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Add fast futex-based thread parker for Linux.

This commit is contained in:
Mara Bos 2020-09-19 14:54:21 +02:00
parent 1464fc3a0c
commit ec13df4ec4
3 changed files with 231 additions and 123 deletions
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119
library/std/src/thread/parker/generic.rs Normal file
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@ -0,0 +1,119 @@
//! Parker implementaiton based on a Mutex and Condvar.
use crate::sync::atomic::AtomicUsize;
use crate::sync::atomic::Ordering::SeqCst;
use crate::sync::{Condvar, Mutex};
use crate::time::Duration;
const EMPTY: usize = 0;
const PARKED: usize = 1;
const NOTIFIED: usize = 2;
pub struct Parker {
state: AtomicUsize,
lock: Mutex<()>,
cvar: Condvar,
}
impl Parker {
pub fn new() -> Self {
Parker { state: AtomicUsize::new(EMPTY), lock: Mutex::new(()), cvar: Condvar::new() }
}
// This implementaiton 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) {
// 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
let mut m = self.lock.lock().unwrap();
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);
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
} // should consume this notification, so prohibit spurious wakeups in next park.
Err(_) => panic!("inconsistent park state"),
}
loop {
m = self.cvar.wait(m).unwrap();
match self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) {
Ok(_) => return, // got a notification
Err(_) => {} // spurious wakeup, go back to sleep
}
}
}
// This implementaiton doesn't require `unsafe`, but other implementations
// may assume this is only called by the thread that owns the Parker.
pub unsafe fn park_timeout(&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;
}
let m = self.lock.lock().unwrap();
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);
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
} // should consume this notification, so prohibit spurious wakeups in next park.
Err(_) => 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.
let (_m, _result) = self.cvar.wait_timeout(m, dur).unwrap();
match self.state.swap(EMPTY, SeqCst) {
NOTIFIED => {} // got a notification, hurray!
PARKED => {} // no notification, alas
n => panic!("inconsistent park_timeout state: {}", n),
}
}
pub fn unpark(&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`.
drop(self.lock.lock().unwrap());
self.cvar.notify_one()
}
}

105
library/std/src/thread/parker/linux.rs Normal file
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use crate::sync::atomic::AtomicI32;
use crate::sync::atomic::Ordering::{Acquire, Relaxed, Release};
use crate::time::Duration;
const PARKED: i32 = -1;
const EMPTY: i32 = 0;
const NOTIFIED: i32 = 1;
pub struct Parker {
state: AtomicI32,
}
impl Parker {
#[inline]
pub const fn new() -> Self {
Parker { state: AtomicI32::new(EMPTY) }
}
// Assumes this is only called by the thread that owns the Parker,
// which means that `self.state != PARKED`.
pub unsafe fn park(&self) {
// Change NOTIFIED=>EMPTY or EMPTY=>PARKED, and directly return in the
// first case.
if self.state.fetch_sub(1, Acquire) == NOTIFIED {
return;
}
loop {
// Wait for something to happen, assuming it's still set to PARKED.
futex_wait(&self.state, PARKED, None);
// Change NOTIFIED=>EMPTY and return in that case.
if self.state.compare_and_swap(NOTIFIED, EMPTY, Acquire) == NOTIFIED {
return;
} else {
// Spurious wake up. We loop to try again.
}
}
}
// Assumes this is only called by the thread that owns the Parker,
// which means that `self.state != PARKED`.
pub unsafe fn park_timeout(&self, timeout: Duration) {
// Change NOTIFIED=>EMPTY or EMPTY=>PARKED, and directly return in the
// first case.
if self.state.fetch_sub(1, Acquire) == NOTIFIED {
return;
}
// Wait for something to happen, assuming it's still set to PARKED.
futex_wait(&self.state, PARKED, Some(timeout));
// This is not just a store, because we need to establish a
// release-acquire ordering with unpark().
if self.state.swap(EMPTY, Acquire) == NOTIFIED {
// Woke up because of unpark().
} else {
// Timeout or spurious wake up.
// We return either way, because we can't easily tell if it was the
// timeout or not.
}
}
pub fn unpark(&self) {
// Change PARKED=>NOTIFIED, EMPTY=>NOTIFIED, or NOTIFIED=>NOTIFIED, and
// wake the thread in the first case.
//
// Note that even NOTIFIED=>NOTIFIED results in a write. This is on
// purpose, to make sure every unpark() has a release-acquire ordering
// with park().
if self.state.swap(NOTIFIED, Release) == PARKED {
futex_wake(&self.state);
}
}
}
fn futex_wait(futex: &AtomicI32, expected: i32, timeout: Option<Duration>) {
let timespec;
let timespec_ptr = match timeout {
Some(timeout) => {
timespec = libc::timespec {
tv_sec: timeout.as_secs() as _,
tv_nsec: timeout.subsec_nanos() as _,
};
&timespec as *const libc::timespec
}
None => crate::ptr::null(),
};
unsafe {
libc::syscall(
libc::SYS_futex,
futex as *const AtomicI32,
libc::FUTEX_WAIT | libc::FUTEX_PRIVATE_FLAG,
expected,
timespec_ptr,
);
}
}
fn futex_wake(futex: &AtomicI32) {
unsafe {
libc::syscall(
libc::SYS_futex,
futex as *const AtomicI32,
libc::FUTEX_WAKE | libc::FUTEX_PRIVATE_FLAG,
1,
);
}
}

130
library/std/src/thread/parker/mod.rs
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@ -1,125 +1,9 @@
//! Parker implementaiton based on a Mutex and Condvar.
//!
//! The implementation currently uses the trivial strategy of a Mutex+Condvar
//! with wakeup flag, which does not actually allow spurious wakeups. In the
//! future, this will be implemented in a more efficient way, perhaps along the lines of
//! http://cr.openjdk.java.net/~stefank/6989984.1/raw_files/new/src/os/linux/vm/os_linux.cpp
//! or futuxes, and in either case may allow spurious wakeups.
use crate::sync::atomic::AtomicUsize;
use crate::sync::atomic::Ordering::SeqCst;
use crate::sync::{Condvar, Mutex};
use crate::time::Duration;
const EMPTY: usize = 0;
const PARKED: usize = 1;
const NOTIFIED: usize = 2;
pub struct Parker {
state: AtomicUsize,
lock: Mutex<()>,
cvar: Condvar,
}
impl Parker {
pub fn new() -> Self {
Parker { state: AtomicUsize::new(EMPTY), lock: Mutex::new(()), cvar: Condvar::new() }
}
// This implementaiton 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) {
// 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
let mut m = self.lock.lock().unwrap();
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);
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
} // should consume this notification, so prohibit spurious wakeups in next park.
Err(_) => panic!("inconsistent park state"),
}
loop {
m = self.cvar.wait(m).unwrap();
match self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) {
Ok(_) => return, // got a notification
Err(_) => {} // spurious wakeup, go back to sleep
}
}
}
// This implementaiton doesn't require `unsafe`, but other implementations
// may assume this is only called by the thread that owns the Parker.
pub unsafe fn park_timeout(&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;
}
let m = self.lock.lock().unwrap();
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);
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
} // should consume this notification, so prohibit spurious wakeups in next park.
Err(_) => 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.
let (_m, _result) = self.cvar.wait_timeout(m, dur).unwrap();
match self.state.swap(EMPTY, SeqCst) {
NOTIFIED => {} // got a notification, hurray!
PARKED => {} // no notification, alas
n => panic!("inconsistent park_timeout state: {}", n),
}
}
pub fn unpark(&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`.
drop(self.lock.lock().unwrap());
self.cvar.notify_one()
cfg_if::cfg_if! {
if #[cfg(any(target_os = "linux", target_os = "android"))] {
mod linux;
pub use linux::Parker;
} else {
mod generic;
pub use generic::Parker;
}
}