runtime: use timer.lock in cleantimers

Continue using timer.lock to simplify timer operations.

[This is one CL in a refactoring stack making very small changes
in each step, so that any subtle bugs that we miss can be more
easily pinpointed to a small change.]

Change-Id: Ic12fd2630e8ac23cddd00fa7e3240a1ac19da596
Reviewed-on: https://go-review.googlesource.com/c/go/+/564126
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/runtime/time.go

@@ -393,31 +393,31 @@ func cleantimers(pp *p) {
 		if t.pp.ptr() != pp {
 			throw("cleantimers: bad p")
 		}
-		switch s := t.status.Load(); s {
-		case timerModified:
-			if !t.status.CompareAndSwap(s, timerLocked) {
-				continue
-			}
-			if t.nextwhen == 0 {
-				dodeltimer0(pp)
-				pp.deletedTimers.Add(-1)
-				if !t.status.CompareAndSwap(timerLocked, timerRemoved) {
-					badTimer()
-				}
-			} else {
-				// Now we can change the when field.
-				t.when = t.nextwhen
-				// Move t to the right position.
-				dodeltimer0(pp)
-				doaddtimer(pp, t)
-				if !t.status.CompareAndSwap(timerLocked, timerWaiting) {
-					badTimer()
-				}
-			}
-		default:
+
+		status := t.status.Load()
+		if status != timerModified {
+			// Fast path: head of timers does not need adjustment.
+			return
+		}
+
+		status, mp := t.lock()
+		if status != timerModified {
 			// Head of timers does not need adjustment.
+			t.unlock(status, mp)
 			return
 		}
+		dodeltimer0(pp)
+		if t.nextwhen == 0 {
+			pp.deletedTimers.Add(-1)
+			status = timerRemoved
+		} else {
+			// Now we can change the when field.
+			t.when = t.nextwhen
+			// Move t to the right position.
+			doaddtimer(pp, t)
+			status = timerWaiting
+		}
+		t.unlock(status, mp)
 	}
 }