clarify RW lock's priority gotcha

[matklad]

In particular, the following program works on Linux, but deadlocks on
mac:

    use std::{
        sync::{Arc, RwLock},
        thread,
        time::Duration,
    };

    fn main() {
        let lock = Arc::new(RwLock::new(()));

        let r1 = thread::spawn({
            let lock = Arc::clone(&lock);
            move || {
                let _rg = lock.read();
                eprintln!("r1/1");
                sleep(1000);

                let _rg = lock.read();
                eprintln!("r1/2");

                sleep(5000);
            }
        });
        sleep(100);
        let w = thread::spawn({
            let lock = Arc::clone(&lock);
            move || {
                let _wg = lock.write();
                eprintln!("w");
            }
        });
        sleep(100);
        let r2 = thread::spawn({
            let lock = Arc::clone(&lock);
            move || {
                let _rg = lock.read();
                eprintln!("r2");
                sleep(2000);
            }
        });

        r1.join().unwrap();
        r2.join().unwrap();
        w.join().unwrap();
    }

    fn sleep(ms: u64) {
        std::thread::sleep(Duration::from_millis(ms))
    }

Context: I was completely mystified by a my CI deadlocking on mac (here), until @azdavis debugged the issue. See a stand-alone reproduciton here: matklad/xshell#15

[rust-highfive]

r? @sfackler

(rust-highfive has picked a reviewer for you, use r? to override)

[sfackler]

@bors r+ rollup

[bors]

📌 Commit 261c952 has been approved by sfackler

[ojeda]

Perhaps add a time-threads diagram with the deadlock example, e.g.:

// Thread 1             |  // Thread 2
let _rg = lock.read();  |
                        |  // will block
                        |  let _wg = lock.write();
// may deadlock         |
let _rg = lock.read();  |