`Arc::strong_count` memory ordering is a potential footgun

In https://github.com/rust-lang/rust/pull/115546 the memory ordering of {Arc,Weak}::{strong,weak}_count got changed to Relaxed, which can cause some hard to debug race bugs.

I tried this code:

use std::cell::UnsafeCell;
use std::sync::Arc;
use std::thread;

struct SendMe<T>(T);
unsafe impl<T> Send for SendMe<T> {}

fn main() {
    let a = Arc::new(UnsafeCell::new(0i32));
    let b = SendMe(a.clone());
    thread::spawn(move || {
        let b = b;
        for _ in 0..100 {
            unsafe { *b.0.get() += 1; }
        }
    });
    while Arc::strong_count(&a) != 1 {}
    // core::sync::atomic::fence(core::sync::atomic::Ordering::Acquire);
    unsafe { *a.get() += 1; }
}

I expected to see this happen: I expected the check strong_count == 0 to be strong enough to avoid a data race in this program. My intuition was that this check implies that no other thread can access the data concurrently.

Instead, this happened: With the fence commented out, the above program has undefined behavior. Miri outputs:

error: Undefined Behavior: Data race detected between (1) Write on thread `<unnamed>` and (2) Read on thread `main` at alloc826+0x10. (2) just happened here
  --> src/main.rs:19:14
   |
19 |     unsafe { *a.get() += 1; }
   |              ^^^^^^^^^^^^^ Data race detected between (1) Write on thread `<unnamed>` and (2) Read on thread `main` at alloc826+0x10. (2) just happened here
   |
help: and (1) occurred earlier here
  --> src/main.rs:14:22
   |
14 |             unsafe { *b.0.get() += 1; }
   |                      ^^^^^^^^^^^^^^^
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: BACKTRACE (of the first span):
   = note: inside `main` at src/main.rs:19:14: 19:27

I think we should either put the Acquire ordering back or add a warning to the docs that such code requires a manual fence.

Meta

1.74 = current beta

cc @SUPERCILEX author of https://github.com/rust-lang/rust/pull/115546

@rustbot label T-libs A-atomic

Review Rust PR #115546 and reproduce the supplied Arc::strong_count example with Miri, including the commented Acquire fence. Determine whether the API's memory ordering or its documentation should change, and validate the chosen resolution against the reported data-race behavior.