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use core::fmt; use core::ptr; use core::mem; use deferred::Deferred; use internal::Local; use collector::Collector; /// 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, Owned}; /// 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); /// } /// ``` /// /// # 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()); /// ``` /// /// The same can be achieved by cloning guards: /// /// ``` /// use crossbeam_epoch as epoch; /// /// let guard1 = epoch::pin(); /// let guard2 = guard1.clone(); /// ``` /// /// [`pin`]: fn.pin.html 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 won't /// until all currently pinned threads get unpinned. In theory, `f` might never be deallocated, /// 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 `defef`. /// guard.defer(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(Deferred::new(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. /// /// ```rust /// 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`. /// guard.defer(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()); /// }); /// } /// } /// ``` /// /// [`unprotected`]: fn.unprotected.html pub unsafe fn defer<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); } } /// 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(); /// unsafe { /// guard.defer(move || { /// println!("This better be printed as soon as possible!"); /// }); /// } /// guard.flush(); /// ``` /// /// [`unprotected`]: fn.unprotected.html 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; /// 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(); /// { /// let p = a.load(SeqCst, &guard); /// assert_eq!(unsafe { p.as_ref() }, Some(&777)); /// } /// ``` /// /// [`unprotected`]: fn.unprotected.html 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)); /// } /// ``` /// /// [`unprotected`]: fn.unprotected.html 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 mut guard1 = epoch::pin(); /// let mut guard2 = epoch::pin(); /// assert!(guard1.collector() == guard2.collector()); /// ``` /// /// [`unprotected`]: fn.unprotected.html 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 Clone for Guard { #[inline] fn clone(&self) -> Guard { match unsafe { self.local.as_ref() } { None => Guard { local: ptr::null() }, Some(local) => local.pin(), } } } impl fmt::Debug for Guard { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.debug_struct("Guard").finish() } } /// 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. /// } /// ``` /// /// 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`]: struct.Atomic.html /// [`defer`]: struct.Guard.html#method.defer #[inline] pub unsafe fn unprotected() -> &'static Guard { // HACK(stjepang): An unprotected guard is just a `Guard` with its field `local` set to null. // Since this function returns a `'static` reference to a `Guard`, we must return a reference // to a global guard. However, it's not possible to create a `static` `Guard` because it does // not implement `Sync`. To get around the problem, we create a static `usize` initialized to // zero and then transmute it into a `Guard`. This is safe because `usize` and `Guard` // (consisting of a single pointer) have the same representation in memory. static UNPROTECTED: usize = 0; &*(&UNPROTECTED as *const _ as *const Guard) }