std::atomic<std::shared_ptr>::wait does not seem to care about control block difference. Is this a bug?

[zwhfly]

This code seems to block forever under MSVC, but returns successfully under gcc/clanglibstdc++.

#include <memory>
#include <atomic>


int main()
{
    auto sp1 = std::make_shared<char>();
    auto holder = [sp1] {};
    auto sp2 = std::make_shared<decltype(holder)>(holder);
    std::shared_ptr<char> sp3{ sp2, sp1.get() };

    std::atomic<std::shared_ptr<char>> asp{ sp1 };
    asp.wait(sp3);

    return 0;
}

It is said here that

(atomic<shared_ptr>::wait) compares load(order) with old and if they are equivalent then blocks ...

Notes: two shared_ptrs are equivalent if they store the same pointer and either share ownership or are both empty.

I thought that sp1 and sp3 do not have the same control block, so they do not share ownership. Right? So the wait call should return immediately.

But by checking the source code, it seems that the STL implementation for std::atomic<std::shared_ptr>::wait compares the object pointer (the char *) only. It does not compare the control block pointer at all.

Is this a bug in STL? Or what am I missing here?

[fsb4000]

Thanks, it looks like a bug for me. https://eel.is/c++draft/util.smartptr.atomic.shared#lib:wait,atomic%3cshared_ptr%3cT%3e%3e

void wait(shared_ptr<T> old, memory_order order = memory_order::seq_cst) const noexcept;

Preconditions: order is neither memory_­order​::​release nor memory_­order​::​acq_­rel.

Effects: Repeatedly performs the following steps, in order:
    Evaluates load(order) and compares it to old.
    If the two are not equivalent, returns.
    Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.

Remarks: Two shared_­ptr objects are equivalent if they store the same pointer and either share ownership or are both empty.

returns successfully under gcc/clang

By the way LLVM libc++ doesn't implement Atomic std::shared_ptr and std::weak_ptr for now: https://godbolt.org/z/vKTE5s6d6

[zwhfly]

Oh I didn't know that godbolt.org uses libstdc++ with clang by default. Have edited the original post.