[cpp] Add example and solution for deadlocks

cpp/multithreading/CMakeLists.txt

@@ -15,4 +15,5 @@ project(
     LANGUAGES   CXX
 )
 
+add_subdirectory(deadlock)
 add_subdirectory(race-condition)

cpp/multithreading/deadlock/CMakeLists.txt

@@ -0,0 +1,26 @@
+################################################################################
+## Author: Shaun Reed                                                         ##
+## Legal: All Content (c) 2022 Shaun Reed, all rights reserved                ##
+## About: An example and solution for deadlocks in C++                        ##
+##                                                                            ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0   ##
+################################################################################
+
+cmake_minimum_required(VERSION 3.16)
+
+# std::scoped_lock requires C++17
+set(CMAKE_CXX_STANDARD 17)
+add_compile_options("-Wall")
+
+project(
+    #[[NAME]]   Deadlock
+    VERSION     1.0
+    DESCRIPTION "Example and solution for deadlocks in C++"
+    LANGUAGES   CXX
+)
+
+add_executable(
+    multithread-deadlock driver.cpp
+)
+
+target_link_libraries(multithread-deadlock pthread)

cpp/multithreading/deadlock/driver.cpp

@@ -0,0 +1,189 @@
+/*##############################################################################
+## Author: Shaun Reed                                                         ##
+## Legal: All Content (c) 2022 Shaun Reed, all rights reserved                ##
+## About: An example and solution for deadlocks in C++                        ##
+##                                                                            ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0   ##
+################################################################################
+*/
+
+#include <chrono>
+#include <iostream>
+#include <mutex>
+#include <sstream>
+#include <thread>
+
+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;
+}
+
+// Helper function to convert std::thread::id to string
+std::string id_string(const std::thread::id & id) {
+  std::stringstream stream;
+  stream << id;
+  return stream.str();
+}
+
+// In the two threads within this function, we have a problem
+// + The mutex locks are acquired in reverse order, so they collide
+// + This is called a deadlock; The program will *never* finish
+void problem() {
+  std::thread thread_A([]()->void {
+    mtx_A.lock();
+    print_safe(id_string(std::this_thread::get_id()) + " thread_A: Locked A");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+    mtx_B.lock(); // We can't lock B! thread_B is using it
+    // The program will never reach this point in execution; We are in deadlock
+    print_safe(id_string(std::this_thread::get_id())
+               + " thread_A: B has been unlocked, we can proceed!\n  Locked B"
+    );
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id())
+               + " thread_A: Unlocking A, B..."
+    );
+    mtx_A.unlock();
+    mtx_B.unlock();
+  });
+
+  std::thread thread_B([]()->void {
+    mtx_B.lock();
+    print_safe(id_string(std::this_thread::get_id()) + " thread_B: Locked B");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+    mtx_A.lock(); // We can't lock A! thread_A is using it
+    // The program will never reach this point in execution; We are in deadlock
+    print_safe(id_string(std::this_thread::get_id())
+               + " thread_B: A has been unlocked, we can proceed!\n  Locked A"
+    );
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id())
+               + " thread_B: Unlocking B, A..."
+    );
+    mtx_B.unlock();
+    mtx_A.unlock();
+  });
+
+  // This offers a way out of the deadlock, so we can proceed to the solution
+  std::this_thread::sleep_for(std::chrono::seconds(2));
+  char input;
+  print_safe("\n"
+             + id_string(std::this_thread::get_id())
+             + " problem(): We are in a deadlock. \n"
+             + "    Enter y/Y to continue to the solution...\n"
+  );
+  while (std::cin >> input) {
+    if (input != 'Y' && input != 'y') continue;
+    else break;
+  }
+  print_safe(id_string(std::this_thread::get_id())
+             + " problem(): Unlocking A, B..."
+  );
+  mtx_A.unlock();
+  mtx_B.unlock();
+
+  thread_A.join();
+  thread_B.join();
+}
+
+// std::lock will lock N mutex locks
+// + If either is in use, execution will block until both are available to lock
+void solution_A() {
+  std::thread thread_A([]()->void {
+    std::lock(mtx_A, mtx_B);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked A, B");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking A, B...");
+    mtx_A.unlock();
+    mtx_B.unlock();
+  });
+
+  std::thread thread_B([]()->void {
+    std::lock(mtx_B, mtx_A);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked B, A");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking B, A...");
+    mtx_B.unlock();
+    mtx_A.unlock();
+  });
+
+  thread_A.join();
+  thread_B.join();
+}
+
+// std::lock_guard is a C++11 object which can be constructed with 1 mutex
+// + When the program leaves the scope of the guard, the mutex is unlocked
+void solution_B() {
+  std::thread thread_A([]()->void {
+    // lock_guard will handle unlocking when program leaves this scope
+    std::lock_guard<std::mutex> guard_A(mtx_A), guard_B(mtx_B);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked A, B");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking A, B...");
+    // We don't need to explicitly unlock either mutex
+  });
+
+  std::thread thread_B([]()->void {
+    std::lock_guard<std::mutex> guard_B(mtx_B), guard_A(mtx_A);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked B, A");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking B, A...");
+    // We don't need to explicitly unlock either mutex
+  });
+
+  thread_A.join();
+  thread_B.join();
+}
+
+// std::scoped_lock is a C++17 object that can be constructed with N mutex
+// + When the program leaves this scope, all N mutex will be unlocked
+void solution_C() {
+  std::thread thread_A([]()->void {
+    // scoped_lock will handle unlocking when program leaves this scope
+    std::scoped_lock scopedLock(mtx_A, mtx_B);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked A, B");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking A, B...");
+    // We don't need to explicitly unlock either mutex
+  });
+
+  std::thread thread_B([]()->void {
+    std::scoped_lock scopedLock(mtx_A, mtx_B);
+    print_safe(id_string(std::this_thread::get_id()) + ": Locked B, A");
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+
+    print_safe(id_string(std::this_thread::get_id()) + ": Unlocking B, A...");
+    // We don't need to explicitly unlock either mutex
+  });
+
+  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();
+
+  print_safe("\nsolution_A, using std::lock\n");
+  solution_A();
+
+  print_safe("\nsolution_B, using std::lock_guard\n");
+  solution_B();
+
+  print_safe("\nsolution_C, using std::scoped_lock\n");
+  solution_C();
+
+  return 0;
+}