← Back to context

Comment by BeeOnRope

7 years ago

Despite that it was written by Alexandrescu, I can say that without a doubt this 2001 article doesn't represent the current state of thought around MT programming and volatile. I'd think of it more as a historical artifact than anything.

16 comments

BeeOnRope

Reply
mehrdadn  7 years ago

The fact that it doesn't doesn't mean it shouldn't be. It's a damn useful method, it just didn't become popular.

  • ChrisLomont  7 years ago

    It’s completely broken. One of the modern Meyers books even has a chapter on not using volatile in the Dr. Dobbs article manner.

    When the article was written, there was no real alternative, and volatile accidentally worked nicely on certain architectures. It failed on others. It absolutely was never designed to do what you’re trying to defend. It’s always been non-portable, implementation and architecture behavior on how it handled memory read/write barriers. Now that there’s proper ways to do barriers portably, the volatile approach is terrible advice.

    C++ 11 addressed this all in a proper manner, after much research and many papers on the matter. Since then, for major compilers on major architectures, the new C++11 features have been implemented correctly. Volatile has zero use for correct multi threading code. It only has use for memory mapped hardware from a single properly synchronized thread.

    Your article, as people keep telling you but you seem unable to accept it, is wrong. It’s now absolutely not portable, it’s inherently broken, and leads to undefined, hard to debug, terrible behavior for threading issues.

    Go dig up the backstory on how C++11 got its threading model and dig up the More Effective C++ chapter on it to learn why your article is bad.

    • mehrdadn  7 years ago

      It sounds like you don't get what the article's point is. The article is NOT using volatile as a barrier mechanism. It's using it as a compiler-enforced type annotation, which you strip away before accessing the variable of interest. It sounds like absolutely nobody here is willing to read the article because they think they already know everything the article could possibly be saying. Fine, I give up, you win. I've summarized it here for you.

      The idea is this you can use volatile like below. It's pretty self-explanatory. Now can you look through this code and tell me where you see such a horrifying lack of memory barriers and undefined behavior? (And don't point out something irrelevant like how I didn't delete the copy constructor.)

        #include <mutex>

  template<class T>
  class lock_ptr
  {
      T *p;
  public:
      ~lock_ptr() { this->p->m.unlock(); }
      lock_ptr(volatile T *p) : p(const_cast<T *>(p)) { this->p->m.lock(); }
      T *operator->() const { return p; }
  };

  class MyClass
  {
      int x;
  public:
      MyClass() : x() { }
      mutable std::mutex m;
      void bar() { ++x; }
      void foo() volatile { return lock_ptr<MyClass>(this)->bar(); }
  };

  void worker(volatile MyClass *p)  // called in multiple threads
  {
      p->foo();  // thread-safe, and compiles fine
      p->bar();  // thread-unsafe, and compile-time error
  }

  #include <future>

  int main()
  {
      MyClass c;
      auto a = std::async(worker, &c);
      auto b = std::async(worker, &c);
      a.wait();
      b.wait();
      return c.x;
  }

      5 replies →

  • BeeOnRope  7 years ago

    It shouldn't be.

    The stuff the article recommends is straight up UB in modern C++. Volatile has never been specified to work properly with threads, but before C++11 when there was no alternative, some limited use in that context, preferably hidden away from the casual user, may have been acceptable. Recommending these techniques today, however, makes no sense.

    • mehrdadn  7 years ago

      It should be.

      The stuff you're taking about is not the same stuff I'm talking about. There's nothing UB about the locking pointer pattern and how it uses volatile. Read the article in full. It has a specific thesis that is just as valid today as it was 20 years ago, and that thesis is NOT the 2001 malpractice you're talking about.

      6 replies →