[Openmp-dev] Status of detached tasks in Clang

[Joseph Schuchart via Openmp-dev]

Thanks Joachim for looking into this. For the record, the patch you 
posted at https://reviews.llvm.org/D79702 seems to fix the race for me. 
I was able to run several different configurations and the issues to be 
gone.

Cheers
Joseph

On 5/8/20 6:15 PM, Joachim Protze wrote:
> Hi all,
> 
> I looked a bit into this issue. As I understand the code, there is a 
> race between __kmp_task_finish and __kmp_fulfill_event:
> 
>  From my perspective, all accesses to taskdata need to happen before
> 
> https://github.com/llvm/llvm-project/blob/master/openmp/runtime/src/kmp_tasking.cpp#L873 
> 
> taskdata->td_flags.proxy = TASK_PROXY;
> 
> taskdata cannot be safely accessed after this line.
> 
> __kmp_task_finish () "passes ownership of taskdata" with this line.
> 
> At this point, __kmp_fulfill_event might release the task at any time.
> 
> Joseph sees a spurious assertion failure in
> https://github.com/llvm/llvm-project/blob/master/openmp/runtime/src/kmp_tasking.cpp#L710 
> 
> 
> This is, when
> https://github.com/llvm/llvm-project/blob/master/openmp/runtime/src/kmp_tasking.cpp#L906 
> 
> is not reached fast enough.
> 
> Can the accesses be safely moved up?
> 
> Best
> Joachim
> 
> 
> Am 08.04.20 um 14:02 schrieb Joseph Schuchart via Openmp-dev:
>> Johannes, Alexey,
>>
>> Thanks for the pointers. I will try to call in to the telco tomorrow.
>>
>> In the meantime: I was able to install trunk and run tests with 
>> detached tasks. However, every once in a while I end up in an assert 
>> inside libomp:
>>
>> OMP: Error #13: Assertion failure at kmp_tasking.cpp(920).
>>
>> The relevant code is in __kmp_task_finish:
>>
>>    if (taskdata->td_flags.destructors_thunk) {
>>      kmp_routine_entry_t destr_thunk = task->data1.destructors;
>>      KMP_ASSERT(destr_thunk); // <- this assert triggers
>>      destr_thunk(gtid, task);
>>    }
>>
>>
>> My first suspicion is that there is a race condition when one thread 
>> fulfills the event while the executing task completes it. I have not 
>> been able to spot this issue in the code but maybe someone familiar 
>> with it has an idea about what might be going wrong?
>>
>> Thanks,
>> Joseph
>>
>> On 4/4/20 6:20 PM, Johannes Doerfert wrote:
>>> Hi Joseph,
>>>
>>>
>>> I'm not sure who is working on this but we might be able to figure 
>>> this out on our bi-weekly call Wednesday.
>>>
>>> Feel free to call but we should figure this out either way.
>>>
>>>
>>> Apologies for the delay,
>>>
>>>    Johannes
>>>
>>>
>>>
>>> On 4/3/20 7:18 AM, Joseph Schuchart via Openmp-dev wrote:
>>>> Dear all,
>>>>
>>>> I am eager to play around with detached tasks in OpenMP. It seems 
>>>> that support has gone into the runtime about a year ago [1] but even 
>>>> with Clang 10 I get a warning that the detach clause is ignored:
>>>>
>>>> $ clang-10 -fopenmp test_detached.c
>>>> test_detached.c:25:47: warning: extra tokens at the end of '#pragma 
>>>> omp task' are ignored [-Wextra-tokens]
>>>>   #pragma omp task detach(event)
>>>>
>>>> The status website lists detached tasks as work in progress [2]. Is 
>>>> there a way to try detached tasks in some experimental version of 
>>>> Clang? Or am I just missing a flag to enable them in the Clang release?
>>>>
>>>> Thanks a lot,
>>>> Joseph
>>>>
>>>> [1] https://reviews.llvm.org/D62485
>>>> [2] https://clang.llvm.org/docs/OpenMPSupport.html
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