test that futexes induce appropriate synchronization
This fails when I remove the `validate_lock_acquire` or `validate_lock_release` from `futex_wake`. So finally we got those code paths actually covered in tests. :)
Prevent futex_wait from actually waiting if a concurrent waker was executed before us
Fixes#2223
Two SC fences were placed in `futex_wake` (after the caller has changed `addr`), and in `futex_wait` (before we read `addr`). This guarantees that `futex_wait` sees the value written to `addr` before the last `futex_wake` call, should one exists, and avoid going into sleep with no one else to wake us up.
ada7b72a87/src/concurrency/weak_memory.rs (L324-L326)
Earlier I proposed to use `fetch_add(0)` to read the latest value in MO, though this isn't the proper way to do it and breaks aliasing: syscall caller may pass in a `*const` from a `&` and Miri complains about write to a `SharedReadOnly` location, causing this test to fail.
ada7b72a87/tests/pass/concurrency/linux-futex.rs (L56-L68)
