Race condition in OpenMP outside parallel block (ThreadSanitizer); false positive?

Question

The following minimal example computes the sum of all the numbers from 1 to 1000 and is parallelized with OpenMP:

#include <iostream>

double sum;

void do_it() {
  const size_t n = 1000;
#pragma omp parallel
  {
#pragma omp for
    for (size_t i = 1; i <= n; ++i) {
#pragma omp atomic
      sum += static_cast<double>(i);
    }
  }
}

int main() {
  sum = 0.;
  do_it();
  std::cout << sum << std::endl;
  return 0;
}

I tried compiling this with either clang++-6.0.0 and g++-5.4.0 and ThreadSanitizer. Both compilers produce a few warnings about race conditions in libomp.so/libgomp.so, which I am assuming are false positives, and the following warning about my code:

==================
WARNING: ThreadSanitizer: data race (pid=22081)
  Read of size 8 at 0x000001555f48 by main thread:
    #0 main /home/arekfu/src/foo/openmp.cc:20 (openmp+0x4be0ce)

  Previous atomic write of size 8 at 0x000001555f48 by thread T11:
    #0 __tsan_atomic64_compare_exchange_val ??:? (openmp+0x476470)
    #1 .omp_outlined._debug__ /home/arekfu/src/foo/openmp.cc:12 (openmp+0x4be011)
    #2 .omp_outlined. /home/arekfu/src/foo/openmp.cc:8 (openmp+0x4be011)
    #3 __kmp_invoke_microtask ??:? (libomp.so.5+0x994b2)

  Location is global '<null>' at 0x000000000000 (openmp+0x000001555f48)

  Thread T11 (tid=22093, running) created by main thread at:
    #0 pthread_create ??:? (openmp+0x4284db)
    #1 __kmpc_threadprivate_register_vec ??:? (libomp.so.5+0x5bc1f)
    #2 __libc_start_main /build/glibc-LK5gWL/glibc-2.23/csu/../csu/libc-start.c:291 (libc.so.6+0x2082f)

SUMMARY: ThreadSanitizer: data race /home/arekfu/src/foo/openmp.cc:20 in main
==================

I cannot see any data race in my code though!

I have also tried replacing the atomic updates with a critical section, like this:

#pragma omp critical
      {
        sum += static_cast<double>(i);
      }

This changes the warning, but the new one does not make much more sense:

==================
WARNING: ThreadSanitizer: data race (pid=27477)
  Write of size 8 at 0x000001555f48 by thread T4:
    #0 .omp_outlined._debug__ /home/arekfu/src/foo/openmp.cc:13 (openmp+0x4be0a2)
    #1 .omp_outlined. /home/arekfu/src/foo/openmp.cc:8 (openmp+0x4be0a2)
    #2 __kmp_invoke_microtask ??:? (libomp.so.5+0x994b2)

  Previous write of size 8 at 0x000001555f48 by thread T3:
    #0 .omp_outlined._debug__ /home/arekfu/src/foo/openmp.cc:13 (openmp+0x4be0a2)
    #1 .omp_outlined. /home/arekfu/src/foo/openmp.cc:8 (openmp+0x4be0a2)
    #2 __kmp_invoke_microtask ??:? (libomp.so.5+0x994b2)

  Location is global '<null>' at 0x000000000000 (openmp+0x000001555f48)

  Thread T4 (tid=27482, running) created by main thread at:
    #0 pthread_create ??:? (openmp+0x42857b)
    #1 __kmpc_threadprivate_register_vec ??:? (libomp.so.5+0x5bc1f)
    #2 __libc_start_main /build/glibc-LK5gWL/glibc-2.23/csu/../csu/libc-start.c:291 (libc.so.6+0x2082f)

  Thread T3 (tid=27481, running) created by main thread at:
    #0 pthread_create ??:? (openmp+0x42857b)
    #1 __kmpc_threadprivate_register_vec ??:? (libomp.so.5+0x5bc1f)
    #2 __libc_start_main /build/glibc-LK5gWL/glibc-2.23/csu/../csu/libc-start.c:291 (libc.so.6+0x2082f)

SUMMARY: ThreadSanitizer: data race /home/arekfu/src/foo/openmp.cc:13 in .omp_outlined._debug__
==================

Are these warnings an indication of real data races, or are they false positives?

Answer 1

The "problem" is the read operation on sum in line 20:

std::cout << sum << std::endl; // here you are reading the value of sum

TSAN cannot infer an inter-thread happens-before relation between this read and the (atomic) updates in the loop. But of course such a relation exists, since all threads are sychronized at the end of the omp block. So yes, this is a false positive.

This post provides more information how to avoid such false positives with OpenMP: Can I use Thread Sanitizer for OpenMP programs?