This is more of a theoretical question. I'm not sure if all concepts, compiler behaviors, etc. are uptodate and still in use, but I'd like to have confirmation if I'm correctly understanding some c...

Consider the following code: #include <atomic> #include <thread> #include <cassert> #include <memory> int i = 0; std::atomic_int a{0}; int main() { std::thread thr1{[] {...

Condition variables are generally used such that the state they refer to is modified under a mutex. However, when the state is just a single set-only flag, there's no need for a mutex to prevent si...

libc++ std::counting_semaphore uses atomic increment with memory_order_release in release method: void release(ptrdiff_t __update = 1) { if(0 < __a.fetch_add(__update, memory_order_release)) ...

I have read Intel 64 and IA-32 Architectures SDM vol 3A, 9.2 MEMORY ORDERING, but there was one question that kept bothering me. If I first write to a memory address, then send an interprocessor in...

To write a ref-counted class, I have seen 2 different approaches: Approach 1: struct RefCounting1 { void ref_up() { m_ref.fetch_add(1, std::memory_order_relaxed); } void release() { if (m_ref...

Just when I thought I got some grip around atomics, I see another article. This is an excerpt from GCC wiki, under Overall Summary: -Thread 1- -Thread 2- -Thread 3- y.store (20); if (x.load() == ...

In Paul McKenny's famous paper "Memory Barriers: A Hardware View for Software Hackers" 3.3 Store Buffers and Memory Barriers To see the second complication, a violation of global memory ...

Consider an atomic read-modify-write operation such as x.exchange(..., std::memory_order_acq_rel). For purposes of ordering with respect to loads and stores to other objects, is this treated as: a...

In MESI protocol you write to the cache line only when holding it in the Exclusive/Modified state. To acquire the Exclusive state, you send an Invalidate request to all the cores holding the same c...

I came across this code for a simple implementation of a barrier (for code that can't use std::experimental::barrier in C++17 or std::barrier in C++20) in C++ Concurrency in Action book. [Edit] A b...

I keep fighting to understand what VarHandle::setOpaque and VarHandle::getOpaque are really doing. It has not been easy so far - there are some things I think I get (but will not present them in th...

For example, functions like futex_wake/futex_wait, epoll_ctl/epoll_wait, pthread_create provide acquire/release semantics. That's to say, I made some changes before calling futex_wake, and then the...

So I have a simple cow_ptr. It looks something like this: template<class T, class Base=std::shared_ptr<T const>> struct cow_ptr:private Base{ using Base::operator*; using Base::opera...

Consider following toy example, especially the result function: #include <atomic> #include <chrono> #include <iostream> #include <thread> class Worker { std::thread th; s...

Here is a simple example of acquire-release semantics used for data synchronization across threads. // thread 1 // thread 2 data = 100; flag.store(true, std::memory_order_release); while(!flag.loa...

I have read about std::memory_order in C++ and understood partially. But I still had some doubts around it. Explanation on std::memory_order_acquire says that, no reads or writes in the current th...

libc++ shared_ptr implementation release() for the sake of simplicity can be depicted as: void release() { if (ref_count.fetch_sub(1, std::memory_order_acq_rel) == 1) { delete this; } } Why do...

I know store buffer and invalidate queues are reasons that cause memory reordering. What I don't know is if Out-of-Order-Execution can cause memory reordering. In my opinion, Out-of-Order-Execution...

I am currently reading C++ Concurrency in Action by Anthony Williams. One of his listing shows this code, and he states that the assertion that z != 0 can fire. #include <atomic> #include &...

I have a ring buffer that looks like: template<class T> class RingBuffer { public: bool Publish(); bool Consume(T& value); bool IsEmpty(std::size_t head, std::size_t tail); bool I...

I'm learning about different memory orders. I have this code, which works and passes GCC's and Clang's thread sanitizers: #include <atomic> #include <iostream> #include <future> ...

I read about Herb's atomic<> Weapons talk and had a question about page 42: He mentioned that (50:00 in the video): (x86) stores are much stronger than they need to be... What I don't unde...

Consider the store buffer litmus test with SC atomics: // Initial std::atomic<int> x(0), y(0); // Thread 1 // Thread 2 x.store(1); y.store(1); auto r1 = y.load(); auto r2 = x.load(); Can th...

My understanding of std::memory_order_acquire and std::memory_order_release is as follows: Acquire means that no memory accesses which appear after the acquire fence can be reordered to before the...