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mod tid;
pub use self::tid::ThreadId;
use owned_alloc::{Cache, OwnedAlloc, UninitAlloc};
use ptr::check_null_align;
use std::{
fmt,
marker::PhantomData,
mem::{forget, replace},
ptr::{null_mut, NonNull},
sync::atomic::{AtomicPtr, Ordering::*},
};
const BITS: usize = 8;
/// Per Object Thread Local Storage. The stored data is not dropped on thread
/// exit. It is only dropped when the structure itself is dropped. After the
/// thread exited, the data might be reused for other threads. This TLS's
/// operation are also wait-free.
///
/// # Example
/// ```
/// extern crate lockfree;
///
/// use lockfree::tls::ThreadLocal;
/// use std::{cell::Cell, sync::Arc, thread};
///
/// let tls = Arc::new(ThreadLocal::<Cell<usize>>::new());
/// let mut threads = Vec::with_capacity(32);
///
/// for i in 1 ..= 32 {
/// let tls = tls.clone();
/// threads.push(thread::spawn(move || {
/// tls.with_default().set(i);
///
/// if tls.get().map(|c| assert_eq!(c.get(), i)).is_none() {
/// // Some OSes mis-run the destructors for their TLS
/// // implementation.
/// eprintln!("Warning: OS mis-reset the global thread state")
/// }
/// }))
/// }
///
/// for thread in threads {
/// thread.join().unwrap();
/// }
/// ```
pub struct ThreadLocal<T> {
top: OwnedAlloc<Table<T>>,
}
impl<T> ThreadLocal<T> {
/// Creates an empty thread local storage.
pub fn new() -> Self {
check_null_align::<Table<T>>();
check_null_align::<Entry<T>>();
Self { top: Table::new_alloc() }
}
/// Removes and drops all entries. The TLS is considered empty then. This
/// method is only available with exclusive references. This method is
/// merely for optimization since the TLS is cleared at drop.
pub fn clear(&mut self) {
let mut tables = Vec::new();
// Method clear means we are also resetting all node pointers to null.
//
// Safe because we store nodes only correctly.
unsafe { self.top.clear(&mut tables) }
while let Some(mut table) = tables.pop() {
// Method free_nodes means we are only freeing node pointers but not
// clearing them.
//
// Safe because we will never refer to these nodes again.
unsafe { table.free_nodes(&mut tables) }
}
}
/// Creates an iterator over immutable refereces of entries.
pub fn iter(&self) -> Iter<T>
where
T: Sync,
{
Iter { curr_table: Some((&self.top, 0)), tables: Vec::new() }
}
/// Creates an iterator over mutable refereces of entries.
pub fn iter_mut(&mut self) -> IterMut<T>
where
T: Send,
{
IterMut { curr_table: Some((&mut self.top, 0)), tables: Vec::new() }
}
/// Accesses the entry for the current thread. No initialization is
/// performed.
#[inline]
pub fn get(&self) -> Option<&T> {
self.get_with_id(ThreadId::current())
}
/// Accesses the entry for the current thread with a given cached ID.
/// Repeated calls with cached IDs should be faster than reloading the ID
/// everytime. No initialization is performed.
pub fn get_with_id(&self, id: ThreadId) -> Option<&T> {
let mut table = &*self.top;
let mut shifted = id.bits();
loop {
// The index of the node for our id.
let index = shifted & (1 << BITS) - 1;
// Load what is in there.
let in_place = table.nodes[index].atomic.load(Acquire);
// Null means there is nothing.
if in_place.is_null() {
break None;
}
// Having in_place's lower bit set to 0 means it is a
// pointer to entry.
if in_place as usize & 1 == 0 {
// This is safe since:
//
// 1. We only store nodes with cleared lower bit if it is an
// entry.
//
// 2. We only delete stuff when we are behind mutable
// references.
let entry = unsafe { &*(in_place as *mut Entry<T>) };
break if entry.id == id {
// We only have an entry for the thread if the ids
// match.
Some(&entry.data)
} else {
None
};
}
// The remaining case (non-null with lower bit set to 1) means
// we have a child table.
// Clear the pointer first lower bit so we can dereference it.
let table_ptr = (in_place as usize & !1) as *mut Table<T>;
// Set it as the table to be checked in the next iteration.
// This is safe since:
//
// 1. We only store nodes with marked lower bit if it is an
// table.
//
// 2. W cleared up the bit above so we can get the original
// pointer.
//
// 3. We only delete stuff when we are behind mutable
// references.
table = unsafe { &*table_ptr };
// Shift our "hash" for the next level.
shifted >>= BITS;
}
}
/// Accesses the entry for the current thread. If necessary, the `init`
/// closure is called to initialize the entry.
#[inline]
pub fn with_init<F>(&self, init: F) -> &T
where
F: FnOnce() -> T,
{
self.with_id_and_init(ThreadId::current(), init)
}
/// Accesses the entry for the current thread with a given cached ID.
/// Repeated calls with cached IDs should be faster than reloading the ID
/// everytime. If necessary, the `init` closure is called to initialize the
/// entry.
pub fn with_id_and_init<F>(&self, id: ThreadId, init: F) -> &T
where
F: FnOnce() -> T,
{
let mut table = &*self.top;
// The depth of the iterations.
let mut depth = 1;
let mut shifted = id.bits();
// The pointer stored in place.
let mut index = shifted & (1 << BITS) - 1;
let mut in_place = table.nodes[index].atomic.load(Acquire);
// Using `LazyInit` to make sure we only initialize if there is no
// entry.
let mut init = LazyInit::Pending(move || Entry { id, data: init() });
let mut tbl_cache = Cache::<OwnedAlloc<Table<T>>>::new();
loop {
if in_place.is_null() {
// Null means we have an empty node and also our thread has
// not stored anything. Let's initialize.
let nnptr = init.get();
// First lower bit set to 0 means this is a pointer to
// entry. This should be guaranteed by the alignment,
// however, always good to ensure it.
debug_assert!(nnptr.as_ptr() as usize & 1 == 0);
// Trying to publish our freshly created entry.
match table.nodes[index].atomic.compare_exchange(
in_place,
nnptr.as_ptr() as *mut (),
AcqRel,
Acquire,
) {
Ok(_) => {
// If the stored value still was null, we succeeded.
// Let's read the entry.
//
// This is safe since... This is the pointer we just
// allocated and we only delete nodes through mutable
// references to the TLS.
break unsafe { &(*nnptr.as_ptr()).data };
},
Err(new) => in_place = new,
}
} else if in_place as usize & 1 == 0 {
// First lower bit set to 0 means we have an entry.
//
// This is safe since:
//
// 1. We only store nodes with cleared lower bit if it is an
// entry.
//
// 2. We only delete stuff when we are behind mutable
// references.
let entry = unsafe { &*(in_place as *mut Entry<T>) };
// If ids match, this is the entry for our thread.
if entry.id == id {
// There is no possible way we have initialized the
// `LazyInit`. It will only happen if we found an empty
// node while searching, and the only way of putting a
// non-empty node is either we put it or some other
// thread (with different id obviously) put it.
debug_assert!(init.is_pending());
// And let's read it...
break &entry.data;
}
// Get a table allocation from the cache.
let new_tbl = tbl_cache.take_or(Table::new_alloc);
// Calculate index for the collided entry.
let other_shifted = entry.id.bits() >> depth * BITS;
let other_index = other_shifted & (1 << BITS) - 1;
// Pre-insert it in the table from the cache.
new_tbl.nodes[other_index].atomic.store(in_place, Relaxed);
// Forget about the owned allocation and turn it into a
// pointer.
let new_tbl_ptr = new_tbl.into_raw();
// Let's try to publish our work.
match table.nodes[index].atomic.compare_exchange(
in_place,
// First lower bit set to 1 means it is a table
// pointer.
(new_tbl_ptr.as_ptr() as usize | 1) as *mut (),
AcqRel,
Release,
) {
Ok(_) => {
// If the old node was still stored, we succeeded.
// Let's set the new table as the table for the next
// iteration.
//
// This is safe since it is the table we just allocated
// and we only delete it through mutable references to
// the TLS.
table = unsafe { &*new_tbl_ptr.as_ptr() };
// We are going one depth further.
depth += 1;
// Shift our "hash" for the next level.
shifted >>= BITS;
// Load new in place pointer.
index = shifted & (1 << BITS) - 1;
in_place = table.nodes[index].atomic.load(Acquire);
},
Err(new) => {
// If we failed, let's rebuild the owned allocation.
//
// This is safe since it is the table we just allocated
// and we don't share it.
let new_tbl =
unsafe { OwnedAlloc::from_raw(new_tbl_ptr) };
// Clear that pre-inserted node.
new_tbl.nodes[other_index]
.atomic
.store(null_mut(), Relaxed);
// Store it into the cache for later.
tbl_cache.store(new_tbl);
in_place = new;
},
}
} else {
// The remaining case (non-null with first lower bit set to
// 1) is a table. Clear the pointer first lower bit so we
// can dereference it.
let table_ptr = (in_place as usize & !1) as *mut Table<T>;
// Set it as table for the next iteration.
//
// 1. We only store nodes with marked lower bit if it is an
// table.
//
// 2. W cleared up the bit above so we can get the original
// pointer.
//
// 3. We only delete stuff when we are behind mutable
// references.
table = unsafe { &*table_ptr };
// We are going one depth further.
depth += 1;
// Shift our "hash" for the next level.
shifted >>= BITS;
// Load new in place pointer.
index = shifted & (1 << BITS) - 1;
in_place = table.nodes[index].atomic.load(Acquire);
}
}
}
/// Accesses the entry for the current thread. If necessary, the entry is
/// initialized with default value.
#[inline]
pub fn with_default(&self) -> &T
where
T: Default,
{
self.with_init(T::default)
}
/// Accesses the entry for the current thread with a given cached ID.
/// Repeated calls with cached IDs should be faster than reloading the ID
/// everytime. If necessary, the entry is initialized with default
/// value.
#[inline]
pub fn with_id_and_default(&self, id: ThreadId) -> &T
where
T: Default,
{
self.with_id_and_init(id, T::default)
}
}
impl<T> Drop for ThreadLocal<T> {
fn drop(&mut self) {
let mut tables = Vec::new();
// Method free_nodes means we are only freeing node pointers but not
// clearing them (no need to clear since nobody will ever use them
// again, we are dropping the TLS).
//
// This is safe because we never load the nodes again.
unsafe { self.top.free_nodes(&mut tables) }
while let Some(mut table) = tables.pop() {
// This is safe because we never load the nodes again.
unsafe { table.free_nodes(&mut tables) }
}
}
}
impl<T> fmt::Debug for ThreadLocal<T>
where
T: fmt::Debug,
{
fn fmt(&self, fmtr: &mut fmt::Formatter) -> fmt::Result {
write!(fmtr, "ThreadLocal {} storage: ", '{')?;
match self.get() {
Some(val) => write!(fmtr, "Some({:?})", val)?,
None => write!(fmtr, "None")?,
}
write!(fmtr, "{}", '}')
}
}
impl<T> Default for ThreadLocal<T> {
fn default() -> Self {
Self::new()
}
}
unsafe impl<T> Send for ThreadLocal<T> {}
unsafe impl<T> Sync for ThreadLocal<T> {}
impl<T> IntoIterator for ThreadLocal<T>
where
T: Send,
{
type IntoIter = IntoIter<T>;
type Item = T;
fn into_iter(self) -> Self::IntoIter {
let raw = self.top.raw();
forget(self);
// Safe since this is the allocation we just forgot about.
let top = unsafe { OwnedAlloc::from_raw(raw) };
IntoIter { curr_table: Some((top, 0)), tables: Vec::new() }
}
}
impl<'tls, T> IntoIterator for &'tls ThreadLocal<T>
where
T: Sync,
{
type IntoIter = Iter<'tls, T>;
type Item = &'tls T;
fn into_iter(self) -> Self::IntoIter {
self.iter()
}
}
impl<'tls, T> IntoIterator for &'tls mut ThreadLocal<T>
where
T: Send,
{
type IntoIter = IterMut<'tls, T>;
type Item = &'tls mut T;
fn into_iter(self) -> Self::IntoIter {
self.iter_mut()
}
}
/// An iterator over immutable references to entries of TLS.
pub struct Iter<'tls, T>
where
T: 'tls,
{
tables: Vec<&'tls Table<T>>,
curr_table: Option<(&'tls Table<T>, usize)>,
}
impl<'tls, T> Iterator for Iter<'tls, T> {
type Item = &'tls T;
fn next(&mut self) -> Option<Self::Item> {
loop {
let (table, index) = self.curr_table.take()?;
match table.nodes.get(index).map(|node| node.atomic.load(Acquire)) {
Some(ptr) if ptr.is_null() => {
self.curr_table = Some((table, index + 1))
},
Some(ptr) if ptr as usize & 1 == 0 => {
let ptr = ptr as *mut Entry<T>;
self.curr_table = Some((table, index + 1));
// This is safe since:
//
// 1. We only store nodes with cleared lower bit if it is an
// entry.
//
// 2. We only delete stuff when we are behind mutable
// references *and* there are no mutable references to the
// TLS as we are a shared one.
break Some(unsafe { &(*ptr).data });
},
Some(ptr) => {
let ptr = (ptr as usize & !1) as *mut Table<T>;
// Set it as table for the next iteration.
//
// 1. We only store nodes with marked lower bit if it is an
// table.
//
// 2. We cleared up the bit above so we can get the original
// pointer.
//
// 3. We only delete stuff when we are behind mutable
// references *and* there are no mutable references to the
// TLS as we are a shared one.
self.tables.push(unsafe { &mut *ptr });
self.curr_table = Some((table, index + 1));
},
None => self.curr_table = self.tables.pop().map(|tbl| (tbl, 0)),
};
}
}
}
/// An iterator over mutable references to entries of TLS.
pub struct IterMut<'tls, T>
where
T: 'tls,
{
tables: Vec<&'tls mut Table<T>>,
curr_table: Option<(&'tls mut Table<T>, usize)>,
}
impl<'tls, T> Iterator for IterMut<'tls, T> {
type Item = &'tls mut T;
fn next(&mut self) -> Option<Self::Item> {
loop {
let (table, index) = self.curr_table.take()?;
match table.nodes.get_mut(index).map(|node| *node.atomic.get_mut())
{
Some(ptr) if ptr.is_null() => {
self.curr_table = Some((table, index + 1))
},
Some(ptr) if ptr as usize & 1 == 0 => {
let ptr = ptr as *mut Entry<T>;
self.curr_table = Some((table, index + 1));
// This is safe since:
//
// 1. We only store nodes with cleared lower bit if it is an
// entry.
//
// 2. We only delete stuff when we are behind mutable
// references *and* we are the only mutable reference to the
// TLS. We are not deleting it.
break Some(unsafe { &mut (*ptr).data });
},
Some(ptr) => {
let ptr = (ptr as usize & !1) as *mut Table<T>;
// Set it as table for the next iteration.
//
// 1. We only store nodes with marked lower bit if it is an
// table.
//
// 2. We cleared up the bit above so we can get the original
// pointer.
//
// 3. We only delete stuff when we are behind mutable
// references *and* we are the only mutable reference to the
// TLS. We are not deleting it.
self.tables.push(unsafe { &mut *ptr });
self.curr_table = Some((table, index + 1));
},
None => self.curr_table = self.tables.pop().map(|tbl| (tbl, 0)),
};
}
}
}
impl<'tls, T> fmt::Debug for IterMut<'tls, T> {
fn fmt(&self, fmtr: &mut fmt::Formatter) -> fmt::Result {
write!(
fmtr,
"IterMut {} tables: {:?}, curr_table: {:?} {}",
'{', self.tables, self.curr_table, '}'
)
}
}
/// An iterator over owned references to entries of TLS.
pub struct IntoIter<T> {
tables: Vec<OwnedAlloc<Table<T>>>,
curr_table: Option<(OwnedAlloc<Table<T>>, usize)>,
}
impl<T> Iterator for IntoIter<T> {
type Item = T;
fn next(&mut self) -> Option<Self::Item> {
loop {
let (mut table, index) = self.curr_table.take()?;
match table.nodes.get_mut(index).map(|node| *node.atomic.get_mut())
{
Some(ptr) if ptr.is_null() => {
self.curr_table = Some((table, index + 1))
},
Some(ptr) if ptr as usize & 1 == 0 => {
let ptr = ptr as *mut Entry<T>;
// This is safe since:
//
// 1. We only store nodes with cleared lower bit if it is an
// entry.
//
// 2. We have ownership over the TLS, so no one else is
// reading or writing or deleting.
let alloc = unsafe {
OwnedAlloc::from_raw(NonNull::new_unchecked(ptr))
};
let (entry, _) = alloc.move_inner();
self.curr_table = Some((table, index + 1));
break Some(entry.data);
},
Some(ptr) => {
let ptr = (ptr as usize & !1) as *mut Table<T>;
// This is safe since:
//
// 1. We only store nodes with marked lower bit if it is an
// table.
//
// 2. We have ownership over the TLS, so no one else is
// reading or writing or deleting.
self.tables.push(unsafe {
OwnedAlloc::from_raw(NonNull::new_unchecked(ptr))
});
self.curr_table = Some((table, index + 1));
},
None => self.curr_table = self.tables.pop().map(|tbl| (tbl, 0)),
};
}
}
}
impl<T> fmt::Debug for IntoIter<T> {
fn fmt(&self, fmtr: &mut fmt::Formatter) -> fmt::Result {
write!(
fmtr,
"IterMut {} tables: {:?}, curr_table: {:?} {}",
'{', self.tables, self.curr_table, '}'
)
}
}
struct Node<T> {
// lower bit marked 0 for Entry, 1 for Table
atomic: AtomicPtr<()>,
_marker: PhantomData<T>,
}
impl<T> Node<T> {
// Unsafe because it is *pretty easy* to make undefined behavior out of this
// because the pointer does not have even a fixed type.
unsafe fn free_ptr(
ptr: *mut (),
tbl_stack: &mut Vec<OwnedAlloc<Table<T>>>,
) {
if ptr.is_null() {
return;
}
if ptr as usize & 1 == 0 {
OwnedAlloc::from_raw(NonNull::new_unchecked(ptr as *mut Entry<T>));
} else {
let table_ptr = (ptr as usize & !1) as *mut Table<T>;
debug_assert!(!table_ptr.is_null());
tbl_stack
.push(OwnedAlloc::from_raw(NonNull::new_unchecked(table_ptr)));
}
}
}
impl<T> fmt::Debug for Node<T> {
fn fmt(&self, fmtr: &mut fmt::Formatter) -> fmt::Result {
write!(fmtr, "Node {} pointer: {:?} {}", '{', self.atomic, '}')
}
}
#[repr(align(/* at least */ 2))]
struct Table<T> {
nodes: [Node<T>; 1 << BITS],
}
impl<T> Table<T> {
#[inline]
fn new_alloc() -> OwnedAlloc<Self> {
// Safe because it calls a correctly a function which correctly
// initializes uninitialized memory with, indeed, uninitialized memory.
unsafe { UninitAlloc::<Self>::new().init_in_place(|this| this.init()) }
}
// Unsafe because passing ininitialized memory may cause leaks.
#[inline]
unsafe fn init(&mut self) {
for node_ref in &mut self.nodes as &mut [_] {
(node_ref as *mut Node<T>).write(Node {
atomic: AtomicPtr::new(null_mut()),
_marker: PhantomData,
})
}
}
// Unsafe because calling this function and using the table again later will
// cause undefined behavior.
#[inline]
unsafe fn free_nodes(&mut self, tbl_stack: &mut Vec<OwnedAlloc<Table<T>>>) {
for node in &mut self.nodes as &mut [Node<T>] {
Node::free_ptr(*node.atomic.get_mut(), tbl_stack);
}
}
// Unsafe because storing the wrong pointers in the table will lead to
// undefined behavior.
#[inline]
unsafe fn clear(&mut self, tbl_stack: &mut Vec<OwnedAlloc<Table<T>>>) {
for node in &mut self.nodes as &mut [Node<T>] {
let ptr = node.atomic.get_mut();
Node::free_ptr(*ptr, tbl_stack);
*ptr = null_mut();
}
}
}
impl<T> fmt::Debug for Table<T> {
fn fmt(&self, fmtr: &mut fmt::Formatter) -> fmt::Result {
write!(
fmtr,
"Table {} nodes: {:?} {}",
'{', &self.nodes as &[Node<T>], '}'
)
}
}
#[repr(align(64))]
struct Entry<T> {
data: T,
id: ThreadId,
}
enum LazyInit<T, F> {
Done(NonNull<T>),
Pending(F),
}
impl<T, F> LazyInit<T, F>
where
F: FnOnce() -> T,
{
fn is_pending(&self) -> bool {
match self {
LazyInit::Pending(_) => true,
_ => false,
}
}
fn get(&mut self) -> NonNull<T> {
let old = replace(self, LazyInit::Done(NonNull::dangling()));
let ptr = match old {
LazyInit::Done(ptr) => ptr,
LazyInit::Pending(init) => OwnedAlloc::new(init()).into_raw(),
};
*self = LazyInit::Done(ptr);
ptr
}
}
#[cfg(test)]
mod test {
use super::ThreadLocal;
use std::{
sync::{Arc, Barrier},
thread,
};
#[test]
fn threads_with_their_id() {
const THREADS: usize = 32;
let tls = Arc::new(ThreadLocal::new());
let mut threads = Vec::with_capacity(THREADS);
// prevent IDs from being reused.
let barrier = Arc::new(Barrier::new(THREADS));
for i in 0 .. THREADS {
let tls = tls.clone();
let barrier = barrier.clone();
threads.push(thread::spawn(move || {
assert_eq!(*tls.with_init(|| i), i);
barrier.wait();
}))
}
for thread in threads {
thread.join().unwrap();
}
}
#[test]
fn iter() {
const THREADS: usize = 32;
let tls = Arc::new(ThreadLocal::new());
let mut threads = Vec::with_capacity(THREADS);
// prevent IDs from being reused.
let barrier = Arc::new(Barrier::new(THREADS));
for i in 0 .. THREADS {
let tls = tls.clone();
let barrier = barrier.clone();
threads.push(thread::spawn(move || {
tls.with_init(|| i);
barrier.wait();
}))
}
for entry in &*tls {
assert!(*entry < THREADS);
}
}
#[test]
fn iter_mut() {
const THREADS: usize = 32;
let tls = Arc::new(ThreadLocal::new());
let mut threads = Vec::with_capacity(THREADS);
// prevent IDs from being reused.
let barrier = Arc::new(Barrier::new(THREADS));
for i in 0 .. THREADS {
let tls = tls.clone();
let barrier = barrier.clone();
threads.push(thread::spawn(move || {
tls.with_init(|| i);
barrier.wait();
}))
}
for thread in threads {
thread.join().unwrap();
}
let mut done = [0; THREADS];
let mut tls = Arc::try_unwrap(tls).unwrap();
for entry in &mut tls {
done[*entry] += 1;
*entry = (*entry + 1) % THREADS;
}
for entry in tls {
done[entry] += 1;
}
for &status in &done as &[_] {
assert_eq!(status, 2);
}
}
}