On Sun, Feb 16, 2025 at 12:38 AM Nate Eldredge via Std-Discussion <
std-discussion_at_[hidden]> wrote:

>
> On Feb 15, 2025, at 15:30, Jennifier Burnett via Std-Discussion <
> std-discussion_at_[hidden]> wrote:
>
> > Basically, I think there's issues with the standard's wording around
> shared_ptr, and I'd prefer to make sure I've actually found something
> before starting a defect report against a decade old library class.
>
> I'm guessing you're the same person who recently asked this on Stack
> Overflow (
> https://stackoverflow.com/questions/79424997/exactly-how-thread-unsafe-is-shared-ptr-supposed-to-be?noredirect=1#comment140073062_79424997).
> I agree with you that there is an issue.
>
> I came up with an example that is, to my mind, a little simpler and more
> explicit (https://godbolt.org/z/PMMKK9d7r):
>
> #include <memory>
> #include <thread>
> #include <iostream>
>
> class Wobble {
> public:
> Wobble() : x(0) { }
> void frob() { x = 42; }
> ~Wobble() {
> std::cout << x << std::endl;
> }
> private:
> int x;
> };
>
> std::shared_ptr<Wobble> p, q;
>
> void thr1() {
> p->frob();
> p.reset();
> }
>
> void thr2() {
> q.reset();
> }
>
> int main() {
> p = q = std::make_shared<Wobble>();
> std::jthread t1(thr1), t2(thr2);
> return 0;
> }
>
> I think we all hope that this program does not contain a data race, and is
> guaranteed to print 42.
>
> To prove that, we need to ensure that frob() happens-before ~Wobble(), to
> avoid a data race on the x member. And I don't see that shared_ptr offers
> any guarantees that allow us to reach that conclusion.
>

It is puzzling that all the standard library implementations have the
acquire fence but the standard doesn't seem to require it. Maybe it does,
and we all just failed to find it.

I'd expect it to say something like this: if D is the invocation of the
destructor that invokes the deleter, and D' is any other invocation of the
destructor of a `shared_ptr` that shares ownership with it, then D'
strongly happens before the invocation of the deleter.


>
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-- 
*Brian Bi*