RFR: 8355938: Addressed rare lost unpark bug 8074773 by pre-loading LockSupport.class

[kabutz]

In 2015, Google discovered a rare disastrous classloading bug in first call to LockSupport.park() that occurred in the AppClassLoader using the Java 7 ConcurrentHashMap, which used ReentrantLock for the synchronization.

Since then, the recommended fix for this bug seems to be this code snippet in any class that directly calls LockSupport.park():


static {
    // Prevent rare disastrous classloading in first call to LockSupport.park.
    // See: https://bugs.openjdk.java.net/browse/JDK-8074773
    Class<?> ensureLoaded = LockSupport.class;
}


Since the bug was logged, we have seen new classes in the JDK that call LockSupport.park(), but that do not have this code snippet. For example:

sun.nio.ch.Poller
jdk.internal.misc.ThreadFlock
java.util.concurrent.ThreadPerTaskExecutor

It should probably also be in:

java.util.concurrent.Exchanger

Considering how hard this bug is to detect and solve, it would probably be safer to add the code snippet into these newer classes.

-------------

Commit messages:
 - Addressed rare lost unpark bug 8074773 by pre-loading LockSupport.class

Changes: https://git.openjdk.org/jdk/pull/24952/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24952&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355938
  Stats: 22 lines in 4 files changed: 21 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/24952.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24952/head:pull/24952

PR: https://git.openjdk.org/jdk/pull/24952