From 698f8c2f01ea549d77d7dc3338a12e04c11057b9 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Wed, 17 Apr 2024 14:02:58 +0200 Subject: Adding upstream version 1.64.0+dfsg1. Signed-off-by: Daniel Baumann --- vendor/crossbeam-utils/src/backoff.rs | 294 ++++++++++++++++++++++++++++++++++ 1 file changed, 294 insertions(+) create mode 100644 vendor/crossbeam-utils/src/backoff.rs (limited to 'vendor/crossbeam-utils/src/backoff.rs') diff --git a/vendor/crossbeam-utils/src/backoff.rs b/vendor/crossbeam-utils/src/backoff.rs new file mode 100644 index 000000000..1012f06b2 --- /dev/null +++ b/vendor/crossbeam-utils/src/backoff.rs @@ -0,0 +1,294 @@ +use crate::primitive::sync::atomic; +use core::cell::Cell; +use core::fmt; + +const SPIN_LIMIT: u32 = 6; +const YIELD_LIMIT: u32 = 10; + +/// Performs exponential backoff in spin loops. +/// +/// Backing off in spin loops reduces contention and improves overall performance. +/// +/// This primitive can execute *YIELD* and *PAUSE* instructions, yield the current thread to the OS +/// scheduler, and tell when is a good time to block the thread using a different synchronization +/// mechanism. Each step of the back off procedure takes roughly twice as long as the previous +/// step. +/// +/// # Examples +/// +/// Backing off in a lock-free loop: +/// +/// ``` +/// use crossbeam_utils::Backoff; +/// use std::sync::atomic::AtomicUsize; +/// use std::sync::atomic::Ordering::SeqCst; +/// +/// fn fetch_mul(a: &AtomicUsize, b: usize) -> usize { +/// let backoff = Backoff::new(); +/// loop { +/// let val = a.load(SeqCst); +/// if a.compare_exchange(val, val.wrapping_mul(b), SeqCst, SeqCst).is_ok() { +/// return val; +/// } +/// backoff.spin(); +/// } +/// } +/// ``` +/// +/// Waiting for an [`AtomicBool`] to become `true`: +/// +/// ``` +/// use crossbeam_utils::Backoff; +/// use std::sync::atomic::AtomicBool; +/// use std::sync::atomic::Ordering::SeqCst; +/// +/// fn spin_wait(ready: &AtomicBool) { +/// let backoff = Backoff::new(); +/// while !ready.load(SeqCst) { +/// backoff.snooze(); +/// } +/// } +/// ``` +/// +/// Waiting for an [`AtomicBool`] to become `true` and parking the thread after a long wait. +/// Note that whoever sets the atomic variable to `true` must notify the parked thread by calling +/// [`unpark()`]: +/// +/// ``` +/// use crossbeam_utils::Backoff; +/// use std::sync::atomic::AtomicBool; +/// use std::sync::atomic::Ordering::SeqCst; +/// use std::thread; +/// +/// fn blocking_wait(ready: &AtomicBool) { +/// let backoff = Backoff::new(); +/// while !ready.load(SeqCst) { +/// if backoff.is_completed() { +/// thread::park(); +/// } else { +/// backoff.snooze(); +/// } +/// } +/// } +/// ``` +/// +/// [`is_completed`]: Backoff::is_completed +/// [`std::thread::park()`]: std::thread::park +/// [`Condvar`]: std::sync::Condvar +/// [`AtomicBool`]: std::sync::atomic::AtomicBool +/// [`unpark()`]: std::thread::Thread::unpark +pub struct Backoff { + step: Cell, +} + +impl Backoff { + /// Creates a new `Backoff`. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_utils::Backoff; + /// + /// let backoff = Backoff::new(); + /// ``` + #[inline] + pub fn new() -> Self { + Backoff { step: Cell::new(0) } + } + + /// Resets the `Backoff`. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_utils::Backoff; + /// + /// let backoff = Backoff::new(); + /// backoff.reset(); + /// ``` + #[inline] + pub fn reset(&self) { + self.step.set(0); + } + + /// Backs off in a lock-free loop. + /// + /// This method should be used when we need to retry an operation because another thread made + /// progress. + /// + /// The processor may yield using the *YIELD* or *PAUSE* instruction. + /// + /// # Examples + /// + /// Backing off in a lock-free loop: + /// + /// ``` + /// use crossbeam_utils::Backoff; + /// use std::sync::atomic::AtomicUsize; + /// use std::sync::atomic::Ordering::SeqCst; + /// + /// fn fetch_mul(a: &AtomicUsize, b: usize) -> usize { + /// let backoff = Backoff::new(); + /// loop { + /// let val = a.load(SeqCst); + /// if a.compare_exchange(val, val.wrapping_mul(b), SeqCst, SeqCst).is_ok() { + /// return val; + /// } + /// backoff.spin(); + /// } + /// } + /// + /// let a = AtomicUsize::new(7); + /// assert_eq!(fetch_mul(&a, 8), 7); + /// assert_eq!(a.load(SeqCst), 56); + /// ``` + #[inline] + pub fn spin(&self) { + for _ in 0..1 << self.step.get().min(SPIN_LIMIT) { + // TODO(taiki-e): once we bump the minimum required Rust version to 1.49+, + // use [`core::hint::spin_loop`] instead. + #[allow(deprecated)] + atomic::spin_loop_hint(); + } + + if self.step.get() <= SPIN_LIMIT { + self.step.set(self.step.get() + 1); + } + } + + /// Backs off in a blocking loop. + /// + /// This method should be used when we need to wait for another thread to make progress. + /// + /// The processor may yield using the *YIELD* or *PAUSE* instruction and the current thread + /// may yield by giving up a timeslice to the OS scheduler. + /// + /// In `#[no_std]` environments, this method is equivalent to [`spin`]. + /// + /// If possible, use [`is_completed`] to check when it is advised to stop using backoff and + /// block the current thread using a different synchronization mechanism instead. + /// + /// [`spin`]: Backoff::spin + /// [`is_completed`]: Backoff::is_completed + /// + /// # Examples + /// + /// Waiting for an [`AtomicBool`] to become `true`: + /// + /// ``` + /// use crossbeam_utils::Backoff; + /// use std::sync::Arc; + /// use std::sync::atomic::AtomicBool; + /// use std::sync::atomic::Ordering::SeqCst; + /// use std::thread; + /// use std::time::Duration; + /// + /// fn spin_wait(ready: &AtomicBool) { + /// let backoff = Backoff::new(); + /// while !ready.load(SeqCst) { + /// backoff.snooze(); + /// } + /// } + /// + /// let ready = Arc::new(AtomicBool::new(false)); + /// let ready2 = ready.clone(); + /// + /// thread::spawn(move || { + /// thread::sleep(Duration::from_millis(100)); + /// ready2.store(true, SeqCst); + /// }); + /// + /// assert_eq!(ready.load(SeqCst), false); + /// spin_wait(&ready); + /// assert_eq!(ready.load(SeqCst), true); + /// ``` + /// + /// [`AtomicBool`]: std::sync::atomic::AtomicBool + #[inline] + pub fn snooze(&self) { + if self.step.get() <= SPIN_LIMIT { + for _ in 0..1 << self.step.get() { + // TODO(taiki-e): once we bump the minimum required Rust version to 1.49+, + // use [`core::hint::spin_loop`] instead. + #[allow(deprecated)] + atomic::spin_loop_hint(); + } + } else { + #[cfg(not(feature = "std"))] + for _ in 0..1 << self.step.get() { + // TODO(taiki-e): once we bump the minimum required Rust version to 1.49+, + // use [`core::hint::spin_loop`] instead. + #[allow(deprecated)] + atomic::spin_loop_hint(); + } + + #[cfg(feature = "std")] + ::std::thread::yield_now(); + } + + if self.step.get() <= YIELD_LIMIT { + self.step.set(self.step.get() + 1); + } + } + + /// Returns `true` if exponential backoff has completed and blocking the thread is advised. + /// + /// # Examples + /// + /// Waiting for an [`AtomicBool`] to become `true` and parking the thread after a long wait: + /// + /// ``` + /// use crossbeam_utils::Backoff; + /// use std::sync::Arc; + /// use std::sync::atomic::AtomicBool; + /// use std::sync::atomic::Ordering::SeqCst; + /// use std::thread; + /// use std::time::Duration; + /// + /// fn blocking_wait(ready: &AtomicBool) { + /// let backoff = Backoff::new(); + /// while !ready.load(SeqCst) { + /// if backoff.is_completed() { + /// thread::park(); + /// } else { + /// backoff.snooze(); + /// } + /// } + /// } + /// + /// let ready = Arc::new(AtomicBool::new(false)); + /// let ready2 = ready.clone(); + /// let waiter = thread::current(); + /// + /// thread::spawn(move || { + /// thread::sleep(Duration::from_millis(100)); + /// ready2.store(true, SeqCst); + /// waiter.unpark(); + /// }); + /// + /// assert_eq!(ready.load(SeqCst), false); + /// blocking_wait(&ready); + /// assert_eq!(ready.load(SeqCst), true); + /// ``` + /// + /// [`AtomicBool`]: std::sync::atomic::AtomicBool + #[inline] + pub fn is_completed(&self) -> bool { + self.step.get() > YIELD_LIMIT + } +} + +impl fmt::Debug for Backoff { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + f.debug_struct("Backoff") + .field("step", &self.step) + .field("is_completed", &self.is_completed()) + .finish() + } +} + +impl Default for Backoff { + fn default() -> Backoff { + Backoff::new() + } +} -- cgit v1.2.3