Stats based on LongAdder can return unexpected values #52411
Description
We expect certain stats values to be non-negative when serializing them over the wire. While investigating a test failure (#52406), I noticed that, due to concurrency, counts from LongAdder can be inexact, and even be negative even though in a properly serialized execution no such negative value should be observable.
Here's a test illustrating the issue:
public void testConcurrentLongAdderAccess() throws InterruptedException {
AtomicBoolean stopped = new AtomicBoolean();
LongAdder longAdder = new LongAdder();
Thread[] writers = new Thread[10];
for (int i = 0; i < writers.length; i++) {
writers[i] = new Thread(() -> {
while(stopped.get() == false) {
longAdder.increment();
longAdder.decrement();
}
});
writers[i].start();
}
Thread[] readers = new Thread[10];
for (int i = 0; i < writers.length; i++) {
readers[i] = new Thread(() -> {
while(stopped.get() == false) {
long val = longAdder.longValue();
assert val >= 0L : "expected positive but got " + val;
}
});
readers[i].start();
}
Thread.sleep(10000);
stopped.set(true);
for (int i = 0; i < writers.length; i++) {
writers[i].join();
}
for (int i = 0; i < readers.length; i++) {
readers[i].join();
}
}