79 lines
2.0 KiB
C++
79 lines
2.0 KiB
C++
#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;
|
|
|
|
}
|