From 1464fc3a0cb2b8a92c54357fcff7c632b334cb29 Mon Sep 17 00:00:00 2001 From: Mara Bos Date: Sat, 19 Sep 2020 12:42:14 +0200 Subject: [PATCH] Move thread parker to a separate module. --- library/std/src/thread/mod.rs | 129 ++++----------------------- library/std/src/thread/parker/mod.rs | 125 ++++++++++++++++++++++++++ 2 files changed, 142 insertions(+), 112 deletions(-) create mode 100644 library/std/src/thread/parker/mod.rs diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs index 8c353e2484e..a5a8d5c9fbb 100644 --- a/library/std/src/thread/mod.rs +++ b/library/std/src/thread/mod.rs @@ -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::num::NonZeroU64; 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 @@ const NOTIFIED: usize = 2; /// /// [`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 @@ impl ThreadId { struct Inner { name: Option, // 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 @@ impl 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 @@ impl 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. diff --git a/library/std/src/thread/parker/mod.rs b/library/std/src/thread/parker/mod.rs new file mode 100644 index 00000000000..c8b9b7b1c79 --- /dev/null +++ b/library/std/src/thread/parker/mod.rs @@ -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() + } +}