| 1 | #include <stdio.h> |
|---|---|
| 2 | #include <thread> |
| 3 | #include <mutex> |
| 4 | #include <condition_variable> |
| 5 | |
| 6 | std::mutex contended_mutex; |
| 7 | |
| 8 | std::mutex control_mutex; |
| 9 | std::condition_variable control_condition; |
| 10 | |
| 11 | std::mutex thread_started_mutex; |
| 12 | std::condition_variable thread_started_condition; |
| 13 | |
| 14 | // This function runs in a thread. The locking dance is to make sure that |
| 15 | // by the time the main thread reaches the pthread_join below, this thread |
| 16 | // has for sure acquired the contended_mutex. So then the call_me_to_get_lock |
| 17 | // function will block trying to get the mutex, and only succeed once it |
| 18 | // signals this thread, then lets it run to wake up from the cond_wait and |
| 19 | // release the mutex. |
| 20 | |
| 21 | void |
| 22 | lock_acquirer_1 (void) |
| 23 | { |
| 24 | std::unique_lock<std::mutex> contended_lock(contended_mutex); |
| 25 | |
| 26 | // Grab this mutex, that will ensure that the main thread |
| 27 | // is in its cond_wait for it (since that's when it drops the mutex. |
| 28 | |
| 29 | thread_started_mutex.lock(); |
| 30 | thread_started_mutex.unlock(); |
| 31 | |
| 32 | // Now signal the main thread that it can continue, we have the contended lock |
| 33 | // so the call to call_me_to_get_lock won't make any progress till this |
| 34 | // thread gets a chance to run. |
| 35 | |
| 36 | std::unique_lock<std::mutex> control_lock(control_mutex); |
| 37 | |
| 38 | thread_started_condition.notify_all(); |
| 39 | |
| 40 | control_condition.wait(lock&: control_lock); |
| 41 | |
| 42 | } |
| 43 | |
| 44 | int |
| 45 | call_me_to_get_lock (int ret_val) |
| 46 | { |
| 47 | control_condition.notify_all(); |
| 48 | contended_mutex.lock(); |
| 49 | return ret_val; |
| 50 | } |
| 51 | |
| 52 | int |
| 53 | get_int() { |
| 54 | return 567; |
| 55 | } |
| 56 | |
| 57 | int main () |
| 58 | { |
| 59 | std::unique_lock<std::mutex> thread_started_lock(thread_started_mutex); |
| 60 | |
| 61 | std::thread thread_1(lock_acquirer_1); |
| 62 | |
| 63 | thread_started_condition.wait(lock&: thread_started_lock); |
| 64 | |
| 65 | control_mutex.lock(); |
| 66 | control_mutex.unlock(); |
| 67 | |
| 68 | // Break here. At this point the other thread will have the contended_mutex, |
| 69 | // and be sitting in its cond_wait for the control condition. So there is |
| 70 | // no way that our by-hand calling of call_me_to_get_lock will proceed |
| 71 | // without running the first thread at least somewhat. |
| 72 | |
| 73 | int result = call_me_to_get_lock(ret_val: get_int()); |
| 74 | thread_1.join(); |
| 75 | |
| 76 | return 0; |
| 77 | |
| 78 | } |
| 79 |