The TSAN checker (https://wiki.openjdk.org/display/tsan) reports data race on java.io.ClassCache$CacheRef.strongReferent, which was introduced in JDK-8280041. The stack trace below are based on JDK 11.0.16.1:

WARNING: ThreadSanitizer: data race (pid=6734)
  Read of size 4 at 0x0000cdcd9d64 by thread T1587:
    #0 java.io.ClassCache$CacheRef.getStrong()Ljava/lang/Object; ClassCache.java:55
    #1 java.io.ClassCache.get(Ljava/lang/Class;)Ljava/lang/Object; ClassCache.java:90
    #2 java.io.ObjectStreamClass.lookup(Ljava/lang/Class;Z)Ljava/io/ObjectStreamClass; ObjectStreamClass.java:336
    #3 java.io.ObjectOutputStream.writeObject0(Ljava/lang/Object;Z)V ObjectOutputStream.java:1125
    #4 java.io.ObjectOutputStream.defaultWriteFields(Ljava/lang/Object;Ljava/io/ObjectStreamClass;)V ObjectOutputStream.java:1543
    #5 java.io.ObjectOutputStream.writeSerialData(Ljava/lang/Object;Ljava/io/ObjectStreamClass;)V ObjectOutputStream.java:1500
    #6 java.io.ObjectOutputStream.writeOrdinaryObject(Ljava/lang/Object;Ljava/io/ObjectStreamClass;Z)V ObjectOutputStream.java:1423
    #7 java.io.ObjectOutputStream.writeObject0(Ljava/lang/Object;Z)V ObjectOutputStream.java:1169
    #8 java.io.ObjectOutputStream.writeObject(Ljava/lang/Object;)V ObjectOutputStream.java:345
    ...

  Previous write of size 4 at 0x0000cdcd9d64 by thread T1589:
    #0 java.io.ClassCache$CacheRef.clearStrong()V ClassCache.java:59
    #1 java.io.ClassCache.get(Ljava/lang/Class;)Ljava/lang/Object; ClassCache.java:92
    #2 java.io.ObjectStreamClass.lookup(Ljava/lang/Class;Z)Ljava/io/ObjectStreamClass; ObjectStreamClass.java:336
    #3 java.io.ObjectOutputStream.writeObject0(Ljava/lang/Object;Z)V ObjectOutputStream.java:1125
    #4 java.io.ObjectOutputStream.defaultWriteFields(Ljava/lang/Object;Ljava/io/ObjectStreamClass;)V ObjectOutputStream.java:1543
    #5 java.io.ObjectOutputStream.writeSerialData(Ljava/lang/Object;Ljava/io/ObjectStreamClass;)V ObjectOutputStream.java:1500
    #6 java.io.ObjectOutputStream.writeOrdinaryObject(Ljava/lang/Object;Ljava/io/ObjectStreamClass;Z)V ObjectOutputStream.java:1423
    #7 java.io.ObjectOutputStream.writeObject0(Ljava/lang/Object;Z)V ObjectOutputStream.java:1169
    #8 java.io.ObjectOutputStream.writeObject(Ljava/lang/Object;)V ObjectOutputStream.java:345
    ...

Update: during code review, we have determined this race is benign, and fixing it could introduce performance penalty (particularly on aarch64). Thus, we leave this data race as-is, and just document it in the source.