[Openmp-dev] Status of detached tasks in Clang

[Joachim Protze via Openmp-dev]

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
>>> _______________________________________________
>>> Openmp-dev mailing list
>>> Openmp-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/openmp-dev
> _______________________________________________
> Openmp-dev mailing list
> Openmp-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/openmp-dev