However, `Arc` is one of those cases where the overhead induced by an exclusive lock is just way too big, so it is worth using a more optimized, unsafe implementation.
As such, you are going to find plenty of atomic accesses in [the source code of `Arc`](https://github.com/rust-lang/rust/blob/c0955a34bcb17f0b31d7b86522a520ebe7fa93ac/src/liballoc/sync.rs#L201).
-And it turns out, as Hai and Jacques-Henri noticed when attempting to prove correctness of [`Arc::get_mut`](https://doc.rust-lang.org/beta/std/sync/struct.Arc.html#method.get_mut), that there is one place where `Relaxed` as used as an ordering, [but it really should have been `Acquire`](https://github.com/rust-lang/rust/pull/52031).
+And it turns out, as Hai and Jacques-Henri noticed when attempting to prove correctness of [`Arc::get_mut`](https://doc.rust-lang.org/stable/std/sync/struct.Arc.html#method.get_mut), that there is one place where `Relaxed` as used as an ordering, [but it really should have been `Acquire`](https://github.com/rust-lang/rust/pull/52031).
Discussing the exact details of the bug would probably fill another blog post (`Arc` is *really* subtle), but the high-level story is exactly like in our example above: Thanks to `Acquire`, an ordering is induced between the code that follows the `get_mut` and the code in another thread that dropped the last other `Arc`, decrementing the reference count to 1.
The PR that fixed the problem contains [some more details in the comments](https://github.com/rust-lang/rust/pull/52031/files).
With `Relaxed`, no such ordering is induced, so we have a data race.
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