Fix spinlock race condition (rwlock)

[trim-the-main]

Readers try to acquire the lock using fetch_add(2) (least significant bit is reserved for the writers) and they check if a writer exists after they perform the addition. If there is a writer they undo their change using fetch_sub(2) . However if the writer unlocks before the fetch_sub(2) of the reader, setting the state to UNLOCKED = 0, then the subsequent fetch_sub of the reader ends up corrupting the state, overflowing and wrapping around.

With this change the unlock_exclusive just clears the writer bit from the state, leaving the reader counts unchanged. The caller must hold the lock so the state must have the writer bit set prior.

$ RUSTFLAGS="--cfg loom" cargo test blocking::rwlock
   
running 2 tests
test blocking::rwlock::tests::read_write ... ok
test blocking::rwlock::tests::write ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 43 filtered out; finished in 0.00s```

[trim-the-main]

I went back to this to see how the loom tests did not catch this. I realized that LOOM_MAX_PREEMPTIONS=2 is not enough to hit this, the test blocking::rwlock::tests::read_write fails with 3 preemptions. I can suggest either bumping this limit to 3 which comes with longer test runs. Or add another test which requires one less preemption to hit this case. Something like:

// Following tests
//     blocking::rwlock::tests::write
//     blocking::rwlock::tests::read_write

#[test]
fn write_read() {
    loom::model(|| {
        let lock = Arc::new(RwLock::<usize>::new(0));
        let r_thread = {
            let reader_lock = lock.clone();
            thread::spawn(move || {
                let guard = reader_lock.read();
                assert!(*guard == 0 || *guard == 1);
            })
        };
        writer(lock)();

        r_thread.join().expect("reader thread mustn't panic")
    });
}

I just changed the order, writer is in the main thread, reader is spawn. This way it requires one less preemption to hit the bug.