vendor/crossbeam-epoch/src/guard.rs

   1 use core::fmt;
   2 use core::mem;
   3
   4 use scopeguard::defer;
   5
   6 use crate::atomic::Shared;
   7 use crate::collector::Collector;
   8 use crate::deferred::Deferred;
   9 use crate::internal::Local;
  10
  11 /// A guard that keeps the current thread pinned.
  12 ///
  13 /// # Pinning
  14 ///
  15 /// The current thread is pinned by calling [`pin`], which returns a new guard:
  16 ///
  17 /// ```
  18 /// use crossbeam_epoch as epoch;
  19 ///
  20 /// // It is often convenient to prefix a call to `pin` with a `&` in order to create a reference.
  21 /// // This is not really necessary, but makes passing references to the guard a bit easier.
  22 /// let guard = &epoch::pin();
  23 /// ```
  24 ///
  25 /// When a guard gets dropped, the current thread is automatically unpinned.
  26 ///
  27 /// # Pointers on the stack
  28 ///
  29 /// Having a guard allows us to create pointers on the stack to heap-allocated objects.
  30 /// For example:
  31 ///
  32 /// ```
  33 /// use crossbeam_epoch::{self as epoch, Atomic};
  34 /// use std::sync::atomic::Ordering::SeqCst;
  35 ///
  36 /// // Create a heap-allocated number.
  37 /// let a = Atomic::new(777);
  38 ///
  39 /// // Pin the current thread.
  40 /// let guard = &epoch::pin();
  41 ///
  42 /// // Load the heap-allocated object and create pointer `p` on the stack.
  43 /// let p = a.load(SeqCst, guard);
  44 ///
  45 /// // Dereference the pointer and print the value:
  46 /// if let Some(num) = unsafe { p.as_ref() } {
  47 ///     println!("The number is {}.", num);
  48 /// }
  49 /// ```
  50 ///
  51 /// # Multiple guards
  52 ///
  53 /// Pinning is reentrant and it is perfectly legal to create multiple guards. In that case, the
  54 /// thread will actually be pinned only when the first guard is created and unpinned when the last
  55 /// one is dropped:
  56 ///
  57 /// ```
  58 /// use crossbeam_epoch as epoch;
  59 ///
  60 /// let guard1 = epoch::pin();
  61 /// let guard2 = epoch::pin();
  62 /// assert!(epoch::is_pinned());
  63 /// drop(guard1);
  64 /// assert!(epoch::is_pinned());
  65 /// drop(guard2);
  66 /// assert!(!epoch::is_pinned());
  67 /// ```
  68 ///
  69 /// [`pin`]: super::pin
  70 pub struct Guard {
  71     pub(crate) local: *const Local,
  72 }
  73
  74 impl Guard {
  75     /// Stores a function so that it can be executed at some point after all currently pinned
  76     /// threads get unpinned.
  77     ///
  78     /// This method first stores `f` into the thread-local (or handle-local) cache. If this cache
  79     /// becomes full, some functions are moved into the global cache. At the same time, some
  80     /// functions from both local and global caches may get executed in order to incrementally
  81     /// clean up the caches as they fill up.
  82     ///
  83     /// There is no guarantee when exactly `f` will be executed. The only guarantee is that it
  84     /// won't be executed until all currently pinned threads get unpinned. In theory, `f` might
  85     /// never run, but the epoch-based garbage collection will make an effort to execute it
  86     /// reasonably soon.
  87     ///
  88     /// If this method is called from an [`unprotected`] guard, the function will simply be
  89     /// executed immediately.
  90     pub fn defer<F, R>(&self, f: F)
  91     where
  92         F: FnOnce() -> R,
  93         F: Send + 'static,
  94     {
  95         unsafe {
  96             self.defer_unchecked(f);
  97         }
  98     }
  99
 100     /// Stores a function so that it can be executed at some point after all currently pinned
 101     /// threads get unpinned.
 102     ///
 103     /// This method first stores `f` into the thread-local (or handle-local) cache. If this cache
 104     /// becomes full, some functions are moved into the global cache. At the same time, some
 105     /// functions from both local and global caches may get executed in order to incrementally
 106     /// clean up the caches as they fill up.
 107     ///
 108     /// There is no guarantee when exactly `f` will be executed. The only guarantee is that it
 109     /// won't be executed until all currently pinned threads get unpinned. In theory, `f` might
 110     /// never run, but the epoch-based garbage collection will make an effort to execute it
 111     /// reasonably soon.
 112     ///
 113     /// If this method is called from an [`unprotected`] guard, the function will simply be
 114     /// executed immediately.
 115     ///
 116     /// # Safety
 117     ///
 118     /// The given function must not hold reference onto the stack. It is highly recommended that
 119     /// the passed function is **always** marked with `move` in order to prevent accidental
 120     /// borrows.
 121     ///
 122     /// ```
 123     /// use crossbeam_epoch as epoch;
 124     ///
 125     /// let guard = &epoch::pin();
 126     /// let message = "Hello!";
 127     /// unsafe {
 128     ///     // ALWAYS use `move` when sending a closure into `defer_unchecked`.
 129     ///     guard.defer_unchecked(move || {
 130     ///         println!("{}", message);
 131     ///     });
 132     /// }
 133     /// ```
 134     ///
 135     /// Apart from that, keep in mind that another thread may execute `f`, so anything accessed by
 136     /// the closure must be `Send`.
 137     ///
 138     /// We intentionally didn't require `F: Send`, because Rust's type systems usually cannot prove
 139     /// `F: Send` for typical use cases. For example, consider the following code snippet, which
 140     /// exemplifies the typical use case of deferring the deallocation of a shared reference:
 141     ///
 142     /// ```ignore
 143     /// let shared = Owned::new(7i32).into_shared(guard);
 144     /// guard.defer_unchecked(move || shared.into_owned()); // `Shared` is not `Send`!
 145     /// ```
 146     ///
 147     /// While `Shared` is not `Send`, it's safe for another thread to call the deferred function,
 148     /// because it's called only after the grace period and `shared` is no longer shared with other
 149     /// threads. But we don't expect type systems to prove this.
 150     ///
 151     /// # Examples
 152     ///
 153     /// When a heap-allocated object in a data structure becomes unreachable, it has to be
 154     /// deallocated. However, the current thread and other threads may be still holding references
 155     /// on the stack to that same object. Therefore it cannot be deallocated before those references
 156     /// get dropped. This method can defer deallocation until all those threads get unpinned and
 157     /// consequently drop all their references on the stack.
 158     ///
 159     /// ```
 160     /// use crossbeam_epoch::{self as epoch, Atomic, Owned};
 161     /// use std::sync::atomic::Ordering::SeqCst;
 162     ///
 163     /// let a = Atomic::new("foo");
 164     ///
 165     /// // Now suppose that `a` is shared among multiple threads and concurrently
 166     /// // accessed and modified...
 167     ///
 168     /// // Pin the current thread.
 169     /// let guard = &epoch::pin();
 170     ///
 171     /// // Steal the object currently stored in `a` and swap it with another one.
 172     /// let p = a.swap(Owned::new("bar").into_shared(guard), SeqCst, guard);
 173     ///
 174     /// if !p.is_null() {
 175     ///     // The object `p` is pointing to is now unreachable.
 176     ///     // Defer its deallocation until all currently pinned threads get unpinned.
 177     ///     unsafe {
 178     ///         // ALWAYS use `move` when sending a closure into `defer_unchecked`.
 179     ///         guard.defer_unchecked(move || {
 180     ///             println!("{} is now being deallocated.", p.deref());
 181     ///             // Now we have unique access to the object pointed to by `p` and can turn it
 182     ///             // into an `Owned`. Dropping the `Owned` will deallocate the object.
 183     ///             drop(p.into_owned());
 184     ///         });
 185     ///     }
 186     /// }
 187     /// ```
 188     pub unsafe fn defer_unchecked<F, R>(&self, f: F)
 189     where
 190         F: FnOnce() -> R,
 191     {
 192         if let Some(local) = self.local.as_ref() {
 193             local.defer(Deferred::new(move || drop(f())), self);
 194         } else {
 195             drop(f());
 196         }
 197     }
 198
 199     /// Stores a destructor for an object so that it can be deallocated and dropped at some point
 200     /// after all currently pinned threads get unpinned.
 201     ///
 202     /// This method first stores the destructor into the thread-local (or handle-local) cache. If
 203     /// this cache becomes full, some destructors are moved into the global cache. At the same
 204     /// time, some destructors from both local and global caches may get executed in order to
 205     /// incrementally clean up the caches as they fill up.
 206     ///
 207     /// There is no guarantee when exactly the destructor will be executed. The only guarantee is
 208     /// that it won't be executed until all currently pinned threads get unpinned. In theory, the
 209     /// destructor might never run, but the epoch-based garbage collection will make an effort to
 210     /// execute it reasonably soon.
 211     ///
 212     /// If this method is called from an [`unprotected`] guard, the destructor will simply be
 213     /// executed immediately.
 214     ///
 215     /// # Safety
 216     ///
 217     /// The object must not be reachable by other threads anymore, otherwise it might be still in
 218     /// use when the destructor runs.
 219     ///
 220     /// Apart from that, keep in mind that another thread may execute the destructor, so the object
 221     /// must be sendable to other threads.
 222     ///
 223     /// We intentionally didn't require `T: Send`, because Rust's type systems usually cannot prove
 224     /// `T: Send` for typical use cases. For example, consider the following code snippet, which
 225     /// exemplifies the typical use case of deferring the deallocation of a shared reference:
 226     ///
 227     /// ```ignore
 228     /// let shared = Owned::new(7i32).into_shared(guard);
 229     /// guard.defer_destroy(shared); // `Shared` is not `Send`!
 230     /// ```
 231     ///
 232     /// While `Shared` is not `Send`, it's safe for another thread to call the destructor, because
 233     /// it's called only after the grace period and `shared` is no longer shared with other
 234     /// threads. But we don't expect type systems to prove this.
 235     ///
 236     /// # Examples
 237     ///
 238     /// When a heap-allocated object in a data structure becomes unreachable, it has to be
 239     /// deallocated. However, the current thread and other threads may be still holding references
 240     /// on the stack to that same object. Therefore it cannot be deallocated before those references
 241     /// get dropped. This method can defer deallocation until all those threads get unpinned and
 242     /// consequently drop all their references on the stack.
 243     ///
 244     /// ```
 245     /// use crossbeam_epoch::{self as epoch, Atomic, Owned};
 246     /// use std::sync::atomic::Ordering::SeqCst;
 247     ///
 248     /// let a = Atomic::new("foo");
 249     ///
 250     /// // Now suppose that `a` is shared among multiple threads and concurrently
 251     /// // accessed and modified...
 252     ///
 253     /// // Pin the current thread.
 254     /// let guard = &epoch::pin();
 255     ///
 256     /// // Steal the object currently stored in `a` and swap it with another one.
 257     /// let p = a.swap(Owned::new("bar").into_shared(guard), SeqCst, guard);
 258     ///
 259     /// if !p.is_null() {
 260     ///     // The object `p` is pointing to is now unreachable.
 261     ///     // Defer its deallocation until all currently pinned threads get unpinned.
 262     ///     unsafe {
 263     ///         guard.defer_destroy(p);
 264     ///     }
 265     /// }
 266     /// ```
 267     pub unsafe fn defer_destroy<T>(&self, ptr: Shared<'_, T>) {
 268         self.defer_unchecked(move || ptr.into_owned());
 269     }
 270
 271     /// Clears up the thread-local cache of deferred functions by executing them or moving into the
 272     /// global cache.
 273     ///
 274     /// Call this method after deferring execution of a function if you want to get it executed as
 275     /// soon as possible. Flushing will make sure it is residing in in the global cache, so that
 276     /// any thread has a chance of taking the function and executing it.
 277     ///
 278     /// If this method is called from an [`unprotected`] guard, it is a no-op (nothing happens).
 279     ///
 280     /// # Examples
 281     ///
 282     /// ```
 283     /// use crossbeam_epoch as epoch;
 284     ///
 285     /// let guard = &epoch::pin();
 286     /// guard.defer(move || {
 287     ///     println!("This better be printed as soon as possible!");
 288     /// });
 289     /// guard.flush();
 290     /// ```
 291     pub fn flush(&self) {
 292         if let Some(local) = unsafe { self.local.as_ref() } {
 293             local.flush(self);
 294         }
 295     }
 296
 297     /// Unpins and then immediately re-pins the thread.
 298     ///
 299     /// This method is useful when you don't want delay the advancement of the global epoch by
 300     /// holding an old epoch. For safety, you should not maintain any guard-based reference across
 301     /// the call (the latter is enforced by `&mut self`). The thread will only be repinned if this
 302     /// is the only active guard for the current thread.
 303     ///
 304     /// If this method is called from an [`unprotected`] guard, then the call will be just no-op.
 305     ///
 306     /// # Examples
 307     ///
 308     /// ```
 309     /// use crossbeam_epoch::{self as epoch, Atomic};
 310     /// use std::sync::atomic::Ordering::SeqCst;
 311     ///
 312     /// let a = Atomic::new(777);
 313     /// let mut guard = epoch::pin();
 314     /// {
 315     ///     let p = a.load(SeqCst, &guard);
 316     ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
 317     /// }
 318     /// guard.repin();
 319     /// {
 320     ///     let p = a.load(SeqCst, &guard);
 321     ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
 322     /// }
 323     /// ```
 324     pub fn repin(&mut self) {
 325         if let Some(local) = unsafe { self.local.as_ref() } {
 326             local.repin();
 327         }
 328     }
 329
 330     /// Temporarily unpins the thread, executes the given function and then re-pins the thread.
 331     ///
 332     /// This method is useful when you need to perform a long-running operation (e.g. sleeping)
 333     /// and don't need to maintain any guard-based reference across the call (the latter is enforced
 334     /// by `&mut self`). The thread will only be unpinned if this is the only active guard for the
 335     /// current thread.
 336     ///
 337     /// If this method is called from an [`unprotected`] guard, then the passed function is called
 338     /// directly without unpinning the thread.
 339     ///
 340     /// # Examples
 341     ///
 342     /// ```
 343     /// use crossbeam_epoch::{self as epoch, Atomic};
 344     /// use std::sync::atomic::Ordering::SeqCst;
 345     /// use std::thread;
 346     /// use std::time::Duration;
 347     ///
 348     /// let a = Atomic::new(777);
 349     /// let mut guard = epoch::pin();
 350     /// {
 351     ///     let p = a.load(SeqCst, &guard);
 352     ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
 353     /// }
 354     /// guard.repin_after(|| thread::sleep(Duration::from_millis(50)));
 355     /// {
 356     ///     let p = a.load(SeqCst, &guard);
 357     ///     assert_eq!(unsafe { p.as_ref() }, Some(&777));
 358     /// }
 359     /// ```
 360     pub fn repin_after<F, R>(&mut self, f: F) -> R
 361     where
 362         F: FnOnce() -> R,
 363     {
 364         if let Some(local) = unsafe { self.local.as_ref() } {
 365             // We need to acquire a handle here to ensure the Local doesn't
 366             // disappear from under us.
 367             local.acquire_handle();
 368             local.unpin();
 369         }
 370
 371         // Ensure the Guard is re-pinned even if the function panics
 372         defer! {
 373             if let Some(local) = unsafe { self.local.as_ref() } {
 374                 mem::forget(local.pin());
 375                 local.release_handle();
 376             }
 377         }
 378
 379         f()
 380     }
 381
 382     /// Returns the `Collector` associated with this guard.
 383     ///
 384     /// This method is useful when you need to ensure that all guards used with
 385     /// a data structure come from the same collector.
 386     ///
 387     /// If this method is called from an [`unprotected`] guard, then `None` is returned.
 388     ///
 389     /// # Examples
 390     ///
 391     /// ```
 392     /// use crossbeam_epoch as epoch;
 393     ///
 394     /// let guard1 = epoch::pin();
 395     /// let guard2 = epoch::pin();
 396     /// assert!(guard1.collector() == guard2.collector());
 397     /// ```
 398     pub fn collector(&self) -> Option<&Collector> {
 399         unsafe { self.local.as_ref().map(|local| local.collector()) }
 400     }
 401 }
 402
 403 impl Drop for Guard {
 404     #[inline]
 405     fn drop(&mut self) {
 406         if let Some(local) = unsafe { self.local.as_ref() } {
 407             local.unpin();
 408         }
 409     }
 410 }
 411
 412 impl fmt::Debug for Guard {
 413     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 414         f.pad("Guard { .. }")
 415     }
 416 }
 417
 418 /// Returns a reference to a dummy guard that allows unprotected access to [`Atomic`]s.
 419 ///
 420 /// This guard should be used in special occasions only. Note that it doesn't actually keep any
 421 /// thread pinned - it's just a fake guard that allows loading from [`Atomic`]s unsafely.
 422 ///
 423 /// Note that calling [`defer`] with a dummy guard will not defer the function - it will just
 424 /// execute the function immediately.
 425 ///
 426 /// If necessary, it's possible to create more dummy guards by cloning: `unprotected().clone()`.
 427 ///
 428 /// # Safety
 429 ///
 430 /// Loading and dereferencing data from an [`Atomic`] using this guard is safe only if the
 431 /// [`Atomic`] is not being concurrently modified by other threads.
 432 ///
 433 /// # Examples
 434 ///
 435 /// ```
 436 /// use crossbeam_epoch::{self as epoch, Atomic};
 437 /// use std::sync::atomic::Ordering::Relaxed;
 438 ///
 439 /// let a = Atomic::new(7);
 440 ///
 441 /// unsafe {
 442 ///     // Load `a` without pinning the current thread.
 443 ///     a.load(Relaxed, epoch::unprotected());
 444 ///
 445 ///     // It's possible to create more dummy guards by calling `clone()`.
 446 ///     let dummy = &epoch::unprotected().clone();
 447 ///
 448 ///     dummy.defer(move || {
 449 ///         println!("This gets executed immediately.");
 450 ///     });
 451 ///
 452 ///     // Dropping `dummy` doesn't affect the current thread - it's just a noop.
 453 /// }
 454 /// ```
 455 ///
 456 /// The most common use of this function is when constructing or destructing a data structure.
 457 ///
 458 /// For example, we can use a dummy guard in the destructor of a Treiber stack because at that
 459 /// point no other thread could concurrently modify the [`Atomic`]s we are accessing.
 460 ///
 461 /// If we were to actually pin the current thread during destruction, that would just unnecessarily
 462 /// delay garbage collection and incur some performance cost, so in cases like these `unprotected`
 463 /// is very helpful.
 464 ///
 465 /// ```
 466 /// use crossbeam_epoch::{self as epoch, Atomic};
 467 /// use std::mem::ManuallyDrop;
 468 /// use std::sync::atomic::Ordering::Relaxed;
 469 ///
 470 /// struct Stack<T> {
 471 ///     head: Atomic<Node<T>>,
 472 /// }
 473 ///
 474 /// struct Node<T> {
 475 ///     data: ManuallyDrop<T>,
 476 ///     next: Atomic<Node<T>>,
 477 /// }
 478 ///
 479 /// impl<T> Drop for Stack<T> {
 480 ///     fn drop(&mut self) {
 481 ///         unsafe {
 482 ///             // Unprotected load.
 483 ///             let mut node = self.head.load(Relaxed, epoch::unprotected());
 484 ///
 485 ///             while let Some(n) = node.as_ref() {
 486 ///                 // Unprotected load.
 487 ///                 let next = n.next.load(Relaxed, epoch::unprotected());
 488 ///
 489 ///                 // Take ownership of the node, then drop its data and deallocate it.
 490 ///                 let mut o = node.into_owned();
 491 ///                 ManuallyDrop::drop(&mut o.data);
 492 ///                 drop(o);
 493 ///
 494 ///                 node = next;
 495 ///             }
 496 ///         }
 497 ///     }
 498 /// }
 499 /// ```
 500 ///
 501 /// [`Atomic`]: super::Atomic
 502 /// [`defer`]: Guard::defer
 503 #[inline]
 504 pub unsafe fn unprotected() -> &'static Guard {
 505     // An unprotected guard is just a `Guard` with its field `local` set to null.
 506     // We make a newtype over `Guard` because `Guard` isn't `Sync`, so can't be directly stored in
 507     // a `static`
 508     struct GuardWrapper(Guard);
 509     unsafe impl Sync for GuardWrapper {}
 510     static UNPROTECTED: GuardWrapper = GuardWrapper(Guard {
 511         local: core::ptr::null(),
 512     });
 513     &UNPROTECTED.0
 514 }