1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271
//! Thread parking without `futex` using the `pthread` synchronization primitives.
use crate::cell::UnsafeCell;
use crate::marker::PhantomPinned;
use crate::pin::Pin;
use crate::ptr::addr_of_mut;
use crate::sync::atomic::AtomicUsize;
use crate::sync::atomic::Ordering::SeqCst;
use crate::time::Duration;
const EMPTY: usize = 0;
const PARKED: usize = 1;
const NOTIFIED: usize = 2;
unsafe fn lock(lock: *mut libc::pthread_mutex_t) {
let r = libc::pthread_mutex_lock(lock);
debug_assert_eq!(r, 0);
}
unsafe fn unlock(lock: *mut libc::pthread_mutex_t) {
let r = libc::pthread_mutex_unlock(lock);
debug_assert_eq!(r, 0);
}
unsafe fn notify_one(cond: *mut libc::pthread_cond_t) {
let r = libc::pthread_cond_signal(cond);
debug_assert_eq!(r, 0);
}
unsafe fn wait(cond: *mut libc::pthread_cond_t, lock: *mut libc::pthread_mutex_t) {
let r = libc::pthread_cond_wait(cond, lock);
debug_assert_eq!(r, 0);
}
const TIMESPEC_MAX: libc::timespec =
libc::timespec { tv_sec: <libc::time_t>::MAX, tv_nsec: 1_000_000_000 - 1 };
unsafe fn wait_timeout(
cond: *mut libc::pthread_cond_t,
lock: *mut libc::pthread_mutex_t,
dur: Duration,
) {
// Use the system clock on systems that do not support pthread_condattr_setclock.
// This unfortunately results in problems when the system time changes.
#[cfg(any(
target_os = "macos",
target_os = "ios",
target_os = "watchos",
target_os = "espidf"
))]
let (now, dur) = {
use crate::cmp::min;
use crate::sys::time::SystemTime;
// OSX implementation of `pthread_cond_timedwait` is buggy
// with super long durations. When duration is greater than
// 0x100_0000_0000_0000 seconds, `pthread_cond_timedwait`
// in macOS Sierra return error 316.
//
// This program demonstrates the issue:
// https://gist.github.com/stepancheg/198db4623a20aad2ad7cddb8fda4a63c
//
// To work around this issue, and possible bugs of other OSes, timeout
// is clamped to 1000 years, which is allowable per the API of `park_timeout`
// because of spurious wakeups.
let dur = min(dur, Duration::from_secs(1000 * 365 * 86400));
let now = SystemTime::now().t;
(now, dur)
};
// Use the monotonic clock on other systems.
#[cfg(not(any(
target_os = "macos",
target_os = "ios",
target_os = "watchos",
target_os = "espidf"
)))]
let (now, dur) = {
use crate::sys::time::Timespec;
(Timespec::now(libc::CLOCK_MONOTONIC), dur)
};
let timeout =
now.checked_add_duration(&dur).and_then(|t| t.to_timespec()).unwrap_or(TIMESPEC_MAX);
let r = libc::pthread_cond_timedwait(cond, lock, &timeout);
debug_assert!(r == libc::ETIMEDOUT || r == 0);
}
pub struct Parker {
state: AtomicUsize,
lock: UnsafeCell<libc::pthread_mutex_t>,
cvar: UnsafeCell<libc::pthread_cond_t>,
// The `pthread` primitives require a stable address, so make this struct `!Unpin`.
_pinned: PhantomPinned,
}
impl Parker {
/// Construct the UNIX parker in-place.
///
/// # Safety
/// The constructed parker must never be moved.
pub unsafe fn new(parker: *mut Parker) {
// Use the default mutex implementation to allow for simpler initialization.
// This could lead to undefined behaviour when deadlocking. This is avoided
// by not deadlocking. Note in particular the unlocking operation before any
// panic, as code after the panic could try to park again.
addr_of_mut!((*parker).state).write(AtomicUsize::new(EMPTY));
addr_of_mut!((*parker).lock).write(UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER));
cfg_if::cfg_if! {
if #[cfg(any(
target_os = "macos",
target_os = "ios",
target_os = "watchos",
target_os = "l4re",
target_os = "android",
target_os = "redox"
))] {
addr_of_mut!((*parker).cvar).write(UnsafeCell::new(libc::PTHREAD_COND_INITIALIZER));
} else if #[cfg(any(target_os = "espidf", target_os = "horizon"))] {
let r = libc::pthread_cond_init(addr_of_mut!((*parker).cvar).cast(), crate::ptr::null());
assert_eq!(r, 0);
} else {
use crate::mem::MaybeUninit;
let mut attr = MaybeUninit::<libc::pthread_condattr_t>::uninit();
let r = libc::pthread_condattr_init(attr.as_mut_ptr());
assert_eq!(r, 0);
let r = libc::pthread_condattr_setclock(attr.as_mut_ptr(), libc::CLOCK_MONOTONIC);
assert_eq!(r, 0);
let r = libc::pthread_cond_init(addr_of_mut!((*parker).cvar).cast(), attr.as_ptr());
assert_eq!(r, 0);
let r = libc::pthread_condattr_destroy(attr.as_mut_ptr());
assert_eq!(r, 0);
}
}
}
// This implementation doesn't require `unsafe`, but other implementations
// may assume this is only called by the thread that owns the Parker.
pub unsafe fn park(self: Pin<&Self>) {
// If we were previously notified then we consume this notification and
// return quickly.
if self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
return;
}
// Otherwise we need to coordinate going to sleep
lock(self.lock.get());
match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
Ok(_) => {}
Err(NOTIFIED) => {
// We must read here, even though we know it will be `NOTIFIED`.
// This is because `unpark` may have been called again since we read
// `NOTIFIED` in the `compare_exchange` above. We must perform an
// acquire operation that synchronizes with that `unpark` to observe
// any writes it made before the call to unpark. To do that we must
// read from the write it made to `state`.
let old = self.state.swap(EMPTY, SeqCst);
unlock(self.lock.get());
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
} // should consume this notification, so prohibit spurious wakeups in next park.
Err(_) => {
unlock(self.lock.get());
panic!("inconsistent park state")
}
}
loop {
wait(self.cvar.get(), self.lock.get());
match self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) {
Ok(_) => break, // got a notification
Err(_) => {} // spurious wakeup, go back to sleep
}
}
unlock(self.lock.get());
}
// This implementation doesn't require `unsafe`, but other implementations
// may assume this is only called by the thread that owns the Parker. Use
// `Pin` to guarantee a stable address for the mutex and condition variable.
pub unsafe fn park_timeout(self: Pin<&Self>, dur: Duration) {
// Like `park` above we have a fast path for an already-notified thread, and
// afterwards we start coordinating for a sleep.
// return quickly.
if self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
return;
}
lock(self.lock.get());
match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
Ok(_) => {}
Err(NOTIFIED) => {
// We must read again here, see `park`.
let old = self.state.swap(EMPTY, SeqCst);
unlock(self.lock.get());
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
} // should consume this notification, so prohibit spurious wakeups in next park.
Err(_) => {
unlock(self.lock.get());
panic!("inconsistent park_timeout state")
}
}
// Wait with a timeout, and if we spuriously wake up or otherwise wake up
// from a notification we just want to unconditionally set the state back to
// empty, either consuming a notification or un-flagging ourselves as
// parked.
wait_timeout(self.cvar.get(), self.lock.get(), dur);
match self.state.swap(EMPTY, SeqCst) {
NOTIFIED => unlock(self.lock.get()), // got a notification, hurray!
PARKED => unlock(self.lock.get()), // no notification, alas
n => {
unlock(self.lock.get());
panic!("inconsistent park_timeout state: {n}")
}
}
}
pub fn unpark(self: Pin<&Self>) {
// To ensure the unparked thread will observe any writes we made
// before this call, we must perform a release operation that `park`
// can synchronize with. To do that we must write `NOTIFIED` even if
// `state` is already `NOTIFIED`. That is why this must be a swap
// rather than a compare-and-swap that returns if it reads `NOTIFIED`
// on failure.
match self.state.swap(NOTIFIED, SeqCst) {
EMPTY => return, // no one was waiting
NOTIFIED => return, // already unparked
PARKED => {} // gotta go wake someone up
_ => panic!("inconsistent state in unpark"),
}
// There is a period between when the parked thread sets `state` to
// `PARKED` (or last checked `state` in the case of a spurious wake
// up) and when it actually waits on `cvar`. If we were to notify
// during this period it would be ignored and then when the parked
// thread went to sleep it would never wake up. Fortunately, it has
// `lock` locked at this stage so we can acquire `lock` to wait until
// it is ready to receive the notification.
//
// Releasing `lock` before the call to `notify_one` means that when the
// parked thread wakes it doesn't get woken only to have to wait for us
// to release `lock`.
unsafe {
lock(self.lock.get());
unlock(self.lock.get());
notify_one(self.cvar.get());
}
}
}
impl Drop for Parker {
fn drop(&mut self) {
unsafe {
libc::pthread_cond_destroy(self.cvar.get_mut());
libc::pthread_mutex_destroy(self.lock.get_mut());
}
}
}
unsafe impl Sync for Parker {}
unsafe impl Send for Parker {}