c++ - Can I use Thread Sanitizer for OpenMP programs?

Question

Consider the following example:

#include <iostream> 

int main () {
    int i = 0;
    #pragma omp parallel
    {
        #pragma omp critical
        {
            ++i;
        }
    }
    std::cout << i;
}

Compiling with g++ -fopenmp -fsanitize=thread and running yields

WARNING: ThreadSanitizer: data race (pid=9576)
Read of size 4 at 0x7ffdc170f600 by thread T1:
#0 main._omp_fn.0 (a.out+0x000000400d20)
#1 gomp_thread_start /build/gcc/src/gcc-5.2.0/libgomp/team.c:118 (libgomp.so.1+0x00000000f42d)

Previous write of size 4 at 0x7ffdc170f600 by thread T2:
#0 main._omp_fn.0 (a.out+0x000000400d35)
#1 gomp_thread_start /build/gcc/src/gcc-5.2.0/libgomp/team.c:118 (libgomp.so.1+0x00000000f42d)

Location is stack of main thread.

Thread T1 (tid=9578, running) created by main thread at:
#0 pthread_create /build/gcc/src/gcc-5.2.0/libsanitizer/tsan/tsan_interceptors.cc:895 (libtsan.so.0+0x000000027a37)
#1 gomp_team_start /build/gcc/src/gcc-5.2.0/libgomp/team.c:796 (libgomp.so.1+0x00000000f98f)
#2 __libc_start_main (libc.so.6+0x00000002060f)

Thread T2 (tid=9579, running) created by main thread at:
#0 pthread_create /build/gcc/src/gcc-5.2.0/libsanitizer/tsan/tsan_interceptors.cc:895 (libtsan.so.0+0x000000027a37)
#1 gomp_team_start /build/gcc/src/gcc-5.2.0/libgomp/team.c:796 (libgomp.so.1+0x00000000f98f)
#2 __libc_start_main (libc.so.6+0x00000002060f)

SUMMARY: ThreadSanitizer: data race ??:0 main._omp_fn.0

As far as I can see, this is a false positive. Is there a way to avoid this?

(Something working with clang and libomp would be fine too.)

Answer 1

Yes, at least with Clang this is relatively easy. You'll need to build libomp (which Clang uses instead of libgomp) with ThreadSanitizer support. This doesn't take that long:

git clone https://github.com/llvm/llvm-project
cd llvm-project
mkdir build
cd build
cmake -DLIBOMP_TSAN_SUPPORT=1 ../openmp
sudo cmake --build . --target install

(sudo and --target install is optional if you adjust the path to libomp.so below)

Now running your example works without any errors if you use this libomp.so instead of the system one:

clang++ -fsanitize=thread -fopenmp main.cpp
env LD_PRELOAD=/usr/local/lib/libomp.so ./a.out