[OpenMP] Runtime assertion failure (barrier on a single thread with target tasks)

[rpereira-dev]

OpenMP runtime assertion failure when executing a barrier on a single thread with target tasks, built from source commit 35fae04

main.c

# include <omp.h>

# define Nz  8
# define DEVICE_ID  0

int
main(void)
{
    # pragma omp parallel
    {
        # pragma omp single
        {
            # pragma omp target teams distribute parallel for nowait device(DEVICE_ID)
            for (int i = 0 ; i < Nz ; ++i)
                {}
        }
        # pragma omp barrier
    }
    return 0;
}

Command line

clang -Wall -Werror -Wextra -fopenmp -O0 -g main.c
OMP_NUM_THREADS=1 ./a.out

Output

Assertion failure at kmp_runtime.cpp(2460): master_th->th.th_task_team == team->t.t_task_team[master_th->th.th_task_state].
OMP: Error #13: Assertion failure at kmp_runtime.cpp(2460).

Insights

• Using OMP_NUM_THREADS>1 lead to successful termination
• Removing the target tasks lead to successful termination
• Removing the explicit barrier lead to successful termination
• Setting LIBOMP_USE_HIDDEN_HELPER_TASK=0 lead to successful termination

I believe this may be related to HHTs

[llvmbot]

@llvm/issue-subscribers-openmp

Author: PEREIRA Romain (rpereira-dev)

OpenMP runtime assertion failure when executing a barrier on a single thread with target tasks, built from source commit 35fae04

main.c

# include <omp.h>

# define Nz  8
# define DEVICE_ID  0

int
main(void)
{
    # pragma omp parallel
    {
        # pragma omp single
        {
            # pragma omp target teams distribute parallel for nowait device(DEVICE_ID)
            for (int i = 0 ; i < Nz ; ++i)
                {}
        }
        # pragma omp barrier
    }
    return 0;
}

Command line

clang -Wall -Werror -Wextra -fopenmp -O0 -g main.c
OMP_NUM_THREADS=1 ./a.out

Output

Assertion failure at kmp_runtime.cpp(2460): master_th->th.th_task_team == team->t.t_task_team[master_th->th.th_task_state].
OMP: Error #<!-- -->13: Assertion failure at kmp_runtime.cpp(2460).

Insights

• Using OMP_NUM_THREADS>1 lead to successful termination
• Removing the target tasks lead to successful termination
• Removing the explicit barrier lead to successful termination
• Setting LIBOMP_USE_HIDDEN_HELPER_TASK=0 lead to successful termination

I believe this may be related to HHTs

[shiltian]

It looks like similar to #50602. The task team "optimization" literally has a lot of inconsistency.
cc @lwshanbd

[lwshanbd]

It looks like similar to #50602. The task team "optimization" literally has a lot of inconsistency. cc @lwshanbd

I am interested in this. Could you assign it to me? @shiltian

[EugeneZelenko]

@lwshanbd: Just create pull request and mention it on this page.

[lwshanbd]

The bug arises from the lack of synchronization for task team when the team is serialized. PR is created.