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