Comment by JonChesterfield
8 hours ago
It's obviously, trivially broken. Stores the index before storing the value, so the other thread reads nonsense whenever the race goes against it.
Also doesn't have fences on the store, has extra branches that shouldn't be there, and is written in really stylistically weird c++.
Maybe an llm that likes a different language more, copying a broken implementation off github? Mostly commenting because the initial replies are "best" and "lol", though I sympathise with one of those.
> It's obviously, trivially broken. Stores the index before storing the value, so the other thread reads nonsense whenever the race goes against it.
Are we reading the same code? The stores are clearly after value accesses.
> Also doesn't have fences on the store
?? It uses acquire/release semantics seemingly correctly. Explicit fences are not required.
Push:
buffer_[head] = value;
head_.store(next_head, std::memory_order_release);
return true;
There's no relationship between the two written variables. Stores to the two are independent and can be reordered. The aq/rel applies to the index, not to the unrelated non-atomic buffer located near the index.
That's backwards: in C++, a release store to head_ and an acquire load of that same atomic do order the prior buffer_ write, even though the data and index live in different locations, so the consumer that sees the new head can't legally see an older value for that slot unless something else is racing on it seperately. If this is broken, the bug is elsewhere.
> There's no relationship between the two written variables. Stores to the two are independent and can be reordered. The aq/rel applies to the index, not to the unrelated non-atomic buffer located near the index.
No, this is incorrect. If you think there's no relationship, you don't understand "release" semantics.
https://en.cppreference.com/w/cpp/atomic/memory_order.html
> A store operation with this memory order performs the release operation: no reads or writes in the current thread can be reordered after this store.
This is just wrong. See https://en.cppreference.com/w/cpp/atomic/memory_order.html. Emphasis mine:
> A store operation with this memory order performs the release operation: no reads or writes in the current thread can be reordered after this store. All writes in the current thread are visible in other threads that acquire the same atomic variable (see Release-Acquire ordering below) and writes that carry a dependency into the atomic variable become visible in other threads that consume the same atomic (see Release-Consume ordering below).
write with release semantic cannot be reordered with any other writes, dependent or not.
Relaxed atomic writes can be reordered in any way.
2 replies →
Sorry, but that's not actually true. There are no data races, the atomics prevent that (note that there are only one consumer and one producer)
Regarding the style, it follows the "almost always auto" idea from Herb Sutter
If you enforce that the buffer size is a power of 2 you just use a mask to do the
part
If it's a power of two, you don't need the branch at all. Let the unsigned index wrap.
5 replies →
Indeed that's true. That extra constraint enables further optimization
It's mentioned in the post, but worth reiterating!
This was, in fact, mentioned in the article.