Do not acquire lock for file.Sync() fsync call

klog.go

@@ -1230,17 +1230,28 @@ func (l *loggingT) lockAndFlushAll() {
 // flushAll flushes all the logs and attempts to "sync" their data to disk.
 // l.mu is held.
 func (l *loggingT) flushAll() {
+	// Remember where we flushed, so we can call sync without holding
+	// the lock.
+	var files []flushSyncWriter
+
 	// Flush from fatal down, in case there's trouble flushing.
 	for s := severity.FatalLog; s >= severity.InfoLog; s-- {
 		file := l.file[s]
 		if file != nil {
 			_ = file.Flush() // ignore error
-			_ = file.Sync()  // ignore error
+			files = append(files, file)
 		}
 	}
 	if logging.loggerOptions.flush != nil {
 		logging.loggerOptions.flush()
 	}
+
+	l.mu.Unlock()
+	// Some environments are slow when syncing and holding the lock might cause contention.
+	for _, file := range files {
+		_ = file.Sync() // ignore error
+	}
+	l.mu.Lock()
 }
 
 // CopyStandardLogTo arranges for messages written to the Go "log" package's