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