m07-concurrency

name	m07-concurrency
description	Mastering C++ Concurrency. Triggers: std::thread, jthread, atomic, mutex, deadlock, race condition, memory model, coroutine, async.

C++ Concurrency

Core Question

How do threads communicate?

• Shared State: Mutexes (std::mutex) or Atomics (std::atomic).
• Coordination: Condition Variables (std::condition_variable) or Latches/Semaphores (C++20).
• Tasks: std::async or Coroutines (C++20).

Error → Design Question

Issue	Design Question
Data Race	Are two threads accessing memory without a Happens-Before edge?
Deadlock	Did you lock Mutex A then B, while another thread locked B then A?
False Sharing	Are independent atomics sitting on the same cache line?
Live Lock	Are threads spinning without progress?

Thinking Prompt

1. Do I need a thread?

   • Short task? → std::async or Thread Pool.
   • Long background? → std::jthread.

2. Is it shared state?

   • Yes? → Protect with std::mutex.
   • Is it small/primitive? → std::atomic.

3. Locks or Atomics?

   • Logic requiring multiple steps? → Mutex (Atomic is only for single instructions).
   • Simple flag/counter? → Atomic.

Trace Up / Down

• Trace Up:
  • Issue: "Random update values in counter."
  • Cause: counter++ is Read-Modify-Write, not atomic. Data race.
  • Fix: std::atomic<int>.

Quick Reference

Tool	C++ Version	Use When
std::jthread	C++20	Standard thread (auto-join).
std::atomic	C++11	Lock-free counters/flags.
std::mutex	C++11	Locking critical sections.
std::shared_mutex	C++17	Read-heavy workloads.
std::latch	C++20	Waiting for N tasks to start.
co_await	C++20	Async I/O (requires library).