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diff --git a/_posts/2015-07-03-std-call-once-bug.md b/_posts/2015-07-03-std-call-once-bug.md deleted file mode 100644 index 0aa0249..0000000 --- a/_posts/2015-07-03-std-call-once-bug.md +++ /dev/null @@ -1,258 +0,0 @@ ---- -title: std::call_once bug in Visual C++ 2012/2013 -excerpt: > - In this post I will describe a nasty bug I've stumbled upon in the C++ - Standard Library implementation shipped with Microsoft Visual Studio - 2012/2013. -category: C++ -custom_css: - - syntax.css ---- -I've recently come across a nasty standard library bug in the implementation -shipped with Microsoft Visual Studio 2012/2013. -[StackOverflow was of no help], so I had to somehow report the bug to the -maintainers. -Oddly enough, Visual Studio's [Connect page] wouldn't let me report one, -complaining about the lack of permissions, even though I was logged in from my -work account, associated with my Visual Studio 2013 installation. - -Fortunately, I've come across the personal website of this amazing guy, -[Stephan T. Lavavej], who appears to be the chief maintainer of Microsoft's -standard library implementation. -He seems to be your go-to guy when it comes to obvious standard library -misbehaviours. - -[StackOverflow was of no help]: https://stackoverflow.com/questions/26477070/concurrent-stdcall-once-calls -[Connect page]: https://connect.microsoft.com/VisualStudio -[Stephan T. Lavavej]: http://nuwen.net/stl.html - -C++11 and singletons --------------------- - -Anyway, the story begins with me trying to implement the singleton pattern -using C++11 facilities like this: - -```c++ -#include <mutex> - -template <typename Derived> -class Singleton { -public: - static Derived& get_instance() { - std::call_once(initialized_flag, &initialize_instance); - return Derived::get_instance_unsafe(); - } - -protected: - Singleton() = default; - ~Singleton() = default; - - static Derived& get_instance_unsafe() { - static Derived instance; - return instance; - } - -private: - static void initialize_instance() { - Derived::get_instance_unsafe(); - } - - static std::once_flag initialized_flag; - - Singleton(const Singleton&) = delete; - Singleton& operator=(const Singleton&) = delete; -}; - -template <typename Derived> -std::once_flag Singleton<Derived>::initialized_flag; -``` - -Neat, huh? -Now other classes can inherit from `Singleton`, implementing the singleton -pattern effortlessly: - -```c++ -class Logger : public Singleton<Logger> { -private: - Logger() = default; - ~Logger() = default; - - friend class Singleton<Logger>; -}; -``` - -Note that the [N2660] standard proposal isn't/wasn't implemented in the -compilers shipped with Visual Studio 2012/2013. -If it was, I wouldn't, of course, need to employ this `std::call_once` -trickery, and the implementation would be much simpler, i.e. something like -this: - -```c++ -class Logger { -public: - static Logger& get_instance() { - static Logger instance; - return instance; - } - -private: - Logger() = default; - ~Logger() = default; -}; -``` - -<div class="alert alert-info" markdown="1"> - -The point is that the `Logger::get_instance` routine above wasn't thread-safe -until C++11. -Imagine what might happen if `Logger`'s constructor takes some time to -initialize the instance. -If a couple of threads then call `get_instance`, the first thread might begin -the initialization process, making the other thread believe that the instance -had already been intialized. -This other thread might then return a reference to the instance which hasn't -yet completed its initialization and is most likely unsafe to use. - -Since C++11 includes the proposal mentioned above, this routine would indeed be -thread-safe in C++11. -Unfortunately, the compilers shipped with Visual Studio 2012/2013 don't/didn't -implement this particular proposal, which caused me to look at -`std::call_once`, which seemed to implement exactly what I needed. - -</div> - -[N2660]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2660.htm - -Problem -------- - -Unfortunately, matters became a bit more complicated when I tried to introduce -two singletons, one having a dependency on the other. -I had `Logger`, like in the example above, and some kind of a "master" -singleton (let's call it `Duke`). -`Duke`'s constructor was complicated and time-consuming, and definetely -required some logging to be done. -I thought that I could simply call `Logger::get_instance` inside `Duke`'s -constructor, and everything looked fine at first glance. - -```c++ -#include <chrono> -#include <thread> - -class Logger : public Singleton<Logger> { -public: - Logger& operator<<(const char* msg) { - // Actual logging is stripped for brevity. - return *this; - } - -private: - Logger() { - // Opening log files, etc. - std::this_thread::sleep_for(std::chrono::seconds{3}); - } - - ~Logger() = default; - - friend class Singleton<Logger>; -}; - -class Duke : public Singleton<Duke> { -private: - Duke() { - Logger::get_instance() << "started Duke's initialization"; - // It's a lot of work to be done. - std::this_thread::sleep_for(std::chrono::seconds{10}); - Logger::get_instance() << "finishing Duke's initialization"; - } - - ~Duke() = default; - - friend class Singleton<Duke>; -}; -``` - -Now, what happens if I have two threads, one using the `Duke` instance, and the -other logging something? -Like in this example: - -```c++ -#include <thread> - -namespace { - -void get_logger() { - entered(__FUNCTION__); - Logger::get_instance(); - exiting(__FUNCTION__); -} - -void get_duke() { - entered(__FUNCTION__); - Duke::get_instance(); - exiting(__FUNCTION__); -} - -} - -int main() { - std::thread t1{&get_duke}; - std::thread t2{&get_logger}; - t1.join(); - t2.join(); - return 0; -} -``` - -`entered` and `exiting` are utility functions to print timestamps. -The implementation is included in the [complete code sample]. -{: .alert .alert-info } - -The first thread is supposed to have the total running time of about 13 -seconds, right? -Three seconds to initialize the `Logger` instance and ten to initialize the -`Duke` instance. -The second thread, similarly, is supposed to be done in about 3 seconds -required for the initialization of `Logger`. - -Weirdly, this program produces the following output when compiled using Visual -Studio 2013's compiler: - - Entered `anonymous-namespace'::get_duke at Fri Jul 03 02:26:16 2015 - Entered `anonymous-namespace'::get_logger at Fri Jul 03 02:26:16 2015 - Exiting `anonymous-namespace'::get_duke at Fri Jul 03 02:26:29 2015 - Exiting `anonymous-namespace'::get_logger at Fri Jul 03 02:26:29 2015 - -Isn't it wrong that the second thread actually took the same 13 seconds as the -first thread? -Better check with some other compiler in case it was me who made a mistake. -Unfortunately, the program behaves as expected when compiled using GCC: - - Entered get_logger at Fri Jul 3 02:27:12 2015 - Entered get_duke at Fri Jul 3 02:27:12 2015 - Exiting get_logger at Fri Jul 3 02:27:15 2015 - Exiting get_duke at Fri Jul 3 02:27:25 2015 - -So it appears that the implementation of `std::call_once` shipped with Visual -Studio 2012/2013 relies on some kind of a global lock, which causes even the -simple example above to misbehave. - -The [complete code sample] to demonstrate the misbehaviour described above can -be found in this blog's repository. - -[complete code sample]: {{ site.github.repository_url }}/tree/master/std_call_once_bug - -Resolution ----------- - -So, since I couldn't submit the bug via Visual Studio's [Connect page], I wrote -to Mr. Lavavej directly, not hoping for an answer. -Amazingly, it took him less than a day to reply. -He told me he was planning to overhaul `std::call_once` for Visual Studio 2015. -Meanwhile, I had to stick to something else; I think I either dropped logging -from `Duke`'s constructor or initialized all the singleton instances manually -before actually using any of them. -In a few months, Mr. Lavavej replied to me that the bug has been fixed in -Visual Studio 2015 RTM. -I would like to thank him for the professionalism and responsibility he's -shown. |