merge branch 'pr-2774'

Kir Kolyshkin (1):
  libct/cg/fs/freezer: fix freezing race

LGTMs: @mrunalp @cyphar
Closes #2774

libcontainer/cgroups/fs/freezer.go

@@ -28,33 +28,54 @@ func (s *FreezerGroup) Apply(path string, d *cgroupData) error {
 
 func (s *FreezerGroup) Set(path string, cgroup *configs.Cgroup) error {
 	switch cgroup.Resources.Freezer {
-	case configs.Frozen, configs.Thawed:
-		for {
-			// In case this loop does not exit because it doesn't get the expected
-			// state, let's write again this state, hoping it's going to be properly
-			// set this time. Otherwise, this loop could run infinitely, waiting for
-			// a state change that would never happen.
-			if err := fscommon.WriteFile(path, "freezer.state", string(cgroup.Resources.Freezer)); err != nil {
+	case configs.Frozen:
+		// As per older kernel docs (freezer-subsystem.txt before
+		// kernel commit ef9fe980c6fcc1821), if FREEZING is seen,
+		// userspace should either retry or thaw. While current
+		// kernel cgroup v1 docs no longer mention a need to retry,
+		// the kernel (tested on v5.4, Ubuntu 20.04) can't reliably
+		// freeze a cgroup while new processes keep appearing in it
+		// (either via fork/clone or by writing new PIDs to
+		// cgroup.procs).
+		//
+		// The number of retries below is chosen to have a decent
+		// chance to succeed even in the worst case scenario (runc
+		// pause/unpause with parallel runc exec).
+		//
+		// Adding any amount of sleep in between retries did not
+		// increase the chances of successful freeze.
+		for i := 0; i < 1000; i++ {
+			if err := fscommon.WriteFile(path, "freezer.state", string(configs.Frozen)); err != nil {
 				return err
 			}
 
-			state, err := s.GetState(path)
+			state, err := fscommon.ReadFile(path, "freezer.state")
 			if err != nil {
 				return err
 			}
-			if state == cgroup.Resources.Freezer {
-				break
+			state = strings.TrimSpace(state)
+			switch state {
+			case "FREEZING":
+				continue
+			case string(configs.Frozen):
+				return nil
+			default:
+				// should never happen
+				return fmt.Errorf("unexpected state %s while freezing", strings.TrimSpace(state))
 			}
-
-			time.Sleep(1 * time.Millisecond)
 		}
+		// Despite our best efforts, it got stuck in FREEZING.
+		// Leaving it in this state is bad and dangerous, so
+		// let's (try to) thaw it back and error out.
+		_ = fscommon.WriteFile(path, "freezer.state", string(configs.Thawed))
+		return errors.New("unable to freeze")
+	case configs.Thawed:
+		return fscommon.WriteFile(path, "freezer.state", string(configs.Thawed))
 	case configs.Undefined:
 		return nil
 	default:
 		return fmt.Errorf("Invalid argument '%s' to freezer.state", string(cgroup.Resources.Freezer))
 	}
-
-	return nil
 }
 
 func (s *FreezerGroup) GetStats(path string, stats *cgroups.Stats) error {