| #include <stdio.h> |
| #include <thread> |
| #include <mutex> |
| #include <condition_variable> |
| |
| std::mutex contended_mutex; |
| |
| std::mutex control_mutex; |
| std::condition_variable control_condition; |
| |
| std::mutex thread_started_mutex; |
| std::condition_variable thread_started_condition; |
| |
| // This function runs in a thread. The locking dance is to make sure that |
| // by the time the main thread reaches the pthread_join below, this thread |
| // has for sure acquired the contended_mutex. So then the call_me_to_get_lock |
| // function will block trying to get the mutex, and only succeed once it |
| // signals this thread, then lets it run to wake up from the cond_wait and |
| // release the mutex. |
| |
| void |
| lock_acquirer_1 (void) |
| { |
| std::unique_lock<std::mutex> contended_lock(contended_mutex); |
| |
| // Grab this mutex, that will ensure that the main thread |
| // is in its cond_wait for it (since that's when it drops the mutex. |
| |
| thread_started_mutex.lock(); |
| thread_started_mutex.unlock(); |
| |
| // Now signal the main thread that it can continue, we have the contended lock |
| // so the call to call_me_to_get_lock won't make any progress till this |
| // thread gets a chance to run. |
| |
| std::unique_lock<std::mutex> control_lock(control_mutex); |
| |
| thread_started_condition.notify_all(); |
| |
| control_condition.wait(control_lock); |
| |
| } |
| |
| int |
| call_me_to_get_lock (int ret_val) |
| { |
| control_condition.notify_all(); |
| contended_mutex.lock(); |
| return ret_val; |
| } |
| |
| int |
| get_int() { |
| return 567; |
| } |
| |
| int main () |
| { |
| std::unique_lock<std::mutex> thread_started_lock(thread_started_mutex); |
| |
| std::thread thread_1(lock_acquirer_1); |
| |
| thread_started_condition.wait(thread_started_lock); |
| |
| control_mutex.lock(); |
| control_mutex.unlock(); |
| |
| // Break here. At this point the other thread will have the contended_mutex, |
| // and be sitting in its cond_wait for the control condition. So there is |
| // no way that our by-hand calling of call_me_to_get_lock will proceed |
| // without running the first thread at least somewhat. |
| |
| int result = call_me_to_get_lock(get_int()); |
| thread_1.join(); |
| |
| return 0; |
| |
| } |