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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
}