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

Answer 2

even with the suppression you will still get false positives on the OpenMP runtime because there are some synchronization mechanism within the runtime that Tsan is not able to understand.

We worked on the OpenMP runtime to make Tsan understand this synchronization points and remove all the false positives.

Take a look to this project:

https://github.com/PRUNER/archer

Let me know if you need more help.

Best,

Simone