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Diffstat (limited to 'third_party/rust/crossbeam-epoch/src/guard.rs')
| -rw-r--r-- | third_party/rust/crossbeam-epoch/src/guard.rs | 520 |
1 files changed, 520 insertions, 0 deletions
diff --git a/third_party/rust/crossbeam-epoch/src/guard.rs b/third_party/rust/crossbeam-epoch/src/guard.rs new file mode 100644 index 0000000000..ba7fe1b119 --- /dev/null +++ b/third_party/rust/crossbeam-epoch/src/guard.rs @@ -0,0 +1,520 @@ +use core::fmt; +use core::mem; + +use scopeguard::defer; + +use crate::atomic::Shared; +use crate::collector::Collector; +use crate::deferred::Deferred; +use crate::internal::Local; + +/// A guard that keeps the current thread pinned. +/// +/// # Pinning +/// +/// The current thread is pinned by calling [`pin`], which returns a new guard: +/// +/// ``` +/// use crossbeam_epoch as epoch; +/// +/// // It is often convenient to prefix a call to `pin` with a `&` in order to create a reference. +/// // This is not really necessary, but makes passing references to the guard a bit easier. +/// let guard = &epoch::pin(); +/// ``` +/// +/// When a guard gets dropped, the current thread is automatically unpinned. +/// +/// # Pointers on the stack +/// +/// Having a guard allows us to create pointers on the stack to heap-allocated objects. +/// For example: +/// +/// ``` +/// use crossbeam_epoch::{self as epoch, Atomic}; +/// use std::sync::atomic::Ordering::SeqCst; +/// +/// // Create a heap-allocated number. +/// let a = Atomic::new(777); +/// +/// // Pin the current thread. +/// let guard = &epoch::pin(); +/// +/// // Load the heap-allocated object and create pointer `p` on the stack. +/// let p = a.load(SeqCst, guard); +/// +/// // Dereference the pointer and print the value: +/// if let Some(num) = unsafe { p.as_ref() } { +/// println!("The number is {}.", num); +/// } +/// # unsafe { drop(a.into_owned()); } // avoid leak +/// ``` +/// +/// # Multiple guards +/// +/// Pinning is reentrant and it is perfectly legal to create multiple guards. In that case, the +/// thread will actually be pinned only when the first guard is created and unpinned when the last +/// one is dropped: +/// +/// ``` +/// use crossbeam_epoch as epoch; +/// +/// let guard1 = epoch::pin(); +/// let guard2 = epoch::pin(); +/// assert!(epoch::is_pinned()); +/// drop(guard1); +/// assert!(epoch::is_pinned()); +/// drop(guard2); +/// assert!(!epoch::is_pinned()); +/// ``` +/// +/// [`pin`]: super::pin +pub struct Guard { + pub(crate) local: *const Local, +} + +impl Guard { + /// Stores a function so that it can be executed at some point after all currently pinned + /// threads get unpinned. + /// + /// This method first stores `f` into the thread-local (or handle-local) cache. If this cache + /// becomes full, some functions are moved into the global cache. At the same time, some + /// functions from both local and global caches may get executed in order to incrementally + /// clean up the caches as they fill up. + /// + /// There is no guarantee when exactly `f` will be executed. The only guarantee is that it + /// won't be executed until all currently pinned threads get unpinned. In theory, `f` might + /// never run, but the epoch-based garbage collection will make an effort to execute it + /// reasonably soon. + /// + /// If this method is called from an [`unprotected`] guard, the function will simply be + /// executed immediately. + pub fn defer<F, R>(&self, f: F) + where + F: FnOnce() -> R, + F: Send + 'static, + { + unsafe { + self.defer_unchecked(f); + } + } + + /// Stores a function so that it can be executed at some point after all currently pinned + /// threads get unpinned. + /// + /// This method first stores `f` into the thread-local (or handle-local) cache. If this cache + /// becomes full, some functions are moved into the global cache. At the same time, some + /// functions from both local and global caches may get executed in order to incrementally + /// clean up the caches as they fill up. + /// + /// There is no guarantee when exactly `f` will be executed. The only guarantee is that it + /// won't be executed until all currently pinned threads get unpinned. In theory, `f` might + /// never run, but the epoch-based garbage collection will make an effort to execute it + /// reasonably soon. + /// + /// If this method is called from an [`unprotected`] guard, the function will simply be + /// executed immediately. + /// + /// # Safety + /// + /// The given function must not hold reference onto the stack. It is highly recommended that + /// the passed function is **always** marked with `move` in order to prevent accidental + /// borrows. + /// + /// ``` + /// use crossbeam_epoch as epoch; + /// + /// let guard = &epoch::pin(); + /// let message = "Hello!"; + /// unsafe { + /// // ALWAYS use `move` when sending a closure into `defer_unchecked`. + /// guard.defer_unchecked(move || { + /// println!("{}", message); + /// }); + /// } + /// ``` + /// + /// Apart from that, keep in mind that another thread may execute `f`, so anything accessed by + /// the closure must be `Send`. + /// + /// We intentionally didn't require `F: Send`, because Rust's type systems usually cannot prove + /// `F: Send` for typical use cases. For example, consider the following code snippet, which + /// exemplifies the typical use case of deferring the deallocation of a shared reference: + /// + /// ```ignore + /// let shared = Owned::new(7i32).into_shared(guard); + /// guard.defer_unchecked(move || shared.into_owned()); // `Shared` is not `Send`! + /// ``` + /// + /// While `Shared` is not `Send`, it's safe for another thread to call the deferred function, + /// because it's called only after the grace period and `shared` is no longer shared with other + /// threads. But we don't expect type systems to prove this. + /// + /// # Examples + /// + /// When a heap-allocated object in a data structure becomes unreachable, it has to be + /// deallocated. However, the current thread and other threads may be still holding references + /// on the stack to that same object. Therefore it cannot be deallocated before those references + /// get dropped. This method can defer deallocation until all those threads get unpinned and + /// consequently drop all their references on the stack. + /// + /// ``` + /// use crossbeam_epoch::{self as epoch, Atomic, Owned}; + /// use std::sync::atomic::Ordering::SeqCst; + /// + /// let a = Atomic::new("foo"); + /// + /// // Now suppose that `a` is shared among multiple threads and concurrently + /// // accessed and modified... + /// + /// // Pin the current thread. + /// let guard = &epoch::pin(); + /// + /// // Steal the object currently stored in `a` and swap it with another one. + /// let p = a.swap(Owned::new("bar").into_shared(guard), SeqCst, guard); + /// + /// if !p.is_null() { + /// // The object `p` is pointing to is now unreachable. + /// // Defer its deallocation until all currently pinned threads get unpinned. + /// unsafe { + /// // ALWAYS use `move` when sending a closure into `defer_unchecked`. + /// guard.defer_unchecked(move || { + /// println!("{} is now being deallocated.", p.deref()); + /// // Now we have unique access to the object pointed to by `p` and can turn it + /// // into an `Owned`. Dropping the `Owned` will deallocate the object. + /// drop(p.into_owned()); + /// }); + /// } + /// } + /// # unsafe { drop(a.into_owned()); } // avoid leak + /// ``` + pub unsafe fn defer_unchecked<F, R>(&self, f: F) + where + F: FnOnce() -> R, + { + if let Some(local) = self.local.as_ref() { + local.defer(Deferred::new(move || drop(f())), self); + } else { + drop(f()); + } + } + + /// Stores a destructor for an object so that it can be deallocated and dropped at some point + /// after all currently pinned threads get unpinned. + /// + /// This method first stores the destructor into the thread-local (or handle-local) cache. If + /// this cache becomes full, some destructors are moved into the global cache. At the same + /// time, some destructors from both local and global caches may get executed in order to + /// incrementally clean up the caches as they fill up. + /// + /// There is no guarantee when exactly the destructor will be executed. The only guarantee is + /// that it won't be executed until all currently pinned threads get unpinned. In theory, the + /// destructor might never run, but the epoch-based garbage collection will make an effort to + /// execute it reasonably soon. + /// + /// If this method is called from an [`unprotected`] guard, the destructor will simply be + /// executed immediately. + /// + /// # Safety + /// + /// The object must not be reachable by other threads anymore, otherwise it might be still in + /// use when the destructor runs. + /// + /// Apart from that, keep in mind that another thread may execute the destructor, so the object + /// must be sendable to other threads. + /// + /// We intentionally didn't require `T: Send`, because Rust's type systems usually cannot prove + /// `T: Send` for typical use cases. For example, consider the following code snippet, which + /// exemplifies the typical use case of deferring the deallocation of a shared reference: + /// + /// ```ignore + /// let shared = Owned::new(7i32).into_shared(guard); + /// guard.defer_destroy(shared); // `Shared` is not `Send`! + /// ``` + /// + /// While `Shared` is not `Send`, it's safe for another thread to call the destructor, because + /// it's called only after the grace period and `shared` is no longer shared with other + /// threads. But we don't expect type systems to prove this. + /// + /// # Examples + /// + /// When a heap-allocated object in a data structure becomes unreachable, it has to be + /// deallocated. However, the current thread and other threads may be still holding references + /// on the stack to that same object. Therefore it cannot be deallocated before those references + /// get dropped. This method can defer deallocation until all those threads get unpinned and + /// consequently drop all their references on the stack. + /// + /// ``` + /// use crossbeam_epoch::{self as epoch, Atomic, Owned}; + /// use std::sync::atomic::Ordering::SeqCst; + /// + /// let a = Atomic::new("foo"); + /// + /// // Now suppose that `a` is shared among multiple threads and concurrently + /// // accessed and modified... + /// + /// // Pin the current thread. + /// let guard = &epoch::pin(); + /// + /// // Steal the object currently stored in `a` and swap it with another one. + /// let p = a.swap(Owned::new("bar").into_shared(guard), SeqCst, guard); + /// + /// if !p.is_null() { + /// // The object `p` is pointing to is now unreachable. + /// // Defer its deallocation until all currently pinned threads get unpinned. + /// unsafe { + /// guard.defer_destroy(p); + /// } + /// } + /// # unsafe { drop(a.into_owned()); } // avoid leak + /// ``` + pub unsafe fn defer_destroy<T>(&self, ptr: Shared<'_, T>) { + self.defer_unchecked(move || ptr.into_owned()); + } + + /// Clears up the thread-local cache of deferred functions by executing them or moving into the + /// global cache. + /// + /// Call this method after deferring execution of a function if you want to get it executed as + /// soon as possible. Flushing will make sure it is residing in in the global cache, so that + /// any thread has a chance of taking the function and executing it. + /// + /// If this method is called from an [`unprotected`] guard, it is a no-op (nothing happens). + /// + /// # Examples + /// + /// ``` + /// use crossbeam_epoch as epoch; + /// + /// let guard = &epoch::pin(); + /// guard.defer(move || { + /// println!("This better be printed as soon as possible!"); + /// }); + /// guard.flush(); + /// ``` + pub fn flush(&self) { + if let Some(local) = unsafe { self.local.as_ref() } { + local.flush(self); + } + } + + /// Unpins and then immediately re-pins the thread. + /// + /// This method is useful when you don't want delay the advancement of the global epoch by + /// holding an old epoch. For safety, you should not maintain any guard-based reference across + /// the call (the latter is enforced by `&mut self`). The thread will only be repinned if this + /// is the only active guard for the current thread. + /// + /// If this method is called from an [`unprotected`] guard, then the call will be just no-op. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_epoch::{self as epoch, Atomic}; + /// use std::sync::atomic::Ordering::SeqCst; + /// + /// let a = Atomic::new(777); + /// let mut guard = epoch::pin(); + /// { + /// let p = a.load(SeqCst, &guard); + /// assert_eq!(unsafe { p.as_ref() }, Some(&777)); + /// } + /// guard.repin(); + /// { + /// let p = a.load(SeqCst, &guard); + /// assert_eq!(unsafe { p.as_ref() }, Some(&777)); + /// } + /// # unsafe { drop(a.into_owned()); } // avoid leak + /// ``` + pub fn repin(&mut self) { + if let Some(local) = unsafe { self.local.as_ref() } { + local.repin(); + } + } + + /// Temporarily unpins the thread, executes the given function and then re-pins the thread. + /// + /// This method is useful when you need to perform a long-running operation (e.g. sleeping) + /// and don't need to maintain any guard-based reference across the call (the latter is enforced + /// by `&mut self`). The thread will only be unpinned if this is the only active guard for the + /// current thread. + /// + /// If this method is called from an [`unprotected`] guard, then the passed function is called + /// directly without unpinning the thread. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_epoch::{self as epoch, Atomic}; + /// use std::sync::atomic::Ordering::SeqCst; + /// use std::thread; + /// use std::time::Duration; + /// + /// let a = Atomic::new(777); + /// let mut guard = epoch::pin(); + /// { + /// let p = a.load(SeqCst, &guard); + /// assert_eq!(unsafe { p.as_ref() }, Some(&777)); + /// } + /// guard.repin_after(|| thread::sleep(Duration::from_millis(50))); + /// { + /// let p = a.load(SeqCst, &guard); + /// assert_eq!(unsafe { p.as_ref() }, Some(&777)); + /// } + /// # unsafe { drop(a.into_owned()); } // avoid leak + /// ``` + pub fn repin_after<F, R>(&mut self, f: F) -> R + where + F: FnOnce() -> R, + { + if let Some(local) = unsafe { self.local.as_ref() } { + // We need to acquire a handle here to ensure the Local doesn't + // disappear from under us. + local.acquire_handle(); + local.unpin(); + } + + // Ensure the Guard is re-pinned even if the function panics + defer! { + if let Some(local) = unsafe { self.local.as_ref() } { + mem::forget(local.pin()); + local.release_handle(); + } + } + + f() + } + + /// Returns the `Collector` associated with this guard. + /// + /// This method is useful when you need to ensure that all guards used with + /// a data structure come from the same collector. + /// + /// If this method is called from an [`unprotected`] guard, then `None` is returned. + /// + /// # Examples + /// + /// ``` + /// use crossbeam_epoch as epoch; + /// + /// let guard1 = epoch::pin(); + /// let guard2 = epoch::pin(); + /// assert!(guard1.collector() == guard2.collector()); + /// ``` + pub fn collector(&self) -> Option<&Collector> { + unsafe { self.local.as_ref().map(|local| local.collector()) } + } +} + +impl Drop for Guard { + #[inline] + fn drop(&mut self) { + if let Some(local) = unsafe { self.local.as_ref() } { + local.unpin(); + } + } +} + +impl fmt::Debug for Guard { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + f.pad("Guard { .. }") + } +} + +/// Returns a reference to a dummy guard that allows unprotected access to [`Atomic`]s. +/// +/// This guard should be used in special occasions only. Note that it doesn't actually keep any +/// thread pinned - it's just a fake guard that allows loading from [`Atomic`]s unsafely. +/// +/// Note that calling [`defer`] with a dummy guard will not defer the function - it will just +/// execute the function immediately. +/// +/// If necessary, it's possible to create more dummy guards by cloning: `unprotected().clone()`. +/// +/// # 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 crossbeam_epoch::{self as epoch, Atomic}; +/// use std::sync::atomic::Ordering::Relaxed; +/// +/// let a = Atomic::new(7); +/// +/// unsafe { +/// // Load `a` without pinning the current thread. +/// a.load(Relaxed, epoch::unprotected()); +/// +/// // It's possible to create more dummy guards by calling `clone()`. +/// let dummy = &epoch::unprotected().clone(); +/// +/// dummy.defer(move || { +/// println!("This gets executed immediately."); +/// }); +/// +/// // Dropping `dummy` doesn't affect the current thread - it's just a noop. +/// } +/// # unsafe { drop(a.into_owned()); } // avoid leak +/// ``` +/// +/// The most common use of this function is when constructing or destructing a data structure. +/// +/// For example, we can use a dummy guard in the destructor of a Treiber stack because at that +/// point no other thread could concurrently modify the [`Atomic`]s we are accessing. +/// +/// If we were to actually pin the current thread during destruction, that would just unnecessarily +/// delay garbage collection and incur some performance cost, so in cases like these `unprotected` +/// is very helpful. +/// +/// ``` +/// use crossbeam_epoch::{self as epoch, Atomic}; +/// use std::mem::ManuallyDrop; +/// use std::sync::atomic::Ordering::Relaxed; +/// +/// struct Stack<T> { +/// head: Atomic<Node<T>>, +/// } +/// +/// struct Node<T> { +/// data: ManuallyDrop<T>, +/// next: Atomic<Node<T>>, +/// } +/// +/// impl<T> Drop for Stack<T> { +/// fn drop(&mut self) { +/// unsafe { +/// // Unprotected load. +/// let mut node = self.head.load(Relaxed, epoch::unprotected()); +/// +/// while let Some(n) = node.as_ref() { +/// // Unprotected load. +/// let next = n.next.load(Relaxed, epoch::unprotected()); +/// +/// // Take ownership of the node, then drop its data and deallocate it. +/// let mut o = node.into_owned(); +/// ManuallyDrop::drop(&mut o.data); +/// drop(o); +/// +/// node = next; +/// } +/// } +/// } +/// } +/// ``` +/// +/// [`Atomic`]: super::Atomic +/// [`defer`]: Guard::defer +#[inline] +pub unsafe fn unprotected() -> &'static Guard { + // An unprotected guard is just a `Guard` with its field `local` set to null. + // We make a newtype over `Guard` because `Guard` isn't `Sync`, so can't be directly stored in + // a `static` + struct GuardWrapper(Guard); + unsafe impl Sync for GuardWrapper {} + static UNPROTECTED: GuardWrapper = GuardWrapper(Guard { + local: core::ptr::null(), + }); + &UNPROTECTED.0 +} |