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

[Jie Fu]

Hi Thomas and Kim,

Thanks for your review and nice suggestion.

Updated: http://cr.openjdk.java.net/~jiefu/8229020/webrev.02/

I had removed the assert and added reviewers in the patch.
I need a sponsor. Could someone help to push it?

Thanks a lot.
Best regards,
Jie

On 2019/8/3 上午4:30, Kim Barrett wrote:
>> On Aug 2, 2019, at 11:39 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
>>
>> 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
> I agree with Thomas; remove the assert.  I'm slightly surprised this
> hasn't come up before with other weakly ordered platforms.
>