RFR: 8229020: Failure on CPUs allowing loads reordering: assert(_tasks[t] == 1) failed: What else?

[Thomas Schatzl]

Hi Jie,

On 02.08.19 02:44, Jie Fu wrote:
> Hi all,
> 
> JBS:    https://bugs.openjdk.java.net/browse/JDK-8229020
> Webrev: http://cr.openjdk.java.net/~jiefu/8229020/webrev.00/
> 
> *Background*
> The failure was first observed on our Loongson CPUs which allow loads 
> reordering with the following test
> ---------------------------------------------------------
> make test 
> TEST="compiler/codecache/stress/UnexpectedDeoptimizationTest.java" 
> CONF=fastdebug
> ---------------------------------------------------------
> 
> *Analysis*
> The failure was caused by the loads reordering on CPUs with weak memory 
> consistency.
> Just imagine the following case:
>    - If the load of _tasks[t] in line 436 is floating up before the load 
> of _tasks[t] in line 432, and
>    - the load in line 436 may read 0 and the load in line 432 may read 1,
>    - then the if-condition in line 433 is false, so Atomic::cmpxchg 
> won't be executed,
>    - then the assert in line 436 fails.
> ---------------------------------------------------------
> 429
> 430 bool SubTasksDone::try_claim_task(uint t) {
> 431   assert(t < _n_tasks, "bad task id.");
> 432   uint old = _tasks[t];
> 433   if (old == 0) {
> 434     old = Atomic::cmpxchg(1u, &_tasks[t], 0u);
> 435   }
> 436   assert(_tasks[t] == 1, "What else?");
> 437   bool res = old == 0;
> 438 #ifdef ASSERT
> 439   if (res) {
> 440     assert(_claimed < _n_tasks, "Too many tasks claimed; missing 
> clear?");
> 441     Atomic::inc(&_claimed);
> 442   }
> 443 #endif
> 444   return res;
> 445 }
> 446
> ---------------------------------------------------------
> 
> *Fix*
> It would be better to insert a memory fence before line 436 to prevent 
> the load from floating up.
> 
> Could you please review it?

   remove the assert. It's old paranoid code adding no further 
information imho.

Thanks,
   Thomas