtio-mem: use page_offline_(start|end) when setting PageOffline()
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/virtio/virtio_mem.c

@@ -1242,12 +1242,19 @@ static void virtio_mem_online_page_cb(struct page *page, unsigned int order)
 			do_online = virtio_mem_bbm_get_bb_state(vm, id) !=
 				    VIRTIO_MEM_BBM_BB_FAKE_OFFLINE;
 		}
+
+		/*
+		 * virtio_mem_set_fake_offline() might sleep, we don't need
+		 * the device anymore. See virtio_mem_remove() how races
+		 * between memory onlining and device removal are handled.
+		 */
+		rcu_read_unlock();
+
 		if (do_online)
 			generic_online_page(page, order);
 		else
 			virtio_mem_set_fake_offline(PFN_DOWN(addr), 1 << order,
 						    false);
-		rcu_read_unlock();
 		return;
 	}
 	rcu_read_unlock();