[PATCH] [libcxx] Fix bug in shared_timed_mutex that could cause a program to hang.

[Eric Fiselier]

>Because the initialism for Stefan's name and that of the Standard Template Library are the same, I'd rather see this named something reflecting what the bug was, instead of the initials of the guy who reported it. This avoids the
>confused question of "isn't every libc++ bug an STL bug?", despite him having made a big name for himself in the community.

Good point. I've changed it.

>Also, did he file a PR for this?

No PR unfortunately. He emailed this bug out to a couple of people.


On Thu, Apr 2, 2015 at 4:06 PM, Eric Fiselier <eric at efcs.ca> wrote:
> Address review comments.
>
>
> http://reviews.llvm.org/D8796
>
> Files:
>   include/shared_mutex
>   test/std/thread/thread.mutex/thread.mutex.requirements/thread.sharedtimedmutex.requirements/thread.sharedtimedmutex.class/try_lock_until_deadlock_bug.pass.cpp
>
> Index: include/shared_mutex
> ===================================================================
> --- include/shared_mutex
> +++ include/shared_mutex
> @@ -193,6 +193,7 @@
>              if (__status == cv_status::timeout)
>              {
>                  __state_ &= ~__write_entered_;
> +                __gate1_.notify_all();
>                  return false;
>              }
>          }
> Index: test/std/thread/thread.mutex/thread.mutex.requirements/thread.sharedtimedmutex.requirements/thread.sharedtimedmutex.class/try_lock_until_deadlock_bug.pass.cpp
> ===================================================================
> --- /dev/null
> +++ test/std/thread/thread.mutex/thread.mutex.requirements/thread.sharedtimedmutex.requirements/thread.sharedtimedmutex.class/try_lock_until_deadlock_bug.pass.cpp
> @@ -0,0 +1,70 @@
> +//===----------------------------------------------------------------------===//
> +//
> +//                     The LLVM Compiler Infrastructure
> +//
> +// This file is dual licensed under the MIT and the University of Illinois Open
> +// Source Licenses. See LICENSE.TXT for details.
> +//
> +//===----------------------------------------------------------------------===//
> +//
> +// UNSUPPORTED: libcpp-has-no-threads
> +// UNSUPPORTED: c++03, c++98, c++11
> +
> +// <shared_mutex>
> +
> +// class shared_timed_mutex;
> +
> +#include <shared_mutex>
> +
> +#include <atomic>
> +#include <chrono>
> +#include <thread>
> +#include <cstdlib>
> +#include <cassert>
> +
> +std::shared_timed_mutex m;
> +
> +const int total_readers = 2;
> +std::atomic<int> readers_started(0);
> +std::atomic<int> readers_finished(0);
> +
> +// Wait for the readers to start then try and acquire the write lock.
> +void writer_one() {
> +  while (readers_started != total_readers) {}
> +  bool b = m.try_lock_for(std::chrono::milliseconds(500));
> +  assert(b == false);
> +}
> +
> +void blocked_reader() {
> +  ++readers_started;
> +  // Wait until writer_one is waiting for the write lock.
> +  while (m.try_lock_shared()) {
> +    m.unlock_shared();
> +  }
> +  // Attempt to get the read lock. writer_one should be blocking us because
> +  // writer_one is blocked by main.
> +  m.lock_shared();
> +  ++readers_finished;
> +  m.unlock_shared();
> +}
> +
> +int main()
> +{
> +  typedef std::chrono::steady_clock Clock;
> +
> +  m.lock_shared();
> +  std::thread t1(writer_one);
> +  // create some readers
> +  std::thread t2(blocked_reader);
> +  std::thread t3(blocked_reader);
> +  // Kill the test after 10 seconds if it hasn't completed.
> +  auto end_point = Clock::now() + std::chrono::seconds(10);
> +  while (readers_finished != total_readers && Clock::now() < end_point) {
> +    std::this_thread::sleep_for(std::chrono::seconds(1));
> +  }
> +  assert(readers_finished == total_readers);
> +  m.unlock_shared();
> +  t1.join();
> +  t2.join();
> +  t3.join();
> +}
>
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