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use crate::ffi::{c_int, c_void};
use crate::pin::Pin;
use crate::ptr::{null, null_mut};
use crate::sync::atomic::{
AtomicU64,
Ordering::{Acquire, Relaxed, Release},
};
use crate::time::Duration;
use libc::{_lwp_self, clockid_t, lwpid_t, time_t, timespec, CLOCK_MONOTONIC};
extern "C" {
fn ___lwp_park60(
clock_id: clockid_t,
flags: c_int,
ts: *mut timespec,
unpark: lwpid_t,
hint: *const c_void,
unparkhint: *const c_void,
) -> c_int;
fn _lwp_unpark(lwp: lwpid_t, hint: *const c_void) -> c_int;
}
/// The thread is not parked and the token is not available.
///
/// Zero cannot be a valid LWP id, since it is used as empty value for the unpark
/// argument in _lwp_park.
const EMPTY: u64 = 0;
/// The token is available. Do not park anymore.
const NOTIFIED: u64 = u64::MAX;
pub struct Parker {
/// The parker state. Contains either one of the two state values above or the LWP
/// id of the parked thread.
state: AtomicU64,
}
impl Parker {
pub unsafe fn new(parker: *mut Parker) {
parker.write(Parker { state: AtomicU64::new(EMPTY) })
}
// Does not actually need `unsafe` or `Pin`, but the pthread implementation does.
pub unsafe fn park(self: Pin<&Self>) {
// If the token has already been made available, we can skip
// a bit of work, so check for it here.
if self.state.load(Acquire) != NOTIFIED {
let parked = _lwp_self() as u64;
let hint = self.state.as_mut_ptr().cast();
if self.state.compare_exchange(EMPTY, parked, Relaxed, Acquire).is_ok() {
// Loop to guard against spurious wakeups.
loop {
___lwp_park60(0, 0, null_mut(), 0, hint, null());
if self.state.load(Acquire) == NOTIFIED {
break;
}
}
}
}
// At this point, the change to NOTIFIED has always been observed with acquire
// ordering, so we can just use a relaxed store here (instead of a swap).
self.state.store(EMPTY, Relaxed);
}
// Does not actually need `unsafe` or `Pin`, but the pthread implementation does.
pub unsafe fn park_timeout(self: Pin<&Self>, dur: Duration) {
if self.state.load(Acquire) != NOTIFIED {
let parked = _lwp_self() as u64;
let hint = self.state.as_mut_ptr().cast();
let mut timeout = timespec {
// Saturate so that the operation will definitely time out
// (even if it is after the heat death of the universe).
tv_sec: dur.as_secs().try_into().ok().unwrap_or(time_t::MAX),
tv_nsec: dur.subsec_nanos().into(),
};
if self.state.compare_exchange(EMPTY, parked, Relaxed, Acquire).is_ok() {
// Timeout needs to be mutable since it is modified on NetBSD 9.0 and
// above.
___lwp_park60(CLOCK_MONOTONIC, 0, &mut timeout, 0, hint, null());
// Use a swap to get acquire ordering even if the token was set after
// the timeout occurred.
self.state.swap(EMPTY, Acquire);
return;
}
}
self.state.store(EMPTY, Relaxed);
}
// Does not actually need `Pin`, but the pthread implementation does.
pub fn unpark(self: Pin<&Self>) {
let state = self.state.swap(NOTIFIED, Release);
if !matches!(state, EMPTY | NOTIFIED) {
let lwp = state as lwpid_t;
let hint = self.state.as_mut_ptr().cast();
// If the parking thread terminated and did not actually park, this will
// probably return an error, which is OK. In the worst case, another
// thread has received the same LWP id. It will then receive a spurious
// wakeup, but those are allowable per the API contract. The same reasoning
// applies if a timeout occurred before this call, but the state was not
// yet reset.
// SAFETY:
// The syscall has no invariants to hold. Only unsafe because it is an
// extern function.
unsafe {
_lwp_unpark(lwp, hint);
}
}
}
}
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