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#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;
}
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