Use normal Mutex instead of RWMutex

There is no benefit of having RWMutex as we have one reader and multiple
writers. In such cases RWMutex has worse performance than Mutex.

staging/src/k8s.io/apiserver/pkg/storage/cacher/watch_progress.go

@@ -42,7 +42,7 @@ func newConditionalProgressRequester(requestWatchProgress WatchProgressRequester
 		requestWatchProgress: requestWatchProgress,
 		contextMetadata:      contextMetadata,
 	}
-	pr.cond = sync.NewCond(pr.mux.RLocker())
+	pr.cond = sync.NewCond(&pr.mux)
 	return pr
 }
 
@@ -59,7 +59,7 @@ type conditionalProgressRequester struct {
 	requestWatchProgress WatchProgressRequester
 	contextMetadata      metadata.MD
 
-	mux     sync.RWMutex
+	mux     sync.Mutex
 	cond    *sync.Cond
 	waiting int
 	stopped bool
@@ -82,8 +82,8 @@ func (pr *conditionalProgressRequester) Run(stopCh <-chan struct{}) {
 	defer timer.Stop()
 	for {
 		stopped := func() bool {
-			pr.mux.RLock()
-			defer pr.mux.RUnlock()
+			pr.mux.Lock()
+			defer pr.mux.Unlock()
 			for pr.waiting == 0 && !pr.stopped {
 				pr.cond.Wait()
 			}
@@ -97,8 +97,8 @@ func (pr *conditionalProgressRequester) Run(stopCh <-chan struct{}) {
 		case <-timer.C():
 			timer.Reset(progressRequestPeriod)
 			shouldRequest := func() bool {
-				pr.mux.RLock()
-				defer pr.mux.RUnlock()
+				pr.mux.Lock()
+				defer pr.mux.Unlock()
 				return pr.waiting > 0 && !pr.stopped
 			}()
 			if !shouldRequest {