klips/cpp/multithreading/livelock/driver.cpp

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/*##############################################################################
## Author: Shaun Reed ##
## Legal: All Content (c) 2022 Shaun Reed, all rights reserved ##
## About: An example and solution for livelocks in C++ ##
## ##
## Contact: shaunrd0@gmail.com | URL: www.shaunreed.com | GitHub: shaunrd0 ##
################################################################################
*/
#include <chrono>
#include <iostream>
#include <mutex>
#include <thread>
#include <vector>
static std::mutex mtx_A, mtx_B, output;
// Helper function to output thread ID and string associated with mutex name
// + This must also be thread-safe, since we want threads to produce output
// + There is no bug or issue here; This is just in support of example output
void print_safe(const std::string & s) {
std::scoped_lock<std::mutex> scopedLock(output);
std::cout << s << std::endl;
}
void problem() {
// Construct a vector with 5 agreed-upon times to synchronize loops in threads
typedef std::chrono::time_point<std::chrono::steady_clock,
std::chrono::steady_clock::duration> time_point;
std::vector<time_point> waitTime(6);
for (uint8_t i = 0; i < 6; i++) {
waitTime[i] = std::chrono::steady_clock::now()+std::chrono::seconds(1+i);
}
std::thread thread_A([waitTime]()->void {
uint8_t count = 0; // Used to select time slot from waitTime vector
bool done = false;
while (!done) {
count++;
std::lock_guard l(mtx_A);
std::cout << std::this_thread::get_id() << " thread_A: Lock A\n";
// Wait until the next time slot to continue
// + Helps to show example of livelock by ensuring B is not available
std::this_thread::sleep_until(waitTime[count]);
std::cout << std::this_thread::get_id() << " thread_A: Requesting B\n";
if (mtx_B.try_lock()) {
done = true;
std::cout << std::this_thread::get_id()
<< " thread_A: Acquired locks for A, B! Done.\n";
}
else {
std::cout << std::this_thread::get_id()
<< " thread_A: Can't lock B, unlocking A\n";
}
}
mtx_B.unlock();
});
std::thread thread_B([waitTime]()->void {
// As an example, enter livelock for only 5 iterations
// + Also used to select time slot from waitTime vector
uint8_t count = 0;
bool done = false;
while (!done && count < 5) {
count++;
std::lock_guard l(mtx_B);
// Wait until the next time slot to continue
// + Helps to show example of livelock by ensuring A is not available
std::this_thread::sleep_until(waitTime[count]);
if (mtx_A.try_lock()) {
// The program will never reach this point in the code
// + The only reason livelock ends is because count > 5
done = true;
}
}
});
thread_A.join();
thread_B.join();
}
// The solution below uses std::scoped_lock to avoid the livelock problem
void solution() {
std::thread thread_A([]()->void {
for (int i = 0; i < 5; i++) {
// Increase wait time with i
// + To encourage alternating lock ownership between threads
std::this_thread::sleep_for(std::chrono::milliseconds(100 * i));
std::scoped_lock l(mtx_A, mtx_B);
std::cout << std::this_thread::get_id()
<< " thread_A: Acquired locks for A, B!" << std::endl;
}
});
std::thread thread_B([]()->void {
for (int i = 0; i < 5; i++) {
std::this_thread::sleep_for(std::chrono::milliseconds(100 * i));
std::scoped_lock l(mtx_B, mtx_A);
std::cout << std::this_thread::get_id()
<< " thread_B: Acquired locks for B, A!" << std::endl;
}
});
thread_A.join();
thread_B.join();
}
int main(const int argc, const char * argv[]) {
std::cout << "main() thread id: " << std::this_thread::get_id() << std::endl;
problem();
std::cout << "\nSolution:\n\n";
solution();
return 0;
}