以下最小示例计算从 1 到 1000 的所有数字的总和，并与 OpenMP 并行化：
#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;
}
我尝试使用clang++-6.0.0和g++-5.4.0以及线程清理器进行编译。两个编译器都会生成一些关于libomp.so/libgomp.so中竞争条件的警告，我假设这些警告是误报，以及以下关于我的代码的警告：
==================
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
==================
不过，我在我的代码中看不到任何数据竞赛！
我还尝试用critical部分替换atomic更新，如下所示：
#pragma omp critical
{
sum += static_cast<double>(i);
}
这会更改警告，但新的警告没有多大意义：
==================
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__
==================
这些警告是真实数据竞争的指示，还是误报？
"问题"是第 20 行中总和的读取操作：
std::cout << sum << std::endl; // here you are reading the value of sum
TSAN 无法推断此读取与循环中的(原子(更新之间的线程间发生前关系。但是当然存在这样的关系，因为所有线程都在omp块的末尾同步。所以是的，这是一个误报。
这篇文章提供了有关如何使用 OpenMP 避免此类误报的更多信息：我可以将线程清理器用于 OpenMP 程序吗？