Fix PUT bucket notification deadlocks (#5734)

This PR fixes two different variant of deadlocks in
notification.

- holding write lock on the bucket competing with read lock
- holding competing locks on read/save notification config

cmd/bucket-notification-handlers.go

@@ -138,14 +138,6 @@ func (api objectAPIHandlers) PutBucketNotificationHandler(w http.ResponseWriter,
 		return
 	}
 
-	// Acquire a write lock on bucket before modifying its configuration.
-	bucketLock := globalNSMutex.NewNSLock(bucketName, "")
-	if err = bucketLock.GetLock(globalOperationTimeout); err != nil {
-		writeErrorResponse(w, toAPIErrorCode(err), r.URL)
-		return
-	}
-	defer bucketLock.Unlock()
-
 	if err = saveNotificationConfig(objectAPI, bucketName, config); err != nil {
 		writeErrorResponse(w, toAPIErrorCode(err), r.URL)
 		return

cmd/notification.go

@@ -496,14 +496,6 @@ func readConfig(objAPI ObjectLayer, configFile string) (*bytes.Buffer, error) {
 func readNotificationConfig(objAPI ObjectLayer, bucketName string) (*event.Config, error) {
 	// Construct path to notification.xml for the given bucket.
 	configFile := path.Join(bucketConfigPrefix, bucketName, bucketNotificationConfig)
-
-	// Get read lock.
-	objLock := globalNSMutex.NewNSLock(minioMetaBucket, configFile)
-	if err := objLock.GetRLock(globalOperationTimeout); err != nil {
-		return nil, err
-	}
-	defer objLock.RUnlock()
-
 	reader, err := readConfig(objAPI, configFile)
 	if err != nil {
 		return nil, err
@@ -519,14 +511,6 @@ func saveNotificationConfig(objAPI ObjectLayer, bucketName string, config *event
 	}
 
 	configFile := path.Join(bucketConfigPrefix, bucketName, bucketNotificationConfig)
-
-	// Get write lock.
-	objLock := globalNSMutex.NewNSLock(minioMetaBucket, configFile)
-	if err := objLock.GetLock(globalOperationTimeout); err != nil {
-		return err
-	}
-	defer objLock.Unlock()
-
 	return saveConfig(objAPI, configFile, data)
 }