Task-based application aborting in kmp_alloc.cpp

Dear devs,

Happy new year to all of you!

I am seeing the following assert triggering in the Clang OpenMP runtime when running an application using OpenMP tasks and taskloops:

Assertion failure at kmp_alloc.cpp(2012): size > 128 * 64.
OMP: Error #13: Assertion failure at kmp_alloc.cpp(2012).

backtrace from DDT:

#19 __kmp_launch_worker (thr=0xa24500) at src/clang/llvm-project/openmp/runtime/src/z_Linux_util.cpp:590 (at 0x0000155554d589ec)
#18 __kmp_launch_thread (this_thr=this_thr@entry=0xa24500) at src/clang/llvm-project/openmp/runtime/src/kmp_runtime.cpp:5742 (at 0x0000155554cf03e2)
#17 __kmp_fork_barrier (gtid=gtid@entry=26, tid=tid@entry=-2) at src/clang/llvm-project/openmp/runtime/src/kmp_barrier.cpp:1979 (at 0x0000155554d25f0c)
#16 __kmp_hyper_barrier_release (bt=bt@entry=bs_forkjoin_barrier, this_thr=this_thr@entry=0xa24500, gtid=gtid@entry=26, tid=tid@entry=-2, propagate_icvs=propagate_icvs@entry=1, itt_sync_obj=itt_sync_obj@entry=0x0) at src/clang/llvm-project/openmp/runtime/src/kmp_barrier.cpp:672 (at 0x0000155554d1c15e)
#15 kmp_flag_64<false, true>::wait (itt_sync_obj=0x0, final_spin=1, this_thr=0xa24500, this=0x15514afdba10) at src/clang/llvm-project/openmp/runtime/src/kmp_wait_release.h:899 (at 0x0000155554d1c15e)
#14 __kmp_wait_template<kmp_flag_64<>, true, false, true>(kmp_info_t *, kmp_flag_64<false, true> *, void *) (this_thr=this_thr@entry=0xa24500, flag=flag@entry=0x15514afdba10, itt_sync_obj=itt_sync_obj@entry=0x0) at src/clang/llvm-project/openmp/runtime/src/kmp_wait_release.h:345 (at 0x0000155554d1bb29)
#13 kmp_flag_64<false, true>::execute_tasks (is_constrained=0, itt_sync_obj=0x0, thread_finished=0x15514afdb94c, final_spin=1, gtid=26, this_thr=0xa24500, this=0x15514afdba10) at src/clang/llvm-project/openmp/runtime/src/kmp_wait_release.h:892 (at 0x0000155554d1bb29)
#12 __kmp_execute_tasks_64<false, true> (thread=thread@entry=0xa24500, gtid=gtid@entry=26, flag=flag@entry=0x15514afdba10, final_spin=final_spin@entry=1, thread_finished=thread_finished@entry=0x15514afdb94c, itt_sync_obj=itt_sync_obj@entry=0x0, is_constrained=0) at src/clang/llvm-project/openmp/runtime/src/kmp_tasking.cpp:3029 (at 0x0000155554d176a9)
#11 __kmp_execute_tasks_template<kmp_flag_64<> > (is_constrained=0, itt_sync_obj=0x0, thread_finished=<optimized out>, final_spin=<optimized

, flag=0x15514afdba10, gtid=26, thread=thread@entry=0xa24500) at

/sw/hawk-rh8/hlrs/non-spack/compiler/gcc/10.2.0/include/c++/10.2.0/bits/atomic_base.h:420 (at 0x0000155554d176a9)
#10 __kmp_invoke_task (gtid=gtid@entry=26, task=0xada040, current_task=current_task@entry=0x9b2280) at src/clang/llvm-project/openmp/runtime/src/kmp_tasking.cpp:1589 (at 0x0000155554d10da7)
#9 __kmp_task_finish<false> (gtid=gtid@entry=26, task=task@entry=0xada040, resumed_task=resumed_task@entry=0x9b2280) at /sw/hawk-rh8/hlrs/non-spack/compiler/gcc/10.2.0/include/c++/10.2.0/bits/atomic_base.h:556 (at 0x0000155554d108e7)
#8 __kmp_release_deps (gtid=gtid@entry=26, task=task@entry=0xad9f00) at src/clang/llvm-project/openmp/runtime/src/kmp_taskdeps.h:106 (at 0x0000155554d0fa53)
#7 __kmp_dephash_free (h=<optimized out>, thread=0xa24500) at src/clang/llvm-project/openmp/runtime/src/kmp_taskdeps.h:80 (at 0x0000155554d0fa53)
#6 __kmp_dephash_free_entries (h=<optimized out>, thread=0xa24500) at src/clang/llvm-project/openmp/runtime/src/kmp_taskdeps.h:62 (at 0x0000155554d0fa53)
#5 __kmp_depnode_list_free (list=0x15513affd580, thread=0xa24500) at src/clang/llvm-project/openmp/runtime/src/kmp_taskdeps.h:47 (at 0x0000155554d0fa53)
#4 ___kmp_fast_free (this_thr=this_thr@entry=0xa24500, ptr=ptr@entry=0x15513affd580, _file_=_file_@entry=0x155554d8b710 "src/clang/llvm-project/openmp/runtime/src/kmp_taskdeps.h", _line_=_line_@entry=47) at src/clang/llvm-project/openmp/runtime/src/kmp_alloc.cpp:2012 (at 0x0000155554cc9098)
#3 __kmp_debug_assert (msg=msg@entry=0x155554d7e46c "size > 128 * 64", file=0x155554d7d087 "kmp_alloc.cpp", file@entry=0x155554d7d038 "src/clang/llvm-project/openmp/runtime/src/kmp_alloc.cpp", line=line@entry=2012) at src/clang/llvm-project/openmp/runtime/src/kmp_debug.cpp:74 (at 0x0000155554ce2b55)
#2 __kmp_fatal (message=...) at src/clang/llvm-project/openmp/runtime/src/kmp_i18n.cpp:868 (at 0x0000155554ce77e0)
#1 __kmp_abort_process () at src/clang/llvm-project/openmp/runtime/src/kmp_runtime.cpp:444 (at 0x0000155554cf1abc)
#0 abort () from /lib64/libc.so.6 (at 0x00001555546e2bce)

Any idea what might cause this? I am having a hard time reproducing this in a small example so I'd appreciate if someone could hint at what might trigger this assertion. This error is reproducible in my C++ port of the NPB BT-MZ benchmark using OpenMP tasks (both tied and untied, no detached tasks or offloading), starting from class E (class D runs fine but is too small for my experiments). I have removed all stack-based array allocations to rule out stack overflows inside tasks.

This is llvmorg-12-init-15664-gba82c0b315 (pulled last night) compiled with GCC 10.2.0 on an AMD EPYC system running 32 threads per process (16 processes in total using MPI).

I can compile and run the application with no issues with GCC 10.2.0. I can reproduce the assert if imposing Clang's libgomp after compiling the application with GCC. I used GCC's ASAN both with GCC's and Clang's libgomp on the application and didn't see any errors. Any advice on what else I could try?

Many thanks in advance,
Joseph

Small update: I tried to disable the use of fast memory by defining USE_FAST_MEMORY to 0 in kmp.h right before the test for whether it is defined. My reasoning is that without fast memory it might be easier to spot race conditions and/or memory corruption (memory free'd by one thread and accessed by another). Unfortunately, I now get a Segfault with the following stacktrace obtained from DDT:

#9 main (argc=<optimized out>, argv=<optimized out>) at src/npb3.3/BT-MZ-CXX-omptasks/bt.cc:210 (at 0x00000000005067e1)
#8 __kmpc_fork_call (loc=0x56ba00, argc=15, microtask=0x50c880 <.omp_outlined.(void) const>) at src/clang/llvm-project/openmp/runtime/src/kmp_csupport.cpp:307 (at 0x00001555546ae30d)
#7 __kmp_fork_call (loc=loc@entry=0x56ba00, gtid=gtid@entry=0, call_context=call_context@entry=fork_context_intel, argc=argc@entry=15, microtask=microtask@entry=0x50c880 <.omp_outlined.(void) const>, invoker=<optimized out>, ap=<optimized out>) at src/clang/llvm-project/openmp/runtime/src/kmp_runtime.cpp:2240 (at 0x00001555546c9ca7)
#6 __kmp_invoke_task_func (gtid=0) at src/clang/llvm-project/openmp/runtime/src/kmp_runtime.cpp:7201 (at 0x00001555546c46d8)
#5 __kmp_invoke_microtask () at src/clang/llvm-project/openmp/runtime/src/z_Linux_asm.S:1166 (at 0x0000155554746f83)
#4 .omp_outlined.(void) const (.global_tid.=<optimized out>, .bound_tid.=<optimized out>, u=..., qbc_ou=..., qbc_in=..., nx=..., nxmax=..., ny=..., nz=..., rho_i=..., us=..., vs=..., ws=..., qs=..., square=..., rhs=..., forcing=...) at src/npb3.3/BT-MZ-CXX-omptasks/bt.cc:210 (at 0x000000000050c992)
#3 .omp_outlined._debug__ (.global_tid.=<optimized out>, .bound_tid.=<optimized out>, u=..., qbc_ou=..., qbc_in=..., nx=..., nxmax=..., ny=..., nz=..., rho_i=..., us=..., vs=..., ws=..., qs=..., square=..., rhs=..., forcing=...) at src/npb3.3/BT-MZ-CXX-omptasks/bt.cc:214 (at 0x000000000050c992)
#2 exch_qbc (u=..., qbc_ou=..., qbc_in=..., nx=..., nxmax=..., ny=..., nz=..., iprn_msg=<optimized out>) at src/npb3.3/BT-MZ-CXX-omptasks/exch_qbc.cc:76 (at 0x00000000005441c8)
#1 __kmpc_omp_task_with_deps (loc_ref=0x56fa80, gtid=0, new_task=0x1551b3708760, ndeps=1, dep_list=0x7fffffff8810, ndeps_noalias=0, noalias_dep_list=0x0) at src/clang/llvm-project/openmp/runtime/src/kmp_taskdeps.cpp:601 (at 0x000015555473ed1f)
#0 __kmp_init_node (node=0x1551b37065d0) at src/clang/llvm-project/openmp/runtime/src/kmp_taskdeps.cpp:601 (at 0x000015555473ed1f)

Clang's ASAN reports:

SUMMARY: AddressSanitizer: SEGV src/clang/llvm-project/openmp/runtime/src/kmp_taskdeps.cpp:37:23 in __kmp_init_node

Interestingly, the address does not seem to be NULL (from looking at it in DDT). It seems that __kmp_thread_malloc is not providing valid memory? Is setting USE_FAST_MEMORY to 0 in kmp.h the right approach to disabling fast memory? I'm not at all familiar with this code base so I'm fishing in muddy waters here...

Cheers
Joseph

For the record: With some help from Joachim I found the culprit in a missing initialization of buckets in __kmp_dephash_extend. This explains why this issue only occured with large problem sizes that presumably led to higher numbers of dependencies, which in turn triggered the hash table extension. A fix has been committed in https://reviews.llvm.org/D95167 (thanks Andrey).

Cheers
Joseph

Glad this is fixed. Thanks a lot for reporting and investigating!
Also many thanks to Joachim and Andrey :slight_smile:

~ Johannes