On Mon, 22 Mar 2021 05:40:10 GMT, Kim Barrett <[email protected]> wrote:
>> test/jdk/java/util/Timer/AutoStop.java line 47:
>>
>>> 45: public void run() {
>>> 46: tdThread = Thread.currentThread();
>>> 47: synchronized(wakeup) {
>>
>> tdThread should be set inside the sync block, and then doesn't need to be
>> declared volatile
>
> Without volatile is the while loop still okay? And the read for the join
> call?
Yes the while loop is okay because the access is within a synchronized block.
When the loop exits tdThread is seen as non-null by the current thread, and
that field is never written to again, so when the current thread does the
join() it has to be using the non-null value of tdThread that it previously saw.
>> test/jdk/java/util/Timer/AutoStop.java line 67:
>>
>>> 65: t.schedule(new TimerTask() {
>>> 66: public void run() {
>>> 67: ++counter;
>>
>> This is not thread-safe. Operations on volatile variables are not atomic.
>
> Only one thread (the timer's thread) is writing, via a sequential series of
> task executions, so the simple increments are fine. I made `counter`
> volatile because it's being written by one thread and read by a different
> thread. Happy to drop the qualifier if that's not needed.
Apologies - I thought there was real concurrency going on there. :)
The volatile is correct for the reason you cited.
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PR: https://git.openjdk.java.net/jdk/pull/3106
