[Bug libstdc++/98108] New: Broken Schwarz counter for iostreams initialization

["i.hamsa at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98108

            Bug ID: 98108
           Summary: Broken Schwarz counter for iostreams initialization
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: i.hamsa at gmail dot com
  Target Milestone: ---

Iostreams initialization doesn't work in multithreaded environments.

To reproduce one needs two source files:

// file1.cc

  #include <thread>  

  void threadfunc();  

  struct StaticThread {
      std::thread t;  
      StaticThread() : t(threadfunc) {}
      ~StaticThread() { t.join(); }
  };  

  static StaticThread thread1, thread2;

//file2.cc

  #include <iostream>  

  void threadfunc() {
      std::cout << "Printing\n";
  }  

  int main() {}

//compile
g++ -pthread file1.cc file2.cc

It is important that <iostream> is NOT included in file1.cc.

The resulting executable always crashes on my machine (Gentoo Linux). Add
<iostream> to file1.cc, the crash disappears. Change the order of source files
in the command line, the crash disappears.

---

The culprit is ios_base::Init::_S_refcount. Basically it is an atomic nifty
counter. The problem is, the nifty counter idiom doesn't work with
multithreading. Making it atomic achieves absolutely nothing. If we have two or
more threads that can enter Init::Init() at the same time, one should proceed
and the rest must WAIT. It is the same thing as initialization of a static
object in a block scope, and should be interlocked in exactly the same way.