Fix data race in PinnedPagableArc type

[Ollie-Pearce]

When PinnedPagableArc is dropped the pin counter is decremented with a Relaxed ordering. This can result in a data race when two threads drop an owned instance of a type duplicated via Clone.

Reproduction:

Test Case:

#[test]
fn racy_test() {
    let backend = InMemoryPagableStorage::new();
    let foo = PinnedPagableArc::new(false, PagableStorageHandle::new(backend.handle()));
    let bar = foo.clone();
    std::thread::scope(|s| {
        s.spawn(move || drop(bar));
        let _read = *foo;
        drop(foo);
    });
}

Running with Miri:

MIRIFLAGS="-Zmiri-many-seeds=0..16" cargo +nightly miri test -p pagable --test repro

Miri output:

error: Undefined Behavior: Data race detected between (1) non-atomic read on thread `racy_test`
and (2) retag write of type `pagable::pagable_arc::PagableArcInnerData<bool>` on thread `unnamed-3`
at alloc768762+0x20
   --> pagable/src/pagable_arc.rs:877:44
    |
877 |                 let data: &mut _ = unsafe { &mut *ptr.data.get() };
    |                                             ^^^^^^^^^^^^^^^^^^^^ (2) just happened here
help: and (1) occurred earlier here
   --> pagable/src/pagable_arc.rs:716:14

Fix:

Synchronise the PinnedPagableArc pin counter with Release/Acquire memory orderings and use a fence before data is unpinned.

This mirrors the synchronisation used by std::sync::Arc.
If you'd like to see another example I found a similar bug in the stabby crate

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[meta-codesync]

@ndmitchell has imported this pull request. If you are a Meta employee, you can view this in D117200708. (Because this pull request was imported automatically, there will not be any future comments.)

[8Keep]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

@ndmitchell merged this pull request in ab8400a.