clarify RW lock's priority gotcha
In particular, the following program works on Linux, but deadlocks on
mac:
```rust
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:🧵:sleep(Duration::from_millis(ms))
}
```
Context: I was completely mystified by a my CI deadlocking on mac ([here](https://github.com/matklad/xshell/pull/7)), until ``@azdavis`` debugged the issue. See a stand-alone reproduciton here: https://github.com/matklad/xshell/pull/15
e38b3eb0b5