Fix race

logger.go

@@ -327,7 +327,9 @@ func setLogLevel(lvl Level, logChange bool) Level {
 			logUnconditionalf(Config.LogFileAndLine, Info, "Log level is now %d %s (was %d %s)", lvl, lvl.String(), prev, prev.String())
 		}
 		setLevel(lvl)
+		jWriter.mutex.Lock()
 		Config.Level = lvl.String()
+		jWriter.mutex.Unlock()
 	}
 	return prev
 }
@@ -368,6 +370,7 @@ func Logf(lvl Level, format string, rest ...interface{}) {
 }
 
 // Used when doing our own logging writing, in JSON/structured mode (and some color variants as well, misnomer).
+// Also reusing that lock to update global Config.Level.
 var (
 	jWriter = jsonWriter{w: os.Stderr, tsBuf: make([]byte, 0, 32)}
 )

logger_test.go

@@ -963,13 +963,14 @@ func TestConcurrentLevelSet(t *testing.T) {
 	var wg sync.WaitGroup
 	wg.Add(int(Fatal - Verbose))
 	for i := Verbose; i < Fatal; i++ {
-		go func() {
-			SetLogLevel(i)
+		go func(lvl Level) {
+			SetLogLevel(lvl)
 			wg.Done()
-		}()
+		}(i)
 	}
 	wg.Wait()
 	SetLogLevel(Info)
+	t.Logf("log level is now %s", GetLogLevel().String())
 }
 
 // --- Benchmarks