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Move thread parker to a separate module.

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This commit is contained in:
Mara Bos 2020-09-19 12:42:14 +02:00
parent c9e5e6a53a
commit 1464fc3a0c
2 changed files with 142 additions and 112 deletions
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129
library/std/src/thread/mod.rs
View File

@ -149,6 +149,8 @@
#[cfg(all(test, not(target_os = "emscripten")))]
mod tests;
mod parker;
use crate::any::Any;
use crate::cell::UnsafeCell;
use crate::ffi::{CStr, CString};
@ -159,15 +161,14 @@
use crate::panic;
use crate::panicking;
use crate::str;
use crate::sync::atomic::AtomicUsize;
use crate::sync::atomic::Ordering::SeqCst;
use crate::sync::{Arc, Condvar, Mutex};
use crate::sync::Arc;
use crate::sys::thread as imp;
use crate::sys_common::mutex;
use crate::sys_common::thread;
use crate::sys_common::thread_info;
use crate::sys_common::{AsInner, IntoInner};
use crate::time::Duration;
use parker::Parker;
////////////////////////////////////////////////////////////////////////////////
// Thread-local storage
@ -667,6 +668,8 @@ pub fn current() -> Thread {
///
/// [`channel`]: crate::sync::mpsc
/// [`join`]: JoinHandle::join
/// [`Condvar`]: crate::sync::Condvar
/// [`Mutex`]: crate::sync::Mutex
#[stable(feature = "rust1", since = "1.0.0")]
pub fn yield_now() {
imp::Thread::yield_now()
@ -712,6 +715,8 @@ pub fn yield_now() {
/// panic!()
/// }
/// ```
///
/// [Mutex]: crate::sync::Mutex
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn panicking() -> bool {
@ -779,11 +784,6 @@ pub fn sleep(dur: Duration) {
imp::Thread::sleep(dur)
}
// constants for park/unpark
const EMPTY: usize = 0;
const PARKED: usize = 1;
const NOTIFIED: usize = 2;
/// Blocks unless or until the current thread's token is made available.
///
/// A call to `park` does not guarantee that the thread will remain parked
@ -870,45 +870,11 @@ pub fn sleep(dur: Duration) {
///
/// [`unpark`]: Thread::unpark
/// [`thread::park_timeout`]: park_timeout
//
// 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.
#[stable(feature = "rust1", since = "1.0.0")]
pub fn park() {
let thread = current();
// If we were previously notified then we consume this notification and
// return quickly.
if thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
return;
}
// Otherwise we need to coordinate going to sleep
let mut m = thread.inner.lock.lock().unwrap();
match thread.inner.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 = thread.inner.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 = thread.inner.cvar.wait(m).unwrap();
match thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) {
Ok(_) => return, // got a notification
Err(_) => {} // spurious wakeup, go back to sleep
}
// SAFETY: park_timeout is called on the parker owned by this thread.
unsafe {
current().inner.parker.park();
}
}
@ -970,35 +936,9 @@ pub fn park_timeout_ms(ms: u32) {
/// ```
#[stable(feature = "park_timeout", since = "1.4.0")]
pub fn park_timeout(dur: Duration) {
let thread = current();
// Like `park` above we have a fast path for an already-notified thread, and
// afterwards we start coordinating for a sleep.
// return quickly.
if thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
return;
}
let m = thread.inner.lock.lock().unwrap();
match thread.inner.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
Ok(_) => {}
Err(NOTIFIED) => {
// We must read again here, see `park`.
let old = thread.inner.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) = thread.inner.cvar.wait_timeout(m, dur).unwrap();
match thread.inner.state.swap(EMPTY, SeqCst) {
NOTIFIED => {} // got a notification, hurray!
PARKED => {} // no notification, alas
n => panic!("inconsistent park_timeout state: {}", n),
// SAFETY: park_timeout is called on the parker owned by this thread.
unsafe {
current().inner.parker.park_timeout(dur);
}
}
@ -1077,11 +1017,7 @@ pub fn as_u64(&self) -> NonZeroU64 {
struct Inner {
name: Option<CString>, // Guaranteed to be UTF-8
id: ThreadId,
// state for thread park/unpark
state: AtomicUsize,
lock: Mutex<()>,
cvar: Condvar,
parker: Parker,
}
#[derive(Clone)]
@ -1115,13 +1051,7 @@ pub(crate) fn new(name: Option<String>) -> Thread {
let cname =
name.map(|n| CString::new(n).expect("thread name may not contain interior null bytes"));
Thread {
inner: Arc::new(Inner {
name: cname,
id: ThreadId::new(),
state: AtomicUsize::new(EMPTY),
lock: Mutex::new(()),
cvar: Condvar::new(),
}),
inner: Arc::new(Inner { name: cname, id: ThreadId::new(), parker: Parker::new() }),
}
}
@ -1157,32 +1087,7 @@ pub(crate) fn new(name: Option<String>) -> Thread {
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
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.inner.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.inner.lock.lock().unwrap());
self.inner.cvar.notify_one()
self.inner.parker.unpark();
}
/// Gets the thread's unique identifier.

125
library/std/src/thread/parker/mod.rs Normal file
View File

@ -0,0 +1,125 @@
//! 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()
}
}