RFR 8173083: VarHandle usages in LockSupport and ThreadLocalRandom result in circularity issues

[Paul Sandoz]

> On 19 Jan 2017, at 19:23, Martin Buchholz <martinrb at google.com> wrote:
> 
> Seems OK.

Thanks.


> It would be nice to somehow get more confidence we don't have remaining circularities.

From my experience usually these kind of circularities show up intermittently but regularly enough. Tricky to induce locally.


>   I agree a specialized concurrent hash table to replace ConcurrentHashMap might help.
> 

Yes, it should reliably break the cycle.


> I was surprised to see below a change in parentheses that seems to change the behavior.
> 
>      final long nextSeed() {
>          Thread t; long r; // read and update per-thread seed
> -        SEED.set(t = Thread.currentThread(),
> -                 (r = (long) SEED.get(t)) + GAMMA);
> +        U.putLong(t = Thread.currentThread(), SEED,
> +                  r = U.getLong(t, SEED) + GAMMA);
>          return r;
>      }

The update reverted to the previous code:

http://hg.openjdk.java.net/jdk9/dev/jdk/file/7901a13a051c/src/java.base/share/classes/java/util/concurrent/ThreadLocalRandom.java <http://hg.openjdk.java.net/jdk9/dev/jdk/file/7901a13a051c/src/java.base/share/classes/java/util/concurrent/ThreadLocalRandom.java>

I added the parentheses to bound the cast.

Thanks,
Paul.

> 
> 
> 
> On Thu, Jan 19, 2017 at 6:52 PM, Paul Sandoz <paul.sandoz at oracle.com <mailto:paul.sandoz at oracle.com>> wrote:
> Hi,
> 
> Unfortunately the switch from Unsafe to VarHandle in LockSupport and ThreadLocalRandom caused some circularity issues (similar to those for AtomicInteger) that have shown up in continuous testing (but not when i tested explicitly using the infrastructure).
> 
> The main problem is MethodType leverages ConcurrentHashMap which under certain conditions uses TLR [*].
> 
> We know we want to replace CHM usage in MethodType with an alternative Peter created, which i think we can pursue post 9 and break the circularity.
> 
> This patch reverts the back to Unsafe usage:
> 
>   http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8173083-vh-to-unsafe-ls-tlr/webrev/index.html <http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8173083-vh-to-unsafe-ls-tlr/webrev/index.html>
> 
> I did not revert the changes to Striped64 and ForkJoinWorkerThread.
> 
> Thanks,
> Paul.
> 
> [*]
> 1) The clinit of LockSupport creates a VarHandle;
> 
> 2) a VarHandle creates MethodTypes;
> 
> 3) that are stored in a CHM;
> 
> 4) which on occasion uses TLR
> 
> 5) that uses a VarHandle whose VarForm is being initalized in 1)
> 
> 
>