cgroup: Fix the bug of getting scsi_debug disk

Signed-off-by: NXu Han <xuhan@redhat.com>

qemu/tests/cgroup.py

@@ -19,6 +19,7 @@ from aexpect import ShellTimeoutError
 from virttest.env_process import preprocess
 from virttest import qemu_monitor
 from virttest import error_context
+from virttest import utils_misc
 
 from virttest.staging import utils_memory
 from virttest.staging.utils_cgroup import Cgroup
@@ -1503,6 +1504,18 @@ def run(test, params, env):
 
             return True
 
+        def _get_scsi_debug_disk():
+            cmd = ("ls -1 /sys/bus/pseudo/drivers/scsi_debug/adapter*/"
+                   "host*/target*/*/block | grep ^sd | head -1")
+            name = process.system_output(cmd, shell=True, verbose=False,
+                                         ignore_status=True).decode()
+            if not name:
+                return None
+            node_path = "/dev/%s" % name
+            if not os.path.exists(node_path):
+                return None
+            return node_path
+
         logging.info("Setup test")
         vm = env.get_all_vms()[0]
         # Try to find suitable monitor
@@ -1549,8 +1562,9 @@ def run(test, params, env):
             process.system("modprobe scsi_debug dev_size_mb=8 add_host=0")
         process.system("echo 1 > /sys/bus/pseudo/drivers/scsi_debug/add_host",
                        shell=True)
-        time.sleep(0.1)
-        disk = process.system_output("ls /dev/sd* | tail -n 1", shell=True)
+        disk = utils_misc.wait_for(_get_scsi_debug_disk, 5)
+        if not disk:
+            test.error("Could not get the device node generated by scsi_debug")
         dev = "%s:%s" % get_maj_min(disk)
         permissions = [
             {'property': 'deny',