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|
use crate::loom::cell::UnsafeCell;
use crate::util::slab::{Address, Entry, Slot, TransferStack, INITIAL_PAGE_SIZE};
use std::fmt;
/// Data accessed only by the thread that owns the shard.
pub(crate) struct Local {
head: UnsafeCell<usize>,
}
/// Data accessed by any thread.
pub(crate) struct Shared<T> {
remote: TransferStack,
size: usize,
prev_sz: usize,
slab: UnsafeCell<Option<Box<[Slot<T>]>>>,
}
/// Returns the size of the page at index `n`
pub(super) fn size(n: usize) -> usize {
INITIAL_PAGE_SIZE << n
}
impl Local {
pub(crate) fn new() -> Self {
Self {
head: UnsafeCell::new(0),
}
}
fn head(&self) -> usize {
self.head.with(|head| unsafe { *head })
}
fn set_head(&self, new_head: usize) {
self.head.with_mut(|head| unsafe {
*head = new_head;
})
}
}
impl<T: Entry> Shared<T> {
pub(crate) fn new(size: usize, prev_sz: usize) -> Shared<T> {
Self {
prev_sz,
size,
remote: TransferStack::new(),
slab: UnsafeCell::new(None),
}
}
/// Allocates storage for this page if it does not allready exist.
///
/// This requires unique access to the page (e.g. it is called from the
/// thread that owns the page, or, in the case of `SingleShard`, while the
/// lock is held). In order to indicate this, a reference to the page's
/// `Local` data is taken by this function; the `Local` argument is not
/// actually used, but requiring it ensures that this is only called when
/// local access is held.
#[cold]
fn alloc_page(&self, _: &Local) {
debug_assert!(self.slab.with(|s| unsafe { (*s).is_none() }));
let mut slab = Vec::with_capacity(self.size);
slab.extend((1..self.size).map(Slot::new));
slab.push(Slot::new(Address::NULL));
self.slab.with_mut(|s| {
// this mut access is safe — it only occurs to initially
// allocate the page, which only happens on this thread; if the
// page has not yet been allocated, other threads will not try
// to access it yet.
unsafe {
*s = Some(slab.into_boxed_slice());
}
});
}
pub(crate) fn alloc(&self, local: &Local) -> Option<Address> {
let head = local.head();
// are there any items on the local free list? (fast path)
let head = if head < self.size {
head
} else {
// if the local free list is empty, pop all the items on the remote
// free list onto the local free list.
self.remote.pop_all()?
};
// if the head is still null, both the local and remote free lists are
// empty --- we can't fit any more items on this page.
if head == Address::NULL {
return None;
}
// do we need to allocate storage for this page?
let page_needs_alloc = self.slab.with(|s| unsafe { (*s).is_none() });
if page_needs_alloc {
self.alloc_page(local);
}
let gen = self.slab.with(|slab| {
let slab = unsafe { &*(slab) }
.as_ref()
.expect("page must have been allocated to alloc!");
let slot = &slab[head];
local.set_head(slot.next());
slot.generation()
});
let index = head + self.prev_sz;
Some(Address::new(index, gen))
}
pub(crate) fn get(&self, addr: Address) -> Option<&T> {
let page_offset = addr.slot() - self.prev_sz;
self.slab
.with(|slab| unsafe { &*slab }.as_ref()?.get(page_offset))
.map(|slot| slot.get())
}
pub(crate) fn remove_local(&self, local: &Local, addr: Address) {
let offset = addr.slot() - self.prev_sz;
self.slab.with(|slab| {
let slab = unsafe { &*slab }.as_ref();
let slot = if let Some(slot) = slab.and_then(|slab| slab.get(offset)) {
slot
} else {
return;
};
if slot.reset(addr.generation()) {
slot.set_next(local.head());
local.set_head(offset);
}
})
}
pub(crate) fn remove_remote(&self, addr: Address) {
let offset = addr.slot() - self.prev_sz;
self.slab.with(|slab| {
let slab = unsafe { &*slab }.as_ref();
let slot = if let Some(slot) = slab.and_then(|slab| slab.get(offset)) {
slot
} else {
return;
};
if !slot.reset(addr.generation()) {
return;
}
self.remote.push(offset, |next| slot.set_next(next));
})
}
}
impl fmt::Debug for Local {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
self.head.with(|head| {
let head = unsafe { *head };
f.debug_struct("Local")
.field("head", &format_args!("{:#0x}", head))
.finish()
})
}
}
impl<T> fmt::Debug for Shared<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Shared")
.field("remote", &self.remote)
.field("prev_sz", &self.prev_sz)
.field("size", &self.size)
// .field("slab", &self.slab)
.finish()
}
}
|
