Can acquire loads reorder with other acquire operations? cppreference says only non-atomic and relaxed are ordered by acquire

Question

According to C++ Reference, mutex.lock() is a memory_order_acquire operation, and mutex.unlock() is a memory_order_release operation.

However, memory_order_acquire and memory_order_release are only effective for non-atomic and relaxed atomic operations.

memory_order: Release-Acquire ordering on cppreference

If an atomic store in thread A is tagged memory_order_release and an atomic load in thread B from the same variable is tagged memory_order_acquire, all memory writes (non-atomic and relaxed atomic) that happened-before the atomic store from the point of view of thread A, become visible side-effects in thread B

Could mutex in C++ guarantee the visibility of atomic operations? An example is as follows. Can the code A reorder before the mu.lock(), and the thread b read x as false?


#include <thread>
#include <atomic>
#include <cassert>
#include <iostream>
#include <unistd.h>

std::atomic<bool> x = {false};
std::mutex mu;

void write_x(){
  mu.lock();
  std::cout << "write_x" << std::endl;
  x.store(true, std::memory_order_release);
  mu.unlock();
}

void read_x() {
  mu.lock();
  std::cout << "read_x" << std::endl;
  assert(x.load(std::memory_order_acquire)); // A
  mu.unlock();
}

int main() {
  std::thread a(write_x);
  usleep(1);
  std::thread b(read_x);

  a.join(); b.join();

  return 0;
}

However, memory_order_acquire and memory_order_release are only effective for non-atomic variables. - no, they order atomic operations as well, including relaxed. — Peter Cordes
No, of course it can't reorder before an acquire operation. Synchronizes-with orders all operations, including other acquire and release operations, not just seq_cst, relaxed, or non-atomic. But your code could still fire the assert (at least in theory) if thread a is slow to start up and thread b takes the lock mu first. — Peter Cordes
I think when cpp-reference says "relaxed atomic" then mean any order more relaxed than seq_cst. It's a confusing way to phrase it, but fairly obvious from context or if you check the actual ISO C++ standard. This is somewhat common usage, although it's very confusing because of std::memory_order_relaxed and should be avoided. — Peter Cordes
@PeterCordes Oh, it is because I misunderstand "relaxed atomic" in cpp-reference. Thanks so much! — Zihe Liu
Oh, I see. non-atomic and relaxed atomic is for emphasizing that release-acquire can make them be visible, not to exclude seq_cst. Thanks very mush for answering so many questions. — Zihe Liu

Peter Cordes Peter Cordes · Accepted Answer · 2020-08-23T06:25:23

TL:DR: "all memory writes" means all, not just the kinds mentions, but the phrasing is confusing. Probably intended just to point out that even non-atomic and relaxed atomic ops are safely visible across a synchronizes-with, but the phrasing is missing the word "including".

Note that cppreference is a wiki that's intended to explain the standard. It's not normative technical language, and sometimes even explains things in different terms than the ISO C++ standard.

It's generally very good, but don't just assume that it's perfect when something seems strange. From surrounding context (and sanity), like the last sentence in the paragraph saying "everything" with no qualifications, it's still fairly obvious that's what was meant.

ISO C++ is clear. An acquire operation that "sees" a release operation creates a synchronizes-with relationship. Everything before the release is visible to code after the acquire operation.

So in terms of a model where operations that access a globally coherent shared state of memory, acquire operations block everything from reordering before them. Including release and seq_cst operations. (Note that this part of cppreference doesn't make any reference to reordering, just to guaranteed visibility or not. Local reordering of accesses to global coherent state is in practice how real CPUs work, so it's often more convenient to describe things that way, like you're doing in the question.)

This means that C++'s definition of acquire and release matches standard terminology without insane magic exceptions. https://preshing.com/20120913/acquire-and-release-semantics/

Note that some people use "relaxed atomics" to describe all orderings weaker than seq_cst. Example: Herb Sutter uses it that way in the talk this question is about.

That might be what was meant in that cppreference definition, but IDK why they'd want to exclude seq_cst. All atomic and non-atomic operations are ordered. So perhaps they did mean mo_relaxed, and just wanted to point out that even those are ordered / visible.

(seq_cst could be said to already order itself wrt. everything else, so "of course" it's ordered with respect to acquire and release operations. But that reason seems unlikely.)

If it was intended for emphasis of the fact that weaker orders were also ordered by it, they should have written "including non-atomic and relaxed atomic". Without the word "including", that phrasing can be read as implying only non-atomic and relaxed-atomic. Only an understanding of the big picture and what would be sane or not can give you a correct reading.

Technical writing that needs to be precisely understood will often use the phrase "including but not limited to".

Also note that your example can still trigger the assert, just not for the reason you were worried about.

If thread a is slow to start up, thread b could enter its critical section first and print + read x before the print+store in the other thread happens.

The usual way to write toy examples like that is a loop that spins on an acquire load until it sees a value, e.g. a flag like data_read stored by a release operation after the store you care about. That way you know the read side runs after an acquire operation that synced-with a release operation in the write side.

Can acquire loads reorder with other acquire operations? cppreference says only non-atomic and relaxed are ordered by acquire

1 Answers