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use super::global::Global; use super::local::LocalState; use crate::deferred::Deferred; use core::fmt; use core::marker::PhantomData; use std::sync::Arc; /// Universal methods for any shield implementation. pub trait Shield<'a>: Clone + fmt::Debug { /// Attempt to synchronize the current thread to allow advancing the global epoch. /// This might be useful to call every once in a while if you plan on holding a `Shield` /// for an extended amount of time as to not stop garbage collection. /// /// This is only effective if this is the only active shield created by this thread. /// Has no effect when called from an [`unprotected`] shield. /// /// [`unprotected`]: fn.unprotected.html fn repin(&mut self); /// Attempt to synchronize the current thread like `Shield::repin` but executing a closure /// during the time the `Shield` is temporarily deactivated. /// /// If this method is called from an [`unprotected`] shield, the closure will be executed /// immediately without unpinning the thread. /// /// [`unprotected`]: fn.unprotected.html fn repin_after<F, R>(&mut self, f: F) -> R where F: FnOnce() -> R; /// Schedule a closure for execution once no shield may hold a reference /// to an object unlinked with the current shield. /// /// If this method is called from an [`unprotected`] shield, the closure will be executed /// immediately. /// /// [`unprotected`]: fn.unprotected.html fn retire<F>(&self, f: F) where F: FnOnce() + 'a; } /// A `FullShield` is largely equivalent to `ThinShield` in terms of functionality. /// They're both shields with the same guarantees and can be user interchangeably. /// The major difference is that `FullShield` implements `Send` and `Sync` while /// `Shield` does not. `FullShield` is provided for scenarios like asynchronous iteration /// over a datastructure which is a big pain if the iterator isn't `Send`. /// /// The downside to this functionality is that they are much more expensive to create and destroy /// and even more so when multiple threads are creating and destroying them at the same time. /// This is due to the fact that full shields require more bookkeeping to handle the fact /// that they may suddently change locals/threads. /// /// Because said bookkeeping is shared across all threads it may become contented /// and incur speed penalties due to inter-processor synchronization but it will still remain wait-free. /// /// For documentation on functionality please check the documentation of the `Shield` trait. pub struct FullShield<'a> { global: &'a Arc<Global>, } impl<'a> FullShield<'a> { pub(crate) fn new(global: &'a Arc<Global>) -> Self { Self { global } } } impl<'a> Shield<'a> for FullShield<'a> { fn repin(&mut self) { // repinning is fine here since we are taking a mutable reference and // therefore this shield is not used for anything else unsafe { self.global.ct.exit(self.global); self.global.ct.enter(self.global); } } fn repin_after<F, R>(&mut self, f: F) -> R where F: FnOnce() -> R, { // see comment on FullShield::repin unsafe { self.global.ct.exit(self.global); let value = f(); self.global.ct.enter(self.global); value } } fn retire<F>(&self, f: F) where F: FnOnce() + 'a, { let deferred = Deferred::new(f); self.global.retire(deferred, self); } } impl<'a> Clone for FullShield<'a> { fn clone(&self) -> Self { Global::full_shield(self.global) } } impl<'a> Drop for FullShield<'a> { fn drop(&mut self) { // this is okay since we shall have called enter upon construction of this shield object unsafe { self.global.ct.exit(self.global); } } } unsafe impl<'a> Send for FullShield<'a> {} unsafe impl<'a> Sync for FullShield<'a> {} impl<'a> fmt::Debug for FullShield<'a> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.pad("FullShield { .. }") } } /// A `ThinShield` locks an epoch and is needed to manipulate protected atomic pointers. /// It is a type level contract so that you are forces to acquire one before manipulating pointers. /// This reduces common mistakes drastically since incorrect code will now fail at compile time. /// /// For documentation on functionality please check the documentation of the `Shield` trait. pub struct ThinShield<'a> { local_state: &'a LocalState, _m0: PhantomData<*mut ()>, } impl<'a> ThinShield<'a> { pub(crate) fn new(local_state: &'a LocalState) -> Self { Self { local_state, _m0: PhantomData, } } } impl<'a> Shield<'a> for ThinShield<'a> { // see comment on FullShield::repin fn repin(&mut self) { unsafe { self.local_state.exit(); self.local_state.enter(); } } fn repin_after<F, R>(&mut self, f: F) -> R where F: FnOnce() -> R, { // see comment on FullShield::repin unsafe { self.local_state.exit(); let value = f(); self.local_state.enter(); value } } fn retire<F>(&self, f: F) where F: FnOnce() + 'a, { let deferred = Deferred::new(f); self.local_state.retire(deferred, self); } } impl<'a> Clone for ThinShield<'a> { fn clone(&self) -> Self { // since we're creating a new shield we need to also record the creation of it unsafe { self.local_state.enter(); } Self::new(self.local_state) } } impl<'a> Drop for ThinShield<'a> { fn drop(&mut self) { // this is okay since we shall have called enter upon construction of this shield object unsafe { self.local_state.exit(); } } } impl<'a> fmt::Debug for ThinShield<'a> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.pad("ThinShield { .. }") } } /// An `UnprotectedShield` is a shield that does not actually lock an epoch, but can still be used to /// manipulate protected atomic pointers. /// Obtaining an `UnprotectedShield` is unsafe, since it allows unsafe access to atomics, and is only /// possible through [`flize::unprotected`]. /// /// For documentation on functionality please check the documentation of [`flize::unprotected`] and the `Shield` trait. /// /// [`flize::unprotected`]: fn.unprotected.html #[derive(Copy, Clone)] pub struct UnprotectedShield { _private: (), } // Doc tests have `compile_fail`, but regular `#[test]`s do not (at least without additional dependencies). /// ```compile_fail /// let u = flize::UnprotectedShield { _private: () }; /// ``` #[allow(unused)] struct UnprotectedCompileFailTests; impl<'a> Shield<'a> for UnprotectedShield { fn repin(&mut self) {} fn repin_after<F, R>(&mut self, f: F) -> R where F: FnOnce() -> R, { f() } fn retire<F>(&self, f: F) where F: FnOnce() + 'a, { f(); } } impl fmt::Debug for UnprotectedShield { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.pad("UnprotectedShield { .. }") } } /// Returns a reference to a dummy shield that allows unprotected access to [`Atomic`]s. /// /// This shield will not keep any thread pinned, it just allows interacting with [`Atomic`]s /// unsafely. /// Thus, neither calling [`repin`] nor [`repin_after`] on a shield returned from this function will /// actually re-pin the current thread. Calling [`repin_after`] or [`retire`] will execute the /// supplied function immediately. /// /// # Safety /// Loading and dereferencing data from an [`Atomic`] using this guard is safe only if the [`Atomic`] /// is not being concurrently modified by other threads. /// /// # Examples /// ``` /// use flize::{self, Atomic, Shared, Shield}; /// use std::sync::atomic::Ordering::Relaxed; /// use std::mem; /// /// let a = { /// let s: Shared<'_, i32> = unsafe { Shared::from_ptr(Box::into_raw(Box::new(7))) }; /// Atomic::new(s) /// }; /// /// unsafe { /// // Load `a` without pinning the current thread. /// let s = a.load(Relaxed, flize::unprotected()); /// assert_eq!(s.as_ref_unchecked(), &7); /// /// // It is possible to create more unprotected shields with `clone()`. /// let unprotected = &flize::unprotected().clone(); /// /// // Swap `a` with a new value (9) without pinning the current thread. /// let s = Shared::from_ptr(Box::into_raw(Box::new(9))); /// let s = a.swap(s, Relaxed, unprotected); /// assert_eq!(a.load(Relaxed, unprotected).as_ref_unchecked(), &9); /// assert_eq!(s.as_ref_unchecked(), &7); /// /// let ptr = a.load(Relaxed, unprotected).as_ptr(); /// unprotected.retire(move || { /// // This is executed immediately, thus `a` now holds an invalid pointer. /// drop(Box::from_raw(ptr)); /// }); /// /// // Dropping `unprotected` doesn't affect the current thread since it did not pin it. /// } /// ``` /// /// [`Atomic`]: struct.Atomic.html /// [`repin`]: trait.Shield.html#method.repin /// [`repin_after`]: trait.Shield.html#method.repin_after /// [`retire`]: trait.Shield.html#method.retire pub unsafe fn unprotected() -> &'static UnprotectedShield { static UNPROTECTED: UnprotectedShield = UnprotectedShield { _private: () }; &UNPROTECTED } /// This is a utility type that allows you to either take a reference to a shield /// and be bound by the lifetime of it or take an owned shield use `'static`. #[derive(Clone, Debug)] pub enum CowShield<'collector, 'shield, S> where S: Shield<'collector>, { Owned(S, PhantomData<&'collector ()>), Borrowed(&'shield S), } impl<'collector, 'shield, S> CowShield<'collector, 'shield, S> where S: Shield<'collector>, { pub fn new_owned(shield: S) -> Self { CowShield::Owned(shield, PhantomData) } pub fn new_borrowed(shield: &'shield S) -> Self { CowShield::Borrowed(shield) } pub fn into_owned(self) -> S { match self { CowShield::Owned(shield, _) => shield, CowShield::Borrowed(shield) => shield.clone(), } } pub fn get(&self) -> &S { match self { CowShield::Owned(shield, _) => shield, CowShield::Borrowed(shield) => shield, } } }