CODEWITHSUNDEEP
c++thread-sanitizer
R_Kapp
R_Kapp

Reputation: 2858

Thread sanitizer gives false negative for "function race"

Consider the following code:

#include <atomic>
#include <iostream>
#include <thread>

std::atomic<int> x, y;
std::atomic<int> r1, r2;

void f()
{
  r1 = y.load();
  x = r1.load();
}

void g()
{
  r2 = x.load();
  y = 42;
}

int main()
{
  x = 0;
  y = 0;
  r1 = 0;
  r2 = 0;

  std::thread t1(f);
  std::thread t2(g);

  t1.join();
  t2.join();

  std::cout << r1 << " " << r2 << std::endl;
}
  1. If I compile this code with compilers/linux-x86_64-2.10.1/gnu7.1.0/bin/g++ -fsanitize=thread -O3 -std=c++11 main.cpp -o a.out, TSan does not provide any warnings and/or threading errors.
  2. However, this code is allowed to produce both 42 0 and 0 0 as output.
    • If g() is executed before f() starts, then r1 = y.load() will have a value of 42
    • If g() is not executed before f() starts, then r1 = y.load() will have a value of 0.
  3. Is this something that I should be expecting TSan to catch, or are my expectations completely wrong here?
    • If my expectations are wrong, what can be done (other than code inspection, which can be very difficult for larger code bases) to find bugs such as this?
    • In the event that there should be some error thrown, is there some specific option that I am perhaps missing (I'm using all defaults as specified in the document here)?

Upvotes: 1

Views: 1650

Answers (1)

NathanOliver
NathanOliver

Reputation: 180955

From clang's documentation

ThreadSanitizer is a tool that detects data races

You don't have a data race since all of your variables are atomic, so there is nothing to report.

Upvotes: 4

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