How to use PiFutex::lock_pi_until() with correct clock?

[rtzoeller]

I'm trying to use PiFutex::lock_pi_until() and am hitting an issue with FUTEX_LOCK_PI and std::time::Instant using different clocks. It's manifesting as PiFutex::lock_pi_until() returning nearly immediately, regardless of the timeout value.

For example,

let futex = linux_futex::PiFutex::<linux_futex::Private>::new(1); // Non-zero -> locked by someone else
println!("{:?}", std::time::Instant::now());
let result = futex.lock_pi_until(std::time::Instant::now() + std::time::Duration::from_secs(2));
println!("{:?}", std::time::Instant::now());
dbg!(result);

outputs

Instant { tv_sec: 12663, tv_nsec: 14076982 }
Instant { tv_sec: 12663, tv_nsec: 14100659 }
[tests/tests.rs:144] result = Err(
    TimedOut,
)

---

FUTEX_LOCK_PI is documented as using CLOCK_REALTIME, while std::time::Instant (used by lock_pi_until()) is implemented with CLOCK_MONOTONIC.

1. How is PiFutex::lock_pi_until() intended to be used? Is there a recommendation for converting between std::time::SystemTime (which uses CLOCK_REALTIME) and std::time::Instant?
2. Have you considered adding support for FUTEX_LOCK_PI2, which allows selecting a clock and could be used with CLOCK_MONOTONIC?

Thanks!

[rtzoeller]

It seems like modifying linux-futex to use FUTEX_LOCK_PI2 instead "just works", but there's a much higher Linux kernel requirement associated with that. I'll at least add it to rust-lang/libc (PR), since it seems to be absent.

[m-ou-se]

Thanks! It's fixed in 8f75c58. lock_pi_until now accepts either an SystemTime or an Instant, and picks either FUTEX_LOCK_PI or FUTEX_LOCK_PI2 depending on that.

Published as 0.1.2 to crates.io just now.

[rtzoeller]

@m-ou-se thanks! This new release seems to fix it.