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// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
use std::alloc::Layout;
use std::ptr::NonNull;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering;
use parking_lot::lock_api::RawMutex;
use crate::arena::raw_arena::RawArena;
use super::alloc;
use super::ptr_byte_add;
use super::ptr_byte_sub;
/// In debug mode we use a signature to ensure that raw pointers are pointing to the correct
/// shape of arena object.
#[cfg(debug_assertions)]
const SIGNATURE: usize = 0x1133224455667788;
pub struct ArenaSharedAtomicReservation<T>(NonNull<ArenaArcData<T>>);
impl<T> Drop for ArenaSharedAtomicReservation<T> {
fn drop(&mut self) {
panic!("A reservation must be completed or forgotten")
}
}
/// Represents an atomic reference-counted pointer into an arena-allocated object.
pub struct ArenaArc<T> {
ptr: NonNull<ArenaArcData<T>>,
}
impl<T> ArenaArc<T> {
/// Offset of the `ptr` field within the `ArenaArc` struct.
const PTR_OFFSET: usize = memoffset::offset_of!(ArenaArc<T>, ptr);
/// Converts a raw pointer to the data into a `NonNull` pointer to `ArenaArcData`.
///
/// # Safety
///
/// This function assumes that the input `ptr` points to the data within an `ArenaArc` object.
/// Improper usage may result in undefined behavior.
#[inline(always)]
unsafe fn data_from_ptr(ptr: NonNull<T>) -> NonNull<ArenaArcData<T>> {
ptr_byte_sub(ptr, Self::PTR_OFFSET)
}
/// Converts a `NonNull` pointer to `ArenaArcData` into a raw pointer to the data.
///
/// # Safety
///
/// This function assumes that the input `ptr` is a valid `NonNull` pointer to `ArenaArcData`.
/// Improper usage may result in undefined behavior.
#[inline(always)]
unsafe fn ptr_from_data(ptr: NonNull<ArenaArcData<T>>) -> NonNull<T> {
ptr_byte_add(ptr, Self::PTR_OFFSET)
}
/// Consumes the `ArenaArc`, forgetting it, and returns a raw pointer to the contained data.
///
/// # Safety
///
/// This function returns a raw pointer without managing the memory, potentially leading to
/// memory leaks if the pointer is not properly handled or deallocated.
#[inline(always)]
pub fn into_raw(arc: ArenaArc<T>) -> NonNull<T> {
let ptr = arc.ptr;
std::mem::forget(arc);
unsafe { Self::ptr_from_data(ptr) }
}
/// Clones the `ArenaArc` reference, increments its reference count, and returns a raw pointer to the contained data.
///
/// This function increments the reference count of the `ArenaArc`.
#[inline(always)]
pub fn clone_into_raw(arc: &ArenaArc<T>) -> NonNull<T> {
unsafe {
arc.ptr.as_ref().ref_count.fetch_add(1, Ordering::Relaxed);
Self::ptr_from_data(arc.ptr)
}
}
/// Constructs an `ArenaArc` from a raw pointer to the contained data.
///
/// This function safely constructs an `ArenaArc` from a raw pointer, assuming the pointer is
/// valid, properly aligned, and was originally created by `into_raw` or `clone_into_raw`.
///
/// # Safety
///
/// This function assumes the provided `ptr` is a valid raw pointer to the data within an `ArenaArc`
/// object. Misuse may lead to undefined behavior, memory unsafety, or data corruption.
#[inline(always)]
pub unsafe fn from_raw(ptr: NonNull<T>) -> ArenaArc<T> {
let ptr = Self::data_from_ptr(ptr);
#[cfg(debug_assertions)]
debug_assert_eq!(ptr.as_ref().signature, SIGNATURE);
ArenaArc { ptr }
}
/// Clones an `ArenaArc` reference from a raw pointer and increments its reference count.
///
/// This method increments the reference count of the `ArenaArc` instance
/// associated with the provided raw pointer, allowing multiple references
/// to the same allocated data.
///
/// # Safety
///
/// This function assumes that the provided `ptr` is a valid raw pointer
/// to the data within an `ArenaArc` object. Improper usage may lead
/// to memory unsafety or data corruption.
#[inline(always)]
pub unsafe fn clone_from_raw(ptr: NonNull<T>) -> ArenaArc<T> {
let ptr = Self::data_from_ptr(ptr);
let this = ptr.as_ref();
this.ref_count.fetch_add(1, Ordering::Relaxed);
ArenaArc { ptr }
}
/// Increments the reference count associated with the raw pointer to an `ArenaArc`-managed data.
///
/// This method manually increases the reference count of the `ArenaArc` instance
/// associated with the provided raw pointer. It allows incrementing the reference count
/// without constructing a full `ArenaArc` instance, ideal for scenarios where direct
/// manipulation of raw pointers is required.
///
/// # Safety
///
/// This method bypasses some safety checks enforced by the `ArenaArc` type. Incorrect usage
/// or mishandling of raw pointers might lead to memory unsafety or data corruption.
/// Use with caution and ensure proper handling of associated data.
#[inline(always)]
pub unsafe fn clone_raw_from_raw(ptr: NonNull<T>) {
let this = Self::data_from_ptr(ptr).as_ref();
this.ref_count.fetch_add(1, Ordering::Relaxed);
}
/// Drops the `ArenaArc` reference pointed to by the raw pointer.
///
/// If the reference count drops to zero, the associated data is returned to the arena.
///
/// # Safety
///
/// This function assumes that the provided `ptr` is a valid raw pointer
/// to the data within an `ArenaArc` object. Improper usage may lead
/// to memory unsafety or data corruption.
#[inline(always)]
pub unsafe fn drop_from_raw(ptr: NonNull<T>) {
let ptr = Self::data_from_ptr(ptr);
let this = ptr.as_ref();
if this.ref_count.fetch_sub(1, Ordering::Relaxed) == 0 {
ArenaSharedAtomic::delete(ptr);
}
}
}
unsafe impl<T: Send + Sync> Send for ArenaArc<T> {}
unsafe impl<T: Send + Sync> Sync for ArenaArc<T> {}
// T is Send + Sync, so ArenaArc is too
static_assertions::assert_impl_all!(ArenaArc<()>: Send, Sync);
// T is !Send & !Sync, so ArenaArc is too
static_assertions::assert_not_impl_any!(ArenaArc<*mut ()>: Send, Sync);
impl<T> ArenaArc<T> {}
impl<T> Drop for ArenaArc<T> {
fn drop(&mut self) {
unsafe {
let this = self.ptr.as_ref();
if this.ref_count.fetch_sub(1, Ordering::Relaxed) == 0 {
ArenaSharedAtomic::delete(self.ptr);
}
}
}
}
impl<T> std::ops::Deref for ArenaArc<T> {
type Target = T;
#[inline(always)]
fn deref(&self) -> &Self::Target {
unsafe { &self.ptr.as_ref().data }
}
}
impl<T> std::convert::AsRef<T> for ArenaArc<T> {
#[inline(always)]
fn as_ref(&self) -> &T {
unsafe { &self.ptr.as_ref().data }
}
}
/// Data structure containing metadata and the actual data within the `ArenaArc`.
struct ArenaArcData<T> {
#[cfg(debug_assertions)]
signature: usize,
ref_count: AtomicUsize,
arena_data: NonNull<ArenaSharedAtomicData<T>>,
data: T,
}
/// An atomic reference-counted pointer into an arena-allocated object
/// with thread-safe allocation and deallocation capabilities.
///
/// This structure ensures atomic access and safe sharing of allocated
/// data across multiple threads while maintaining reference counting
/// to manage the memory deallocation when no longer needed.
///
/// It combines a thread-safe `RawArena` for allocation and deallocation
/// and provides a mutex to guarantee exclusive access to the internal
/// data for safe multi-threaded operation.
///
/// The `ArenaSharedAtomic` allows multiple threads to allocate, share,
/// and deallocate objects within the arena, ensuring safety and atomicity
/// during these operations.
pub struct ArenaSharedAtomic<T> {
ptr: NonNull<ArenaSharedAtomicData<T>>,
}
unsafe impl<T> Send for ArenaSharedAtomic<T> {}
// The arena itself may not be shared so that we can guarantee all [`RawArena`]
// access happens on the owning thread.
static_assertions::assert_impl_any!(ArenaSharedAtomic<()>: Send);
static_assertions::assert_not_impl_any!(ArenaSharedAtomic<()>: Sync);
/// Data structure containing a mutex and the `RawArena` for atomic access in the `ArenaSharedAtomic`.
struct ArenaSharedAtomicData<T> {
/// A mutex ensuring thread-safe access to the internal raw arena and refcount.
mutex: parking_lot::RawMutex,
protected: ArenaSharedAtomicDataProtected<T>,
}
struct ArenaSharedAtomicDataProtected<T> {
raw_arena: RawArena<ArenaArcData<T>>,
ref_count: usize,
}
impl<T> ArenaSharedAtomic<T> {
/// Returns the constant overhead per allocation to assist with making allocations
/// page-aligned.
pub const fn overhead() -> usize {
Self::allocation_size() - std::mem::size_of::<T>()
}
/// Returns the size of each allocation.
pub const fn allocation_size() -> usize {
RawArena::<ArenaArcData<T>>::allocation_size()
}
pub fn with_capacity(capacity: usize) -> Self {
unsafe {
let ptr = alloc();
std::ptr::write(
ptr.as_ptr(),
ArenaSharedAtomicData {
mutex: parking_lot::RawMutex::INIT,
protected: ArenaSharedAtomicDataProtected {
raw_arena: RawArena::with_capacity(capacity),
ref_count: 0,
},
},
);
Self { ptr }
}
}
#[inline(always)]
unsafe fn lock<'s>(
arena: NonNull<ArenaSharedAtomicData<T>>,
) -> &'s mut ArenaSharedAtomicDataProtected<T> {
let mutex = &*std::ptr::addr_of!((*arena.as_ptr()).mutex);
while !mutex.try_lock() {
std::thread::yield_now();
}
&mut *std::ptr::addr_of_mut!((*arena.as_ptr()).protected)
}
#[inline(always)]
unsafe fn unlock(arena: NonNull<ArenaSharedAtomicData<T>>) {
let mutex = &*std::ptr::addr_of!((*arena.as_ptr()).mutex);
mutex.unlock()
}
#[cold]
#[inline(never)]
unsafe fn drop_data(arena: NonNull<ArenaSharedAtomicData<T>>) {
let arena = arena.as_ptr();
std::ptr::drop_in_place(arena);
std::alloc::dealloc(arena as _, Layout::new::<ArenaSharedAtomicData<T>>());
}
#[inline(always)]
unsafe fn delete(data: NonNull<ArenaArcData<T>>) {
let ptr = (*data.as_ptr()).arena_data;
// We cannot materialize a reference to arena_data until we have the lock
let this = Self::lock(ptr);
this.raw_arena.recycle(data);
if this.ref_count == 0 {
Self::drop_data(ptr);
} else {
this.ref_count -= 1;
Self::unlock(ptr);
}
}
/// Allocates a new object in the arena and returns an `ArenaArc` pointing to it.
///
/// This method creates a new instance of type `T` within the `RawArena`. The provided `data`
/// is initialized within the arena, and an `ArenaArc` is returned to manage this allocated data.
/// The `ArenaArc` serves as an atomic, reference-counted pointer to the allocated data within
/// the arena, ensuring safe concurrent access across multiple threads while maintaining the
/// reference count for memory management.
///
/// The allocation process employs a mutex to ensure thread-safe access to the arena, allowing
/// only one thread at a time to modify the internal state, including allocating and deallocating memory.
///
/// # Safety
///
/// The provided `data` is allocated within the arena and managed by the `ArenaArc`. Improper handling
/// or misuse of the returned `ArenaArc` pointer may lead to memory leaks or memory unsafety.
///
/// # Example
///
/// ```rust
/// # use deno_core::arena::ArenaSharedAtomic;
///
/// // Define a struct that will be allocated within the arena
/// struct MyStruct {
/// data: usize,
/// }
///
/// // Create a new instance of ArenaSharedAtomic with a specified base capacity
/// let arena: ArenaSharedAtomic<MyStruct> = ArenaSharedAtomic::with_capacity(16);
///
/// // Allocate a new MyStruct instance within the arena
/// let data_instance = MyStruct { data: 42 };
/// let allocated_arc = arena.allocate(data_instance);
///
/// // Now, allocated_arc can be used as a managed reference to the allocated data
/// assert_eq!(allocated_arc.data, 42); // Validate the data stored in the allocated arc
/// ```
pub fn allocate(&self, data: T) -> ArenaArc<T> {
let ptr = unsafe {
let this = Self::lock(self.ptr);
let ptr = this.raw_arena.allocate();
this.ref_count += 1;
std::ptr::write(
ptr.as_ptr(),
ArenaArcData {
#[cfg(debug_assertions)]
signature: SIGNATURE,
arena_data: self.ptr,
ref_count: AtomicUsize::default(),
data,
},
);
Self::unlock(self.ptr);
ptr
};
ArenaArc { ptr }
}
/// Allocates a new object in the arena and returns an `ArenaArc` pointing to it. If no space
/// is available, returns the original object.
///
/// This method creates a new instance of type `T` within the `RawArena`. The provided `data`
/// is initialized within the arena, and an `ArenaArc` is returned to manage this allocated data.
/// The `ArenaArc` serves as an atomic, reference-counted pointer to the allocated data within
/// the arena, ensuring safe concurrent access across multiple threads while maintaining the
/// reference count for memory management.
///
/// The allocation process employs a mutex to ensure thread-safe access to the arena, allowing
/// only one thread at a time to modify the internal state, including allocating and deallocating memory.
pub fn allocate_if_space(&self, data: T) -> Result<ArenaArc<T>, T> {
let ptr = unsafe {
let this = Self::lock(self.ptr);
let Some(ptr) = this.raw_arena.allocate_if_space() else {
return Err(data);
};
this.ref_count += 1;
Self::unlock(self.ptr);
std::ptr::write(
ptr.as_ptr(),
ArenaArcData {
#[cfg(debug_assertions)]
signature: SIGNATURE,
arena_data: self.ptr,
ref_count: AtomicUsize::default(),
data,
},
);
ptr
};
Ok(ArenaArc { ptr })
}
/// Attempt to reserve space in this arena.
///
/// # Safety
///
/// Reservations must be either completed or forgotten, and must be provided to the same
/// arena that created them.
#[inline(always)]
pub unsafe fn reserve_space(
&self,
) -> Option<ArenaSharedAtomicReservation<T>> {
let this = Self::lock(self.ptr);
let ptr = this.raw_arena.allocate_if_space()?;
this.ref_count += 1;
Self::unlock(self.ptr);
Some(ArenaSharedAtomicReservation(ptr))
}
/// Forget a reservation.
///
/// # Safety
///
/// Reservations must be either completed or forgotten, and must be provided to the same
/// arena that created them.
pub unsafe fn forget_reservation(
&self,
reservation: ArenaSharedAtomicReservation<T>,
) {
let ptr = reservation.0;
std::mem::forget(reservation);
let this = Self::lock(self.ptr);
this.ref_count -= 1;
this.raw_arena.recycle_without_drop(ptr);
Self::unlock(self.ptr);
}
/// Complete a reservation.
///
/// # Safety
///
/// Reservations must be either completed or forgotten, and must be provided to the same
/// arena that created them.
#[inline(always)]
pub unsafe fn complete_reservation(
&self,
reservation: ArenaSharedAtomicReservation<T>,
data: T,
) -> ArenaArc<T> {
let ptr = reservation.0;
std::mem::forget(reservation);
let ptr = {
std::ptr::write(
ptr.as_ptr(),
ArenaArcData {
#[cfg(debug_assertions)]
signature: SIGNATURE,
arena_data: self.ptr,
ref_count: AtomicUsize::default(),
data,
},
);
ptr
};
ArenaArc { ptr }
}
}
impl<T> Drop for ArenaSharedAtomic<T> {
fn drop(&mut self) {
unsafe {
let this = Self::lock(self.ptr);
if this.ref_count == 0 {
Self::drop_data(self.ptr);
} else {
this.ref_count -= 1;
Self::unlock(self.ptr);
}
}
}
}
#[cfg(test)]
mod tests {
use super::*;
use std::cell::RefCell;
use std::sync::Arc;
use std::sync::Mutex;
#[test]
fn test_raw() {
let arena: ArenaSharedAtomic<RefCell<usize>> =
ArenaSharedAtomic::with_capacity(16);
let arc = arena.allocate(Default::default());
let raw = ArenaArc::into_raw(arc);
_ = unsafe { ArenaArc::from_raw(raw) };
}
#[test]
fn test_clone_into_raw() {
let arena: ArenaSharedAtomic<RefCell<usize>> =
ArenaSharedAtomic::with_capacity(16);
let arc = arena.allocate(Default::default());
let raw = ArenaArc::clone_into_raw(&arc);
_ = unsafe { ArenaArc::from_raw(raw) };
}
#[test]
fn test_allocate_drop_arc_first() {
let arena: ArenaSharedAtomic<RefCell<usize>> =
ArenaSharedAtomic::with_capacity(16);
let arc = arena.allocate(Default::default());
*arc.borrow_mut() += 1;
drop(arc);
drop(arena);
}
#[test]
fn test_allocate_drop_arena_first() {
let arena: ArenaSharedAtomic<RefCell<usize>> =
ArenaSharedAtomic::with_capacity(16);
let arc = arena.allocate(Default::default());
*arc.borrow_mut() += 1;
drop(arena);
drop(arc);
}
#[test]
fn test_threaded() {
let arena: Arc<Mutex<ArenaSharedAtomic<RefCell<usize>>>> =
Arc::new(Mutex::new(ArenaSharedAtomic::with_capacity(2000)));
const THREADS: usize = 20;
const ITERATIONS: usize = 100;
let mut handles = Vec::new();
let barrier = Arc::new(std::sync::Barrier::new(THREADS));
for _ in 0..THREADS {
let arena = Arc::clone(&arena);
let barrier = Arc::clone(&barrier);
let handle = std::thread::spawn(move || {
barrier.wait();
for _ in 0..ITERATIONS {
let arc = arena.lock().unwrap().allocate(RefCell::new(0));
*arc.borrow_mut() += 1;
drop(arc); // Ensure the Arc is dropped at the end of the iteration
}
});
handles.push(handle);
}
for handle in handles {
handle.join().unwrap();
}
}
}