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//! Implements a container type providing RefCell-like semantics for objects
//! shared across threads.
//!
//! RwLock is traditionally considered to be the |Sync| analogue of RefCell.
//! However, for consumers that can guarantee that they will never mutably
//! borrow the contents concurrently with immutable borrows, an RwLock is
//! overkill, and has key disadvantages:
//! * Performance: Even the fastest existing implementation of RwLock (that of
//! parking_lot) performs at least two atomic operations during immutable
//! borrows. This makes mutable borrows significantly cheaper than immutable
//! borrows, leading to weird incentives when writing performance-critical
//! code.
//! * Features: Implementing AtomicRefCell on top of RwLock makes it impossible
//! to implement useful things like AtomicRef{,Mut}::map.
//!
//! As such, we re-implement RefCell semantics from scratch with a single atomic
//! reference count. The primary complication of this scheme relates to keeping
//! things in a consistent state when one thread performs an illegal borrow and
//! panics. Since an AtomicRefCell can be accessed by multiple threads, and since
//! panics are recoverable, we need to ensure that an illegal (panicking) access by
//! one thread does not lead to undefined behavior on other, still-running threads.
//!
//! So we represent things as follows:
//! * Any value with the high bit set (so half the total refcount space) indicates
//! a mutable borrow.
//! * Mutable borrows perform an atomic compare-and-swap, swapping in the high bit
//! if the current value is zero. If the current value is non-zero, the thread
//! panics and the value is left undisturbed.
//! * Immutable borrows perform an atomic increment. If the new value has the high
//! bit set, the thread panics. The incremented refcount is left as-is, since it
//! still represents a valid mutable borrow. When the mutable borrow is released,
//! the refcount is set unconditionally to zero, clearing any stray increments by
//! panicked threads.
//!
//! There are a few additional purely-academic complications to handle overflow,
//! which are documented in the implementation.
//!
//! The rest of this module is mostly derived by copy-pasting the implementation of
//! RefCell and fixing things up as appropriate. Certain non-threadsafe methods
//! have been removed. We segment the concurrency logic from the rest of the code to
//! keep the tricky parts small and easy to audit.
#![no_std]
#![allow(unsafe_code)]
#![deny(missing_docs)]
use core::cell::UnsafeCell;
use core::cmp;
use core::fmt;
use core::fmt::{Debug, Display};
use core::ops::{Deref, DerefMut};
use core::sync::atomic;
use core::sync::atomic::AtomicUsize;
/// A threadsafe analogue to RefCell.
pub struct AtomicRefCell<T: ?Sized> {
borrow: AtomicUsize,
value: UnsafeCell<T>,
}
/// An error returned by [`AtomicRefCell::try_borrow`](struct.AtomicRefCell.html#method.try_borrow).
pub struct BorrowError {
_private: (),
}
impl Debug for BorrowError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("BorrowError").finish()
}
}
impl Display for BorrowError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
Display::fmt("already mutably borrowed", f)
}
}
/// An error returned by [`AtomicRefCell::try_borrow_mut`](struct.AtomicRefCell.html#method.try_borrow_mut).
pub struct BorrowMutError {
_private: (),
}
impl Debug for BorrowMutError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("BorrowMutError").finish()
}
}
impl Display for BorrowMutError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
Display::fmt("already borrowed", f)
}
}
impl<T> AtomicRefCell<T> {
/// Creates a new `AtomicRefCell` containing `value`.
#[inline]
pub const fn new(value: T) -> AtomicRefCell<T> {
AtomicRefCell {
borrow: AtomicUsize::new(0),
value: UnsafeCell::new(value),
}
}
/// Consumes the `AtomicRefCell`, returning the wrapped value.
#[inline]
pub fn into_inner(self) -> T {
debug_assert!(self.borrow.load(atomic::Ordering::Acquire) == 0);
self.value.into_inner()
}
}
impl<T: ?Sized> AtomicRefCell<T> {
/// Immutably borrows the wrapped value.
#[inline]
pub fn borrow(&self) -> AtomicRef<T> {
match AtomicBorrowRef::try_new(&self.borrow) {
Ok(borrow) => AtomicRef {
value: unsafe { &*self.value.get() },
borrow,
},
Err(s) => panic!("{}", s),
}
}
/// Attempts to immutably borrow the wrapped value, but instead of panicking
/// on a failed borrow, returns `Err`.
#[inline]
pub fn try_borrow(&self) -> Result<AtomicRef<T>, BorrowError> {
match AtomicBorrowRef::try_new(&self.borrow) {
Ok(borrow) => Ok(AtomicRef {
value: unsafe { &*self.value.get() },
borrow,
}),
Err(_) => Err(BorrowError { _private: () }),
}
}
/// Mutably borrows the wrapped value.
#[inline]
pub fn borrow_mut(&self) -> AtomicRefMut<T> {
match AtomicBorrowRefMut::try_new(&self.borrow) {
Ok(borrow) => AtomicRefMut {
value: unsafe { &mut *self.value.get() },
borrow,
},
Err(s) => panic!("{}", s),
}
}
/// Attempts to mutably borrow the wrapped value, but instead of panicking
/// on a failed borrow, returns `Err`.
#[inline]
pub fn try_borrow_mut(&self) -> Result<AtomicRefMut<T>, BorrowMutError> {
match AtomicBorrowRefMut::try_new(&self.borrow) {
Ok(borrow) => Ok(AtomicRefMut {
value: unsafe { &mut *self.value.get() },
borrow,
}),
Err(_) => Err(BorrowMutError { _private: () }),
}
}
/// Returns a raw pointer to the underlying data in this cell.
///
/// External synchronization is needed to avoid data races when dereferencing
/// the pointer.
#[inline]
pub fn as_ptr(&self) -> *mut T {
self.value.get()
}
/// Returns a mutable reference to the wrapped value.
///
/// No runtime checks take place (unless debug assertions are enabled)
/// because this call borrows `AtomicRefCell` mutably at compile-time.
#[inline]
pub fn get_mut(&mut self) -> &mut T {
debug_assert!(self.borrow.load(atomic::Ordering::Acquire) == 0);
unsafe { &mut *self.value.get() }
}
}
//
// Core synchronization logic. Keep this section small and easy to audit.
//
const HIGH_BIT: usize = !(::core::usize::MAX >> 1);
const MAX_FAILED_BORROWS: usize = HIGH_BIT + (HIGH_BIT >> 1);
struct AtomicBorrowRef<'b> {
borrow: &'b AtomicUsize,
}
impl<'b> AtomicBorrowRef<'b> {
#[inline]
fn try_new(borrow: &'b AtomicUsize) -> Result<Self, &'static str> {
let new = borrow.fetch_add(1, atomic::Ordering::Acquire) + 1;
if new & HIGH_BIT != 0 {
// If the new count has the high bit set, that almost certainly
// means there's an pre-existing mutable borrow. In that case,
// we simply leave the increment as a benign side-effect and
// return `Err`. Once the mutable borrow is released, the
// count will be reset to zero unconditionally.
//
// The overflow check here ensures that an unbounded number of
// immutable borrows during the scope of one mutable borrow
// will soundly trigger a panic (or abort) rather than UB.
Self::check_overflow(borrow, new);
Err("already mutably borrowed")
} else {
Ok(AtomicBorrowRef { borrow: borrow })
}
}
#[cold]
#[inline(never)]
fn check_overflow(borrow: &'b AtomicUsize, new: usize) {
if new == HIGH_BIT {
// We overflowed into the reserved upper half of the refcount
// space. Before panicking, decrement the refcount to leave things
// in a consistent immutable-borrow state.
//
// This can basically only happen if somebody forget()s AtomicRefs
// in a tight loop.
borrow.fetch_sub(1, atomic::Ordering::Release);
panic!("too many immutable borrows");
} else if new >= MAX_FAILED_BORROWS {
// During the mutable borrow, an absurd number of threads have
// attempted to increment the refcount with immutable borrows.
// To avoid hypothetically wrapping the refcount, we abort the
// process once a certain threshold is reached.
//
// This requires billions of borrows to fail during the scope of
// one mutable borrow, and so is very unlikely to happen in a real
// program.
//
// To avoid a potential unsound state after overflowing, we make
// sure the entire process aborts.
//
// Right now, there's no stable way to do that without `std`:
// As a workaround, we cause an abort by making this thread panic
// during the unwinding of another panic.
//
// On platforms where the panic strategy is already 'abort', the
// ForceAbort object here has no effect, as the program already
// panics before it is dropped.
struct ForceAbort;
impl Drop for ForceAbort {
fn drop(&mut self) {
panic!("Aborting to avoid unsound state of AtomicRefCell");
}
}
let _abort = ForceAbort;
panic!("Too many failed borrows");
}
}
}
impl<'b> Drop for AtomicBorrowRef<'b> {
#[inline]
fn drop(&mut self) {
let old = self.borrow.fetch_sub(1, atomic::Ordering::Release);
// This assertion is technically incorrect in the case where another
// thread hits the hypothetical overflow case, since we might observe
// the refcount before it fixes it up (and panics). But that never will
// never happen in a real program, and this is a debug_assert! anyway.
debug_assert!(old & HIGH_BIT == 0);
}
}
struct AtomicBorrowRefMut<'b> {
borrow: &'b AtomicUsize,
}
impl<'b> Drop for AtomicBorrowRefMut<'b> {
#[inline]
fn drop(&mut self) {
self.borrow.store(0, atomic::Ordering::Release);
}
}
impl<'b> AtomicBorrowRefMut<'b> {
#[inline]
fn try_new(borrow: &'b AtomicUsize) -> Result<AtomicBorrowRefMut<'b>, &'static str> {
// Use compare-and-swap to avoid corrupting the immutable borrow count
// on illegal mutable borrows.
let old = match borrow.compare_exchange(
0,
HIGH_BIT,
atomic::Ordering::Acquire,
atomic::Ordering::Relaxed,
) {
Ok(x) => x,
Err(x) => x,
};
if old == 0 {
Ok(AtomicBorrowRefMut { borrow })
} else if old & HIGH_BIT == 0 {
Err("already immutably borrowed")
} else {
Err("already mutably borrowed")
}
}
}
unsafe impl<T: ?Sized + Send> Send for AtomicRefCell<T> {}
unsafe impl<T: ?Sized + Send + Sync> Sync for AtomicRefCell<T> {}
//
// End of core synchronization logic. No tricky thread stuff allowed below
// this point.
//
impl<T: Clone> Clone for AtomicRefCell<T> {
#[inline]
fn clone(&self) -> AtomicRefCell<T> {
AtomicRefCell::new(self.borrow().clone())
}
}
impl<T: Default> Default for AtomicRefCell<T> {
#[inline]
fn default() -> AtomicRefCell<T> {
AtomicRefCell::new(Default::default())
}
}
impl<T: ?Sized + PartialEq> PartialEq for AtomicRefCell<T> {
#[inline]
fn eq(&self, other: &AtomicRefCell<T>) -> bool {
*self.borrow() == *other.borrow()
}
}
impl<T: ?Sized + Eq> Eq for AtomicRefCell<T> {}
impl<T: ?Sized + PartialOrd> PartialOrd for AtomicRefCell<T> {
#[inline]
fn partial_cmp(&self, other: &AtomicRefCell<T>) -> Option<cmp::Ordering> {
self.borrow().partial_cmp(&*other.borrow())
}
}
impl<T: ?Sized + Ord> Ord for AtomicRefCell<T> {
#[inline]
fn cmp(&self, other: &AtomicRefCell<T>) -> cmp::Ordering {
self.borrow().cmp(&*other.borrow())
}
}
impl<T> From<T> for AtomicRefCell<T> {
fn from(t: T) -> AtomicRefCell<T> {
AtomicRefCell::new(t)
}
}
impl<'b> Clone for AtomicBorrowRef<'b> {
#[inline]
fn clone(&self) -> AtomicBorrowRef<'b> {
AtomicBorrowRef::try_new(self.borrow).unwrap()
}
}
/// A wrapper type for an immutably borrowed value from an `AtomicRefCell<T>`.
pub struct AtomicRef<'b, T: ?Sized + 'b> {
value: &'b T,
borrow: AtomicBorrowRef<'b>,
}
impl<'b, T: ?Sized> Deref for AtomicRef<'b, T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
self.value
}
}
impl<'b, T: ?Sized> AtomicRef<'b, T> {
/// Copies an `AtomicRef`.
#[inline]
pub fn clone(orig: &AtomicRef<'b, T>) -> AtomicRef<'b, T> {
AtomicRef {
value: orig.value,
borrow: orig.borrow.clone(),
}
}
/// Make a new `AtomicRef` for a component of the borrowed data.
#[inline]
pub fn map<U: ?Sized, F>(orig: AtomicRef<'b, T>, f: F) -> AtomicRef<'b, U>
where
F: FnOnce(&T) -> &U,
{
AtomicRef {
value: f(orig.value),
borrow: orig.borrow,
}
}
/// Make a new `AtomicRef` for an optional component of the borrowed data.
#[inline]
pub fn filter_map<U: ?Sized, F>(orig: AtomicRef<'b, T>, f: F) -> Option<AtomicRef<'b, U>>
where
F: FnOnce(&T) -> Option<&U>,
{
Some(AtomicRef {
value: f(orig.value)?,
borrow: orig.borrow,
})
}
}
impl<'b, T: ?Sized> AtomicRefMut<'b, T> {
/// Make a new `AtomicRefMut` for a component of the borrowed data, e.g. an enum
/// variant.
#[inline]
pub fn map<U: ?Sized, F>(orig: AtomicRefMut<'b, T>, f: F) -> AtomicRefMut<'b, U>
where
F: FnOnce(&mut T) -> &mut U,
{
AtomicRefMut {
value: f(orig.value),
borrow: orig.borrow,
}
}
/// Make a new `AtomicRefMut` for an optional component of the borrowed data.
#[inline]
pub fn filter_map<U: ?Sized, F>(orig: AtomicRefMut<'b, T>, f: F) -> Option<AtomicRefMut<'b, U>>
where
F: FnOnce(&mut T) -> Option<&mut U>,
{
Some(AtomicRefMut {
value: f(orig.value)?,
borrow: orig.borrow,
})
}
}
/// A wrapper type for a mutably borrowed value from an `AtomicRefCell<T>`.
pub struct AtomicRefMut<'b, T: ?Sized + 'b> {
value: &'b mut T,
borrow: AtomicBorrowRefMut<'b>,
}
impl<'b, T: ?Sized> Deref for AtomicRefMut<'b, T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
self.value
}
}
impl<'b, T: ?Sized> DerefMut for AtomicRefMut<'b, T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
self.value
}
}
impl<'b, T: ?Sized + Debug + 'b> Debug for AtomicRef<'b, T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
self.value.fmt(f)
}
}
impl<'b, T: ?Sized + Debug + 'b> Debug for AtomicRefMut<'b, T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
self.value.fmt(f)
}
}
impl<T: ?Sized + Debug> Debug for AtomicRefCell<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "AtomicRefCell {{ ... }}")
}
}