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style: Add support for static references to servo_arc::Arc. r=emilio

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Differential Revision: https://phabricator.services.mozilla.com/D17186
This commit is contained in:
Cameron McCormack 2019-03-30 00:15:57 +00:00 • committed by Emilio Cobos Álvarez
parent b71a601a36
commit f889b303da
2 changed files with 158 additions and 37 deletions
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192
components/servo_arc/lib.rs
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@ -16,6 +16,8 @@
//! * We can add methods to support our custom use cases [1].
//! * We have support for dynamically-sized types (see from_header_and_iter).
//! * We have support for thin arcs to unsized types (see ThinArc).
//! * We have support for references to static data, which don't do any
//! refcounting.
//!
//! [1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1360883
@ -32,6 +34,7 @@ use nodrop::NoDrop;
#[cfg(feature = "servo")]
use serde::{Deserialize, Serialize};
use stable_deref_trait::{CloneStableDeref, StableDeref};
use std::alloc::Layout;
use std::borrow;
use std::cmp::Ordering;
use std::convert::From;
@ -74,6 +77,10 @@ macro_rules! offset_of {
/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
const MAX_REFCOUNT: usize = (isize::MAX) as usize;
/// Special refcount value that means the data is not reference counted,
/// and that the `Arc` is really acting as a read-only static reference.
const STATIC_REFCOUNT: usize = usize::MAX;
/// An atomically reference counted shared pointer
///
/// See the documentation for [`Arc`] in the standard library. Unlike the
@ -194,6 +201,32 @@ impl<T> Arc<T> {
}
}
/// Create a new static Arc<T> (one that won't reference count the object)
/// and place it in the allocation provided by the specified `alloc`
/// function.
///
/// `alloc` must return a pointer into a static allocation suitable for
/// storing data with the `Layout` passed into it. The pointer returned by
/// `alloc` will not be freed.
#[inline]
pub unsafe fn new_static<F>(alloc: F, data: T) -> Arc<T>
where
F: FnOnce(Layout) -> *mut u8,
{
let ptr = alloc(Layout::new::<ArcInner<T>>()) as *mut ArcInner<T>;
let x = ArcInner {
count: atomic::AtomicUsize::new(STATIC_REFCOUNT),
data,
};
ptr::write(ptr, x);
Arc {
p: ptr::NonNull::new_unchecked(ptr),
}
}
/// Produce a pointer to the data that can be converted back
/// to an Arc. This is basically an `&Arc<T>`, without the extra indirection.
/// It has the benefits of an `&T` but also knows about the underlying refcount
@ -225,8 +258,14 @@ impl<T> Arc<T> {
/// Returns the address on the heap of the Arc itself -- not the T within it -- for memory
/// reporting.
///
/// If this is a static reference, this returns null.
pub fn heap_ptr(&self) -> *const c_void {
self.p.as_ptr() as *const ArcInner<T> as *const c_void
if self.inner().count.load(Relaxed) == STATIC_REFCOUNT {
ptr::null()
} else {
self.p.as_ptr() as *const ArcInner<T> as *const c_void
}
}
}
@ -262,30 +301,34 @@ impl<T: ?Sized> Arc<T> {
impl<T: ?Sized> Clone for Arc<T> {
#[inline]
fn clone(&self) -> Self {
// Using a relaxed ordering is alright here, as knowledge of the
// original reference prevents other threads from erroneously deleting
// the object.
//
// As explained in the [Boost documentation][1], Increasing the
// reference counter can always be done with memory_order_relaxed: New
// references to an object can only be formed from an existing
// reference, and passing an existing reference from one thread to
// another must already provide any required synchronization.
//
// [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
let old_size = self.inner().count.fetch_add(1, Relaxed);
// Using a relaxed ordering to check for STATIC_REFCOUNT is safe, since
// `count` never changes between STATIC_REFCOUNT and other values.
if self.inner().count.load(Relaxed) != STATIC_REFCOUNT {
// Using a relaxed ordering is alright here, as knowledge of the
// original reference prevents other threads from erroneously deleting
// the object.
//
// As explained in the [Boost documentation][1], Increasing the
// reference counter can always be done with memory_order_relaxed: New
// references to an object can only be formed from an existing
// reference, and passing an existing reference from one thread to
// another must already provide any required synchronization.
//
// [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
let old_size = self.inner().count.fetch_add(1, Relaxed);
// However we need to guard against massive refcounts in case someone
// is `mem::forget`ing Arcs. If we don't do this the count can overflow
// and users will use-after free. We racily saturate to `isize::MAX` on
// the assumption that there aren't ~2 billion threads incrementing
// the reference count at once. This branch will never be taken in
// any realistic program.
//
// We abort because such a program is incredibly degenerate, and we
// don't care to support it.
if old_size > MAX_REFCOUNT {
process::abort();
// However we need to guard against massive refcounts in case someone
// is `mem::forget`ing Arcs. If we don't do this the count can overflow
// and users will use-after free. We racily saturate to `isize::MAX` on
// the assumption that there aren't ~2 billion threads incrementing
// the reference count at once. This branch will never be taken in
// any realistic program.
//
// We abort because such a program is incredibly degenerate, and we
// don't care to support it.
if old_size > MAX_REFCOUNT {
process::abort();
}
}
unsafe {
@ -351,7 +394,8 @@ impl<T: ?Sized> Arc<T> {
}
}
/// Whether or not the `Arc` is uniquely owned (is the refcount 1?)
/// Whether or not the `Arc` is uniquely owned (is the refcount 1?) and not
/// a static reference.
#[inline]
pub fn is_unique(&self) -> bool {
// See the extensive discussion in [1] for why this needs to be Acquire.
@ -364,6 +408,12 @@ impl<T: ?Sized> Arc<T> {
impl<T: ?Sized> Drop for Arc<T> {
#[inline]
fn drop(&mut self) {
// Using a relaxed ordering to check for STATIC_REFCOUNT is safe, since
// `count` never changes between STATIC_REFCOUNT and other values.
if self.inner().count.load(Relaxed) == STATIC_REFCOUNT {
return;
}
// Because `fetch_sub` is already atomic, we do not need to synchronize
// with other threads unless we are going to delete the object.
if self.inner().count.fetch_sub(1, Release) != 1 {
@ -528,10 +578,20 @@ fn divide_rounding_up(dividend: usize, divisor: usize) -> usize {
impl<H, T> Arc<HeaderSlice<H, [T]>> {
/// Creates an Arc for a HeaderSlice using the given header struct and
/// iterator to generate the slice. The resulting Arc will be fat.
/// iterator to generate the slice.
///
/// `is_static` indicates whether to create a static Arc.
///
/// `alloc` is used to get a pointer to the memory into which the
/// dynamically sized ArcInner<HeaderSlice<H, T>> value will be
/// written. If `is_static` is true, then `alloc` must return a
/// pointer into some static memory allocation. If it is false,
/// then `alloc` must return an allocation that can be dellocated
/// by calling Box::from_raw::<ArcInner<HeaderSlice<H, T>>> on it.
#[inline]
pub fn from_header_and_iter<I>(header: H, mut items: I) -> Self
fn from_header_and_iter_alloc<F, I>(alloc: F, header: H, mut items: I, is_static: bool) -> Self
where
F: FnOnce(Layout) -> *mut u8,
I: Iterator<Item = T> + ExactSizeIterator,
{
use std::mem::size_of;
@ -565,22 +625,20 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
let ptr: *mut ArcInner<HeaderSlice<H, [T]>>;
unsafe {
// Allocate the buffer. We use Vec because the underlying allocation
// machinery isn't available in stable Rust.
//
// To avoid alignment issues, we allocate words rather than bytes,
// rounding up to the nearest word size.
let buffer = if mem::align_of::<T>() <= mem::align_of::<usize>() {
Self::allocate_buffer::<usize>(size)
// Allocate the buffer.
let layout = if mem::align_of::<T>() <= mem::align_of::<usize>() {
Layout::from_size_align_unchecked(size, mem::align_of::<usize>())
} else if mem::align_of::<T>() <= mem::align_of::<u64>() {
// On 32-bit platforms <T> may have 8 byte alignment while usize has 4 byte aligment.
// Use u64 to avoid over-alignment.
// This branch will compile away in optimized builds.
Self::allocate_buffer::<u64>(size)
Layout::from_size_align_unchecked(size, mem::align_of::<u64>())
} else {
panic!("Over-aligned type not handled");
};
let buffer = alloc(layout);
// Synthesize the fat pointer. We do this by claiming we have a direct
// pointer to a [T], and then changing the type of the borrow. The key
// point here is that the length portion of the fat pointer applies
@ -594,7 +652,12 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
//
// Note that any panics here (i.e. from the iterator) are safe, since
// we'll just leak the uninitialized memory.
ptr::write(&mut ((*ptr).count), atomic::AtomicUsize::new(1));
let count = if is_static {
atomic::AtomicUsize::new(STATIC_REFCOUNT)
} else {
atomic::AtomicUsize::new(1)
};
ptr::write(&mut ((*ptr).count), count);
ptr::write(&mut ((*ptr).data.header), header);
let mut current: *mut T = &mut (*ptr).data.slice[0];
for _ in 0..num_items {
@ -628,8 +691,37 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
}
}
/// Creates an Arc for a HeaderSlice using the given header struct and
/// iterator to generate the slice. The resulting Arc will be fat.
#[inline]
pub fn from_header_and_iter<I>(header: H, items: I) -> Self
where
I: Iterator<Item = T> + ExactSizeIterator,
{
Arc::from_header_and_iter_alloc(
|layout| {
// align will only ever be align_of::<usize>() or align_of::<u64>()
let align = layout.align();
unsafe {
if align == mem::align_of::<usize>() {
Self::allocate_buffer::<usize>(layout.size())
} else {
assert_eq!(align, mem::align_of::<u64>());
Self::allocate_buffer::<u64>(layout.size())
}
}
},
header,
items,
/* is_static = */ false,
)
}
#[inline]
unsafe fn allocate_buffer<W>(size: usize) -> *mut u8 {
// We use Vec because the underlying allocation machinery isn't
// available in stable Rust. To avoid alignment issues, we allocate
// words rather than bytes, rounding up to the nearest word size.
let words_to_allocate = divide_rounding_up(size, mem::size_of::<W>());
let mut vec = Vec::<W>::with_capacity(words_to_allocate);
vec.set_len(words_to_allocate);
@ -730,11 +822,37 @@ impl<H, T> ThinArc<H, T> {
Arc::into_thin(Arc::from_header_and_iter(header, items))
}
/// Create a static `ThinArc` for a HeaderSlice using the given header
/// struct and iterator to generate the slice, placing it in the allocation
/// provided by the specified `alloc` function.
///
/// `alloc` must return a pointer into a static allocation suitable for
/// storing data with the `Layout` passed into it. The pointer returned by
/// `alloc` will not be freed.
pub unsafe fn static_from_header_and_iter<F, I>(alloc: F, header: H, items: I) -> Self
where
F: FnOnce(Layout) -> *mut u8,
I: Iterator<Item = T> + ExactSizeIterator,
{
let header = HeaderWithLength::new(header, items.len());
Arc::into_thin(Arc::from_header_and_iter_alloc(
alloc, header, items, /* is_static = */ true,
))
}
/// Returns the address on the heap of the ThinArc itself -- not the T
/// within it -- for memory reporting.
///
/// If this is a static ThinArc, this returns null.
#[inline]
pub fn heap_ptr(&self) -> *const c_void {
self.ptr as *const ArcInner<T> as *const c_void
let is_static =
ThinArc::with_arc(self, |a| a.inner().count.load(Relaxed) == STATIC_REFCOUNT);
if is_static {
ptr::null()
} else {
self.ptr as *const ArcInner<T> as *const c_void
}
}
}