Source code
Revision control
Copy as Markdown
Other Tools
//! This mod provides the logic for the inner tree structure of the CancellationToken.
//!
//! CancellationTokens are only light handles with references to TreeNode.
//! All the logic is actually implemented in the TreeNode.
//!
//! A TreeNode is part of the cancellation tree and may have one parent and an arbitrary number of
//! children.
//!
//! A TreeNode can receive the request to perform a cancellation through a CancellationToken.
//! This cancellation request will cancel the node and all of its descendants.
//!
//! As soon as a node cannot get cancelled any more (because it was already cancelled or it has no
//! more CancellationTokens pointing to it any more), it gets removed from the tree, to keep the
//! tree as small as possible.
//!
//! # Invariants
//!
//! Those invariants shall be true at any time.
//!
//! 1. A node that has no parents and no handles can no longer be cancelled.
//! This is important during both cancellation and refcounting.
//!
//! 2. If node B *is* or *was* a child of node A, then node B was created *after* node A.
//! This is important for deadlock safety, as it is used for lock order.
//! Node B can only become the child of node A in two ways:
//! - being created with `child_node()`, in which case it is trivially true that
//! node A already existed when node B was created
//! - being moved A->C->B to A->B because node C was removed in `decrease_handle_refcount()`
//! or `cancel()`. In this case the invariant still holds, as B was younger than C, and C
//! was younger than A, therefore B is also younger than A.
//!
//! 3. If two nodes are both unlocked and node A is the parent of node B, then node B is a child of
//! node A. It is important to always restore that invariant before dropping the lock of a node.
//!
//! # Deadlock safety
//!
//! We always lock in the order of creation time. We can prove this through invariant #2.
//! Specifically, through invariant #2, we know that we always have to lock a parent
//! before its child.
//!
use crate::loom::sync::{Arc, Mutex, MutexGuard};
/// A node of the cancellation tree structure
///
/// The actual data it holds is wrapped inside a mutex for synchronization.
pub(crate) struct TreeNode {
inner: Mutex<Inner>,
waker: tokio::sync::Notify,
}
impl TreeNode {
pub(crate) fn new() -> Self {
Self {
inner: Mutex::new(Inner {
parent: None,
parent_idx: 0,
children: vec![],
is_cancelled: false,
num_handles: 1,
}),
waker: tokio::sync::Notify::new(),
}
}
pub(crate) fn notified(&self) -> tokio::sync::futures::Notified<'_> {
self.waker.notified()
}
}
/// The data contained inside a TreeNode.
///
/// This struct exists so that the data of the node can be wrapped
/// in a Mutex.
struct Inner {
parent: Option<Arc<TreeNode>>,
parent_idx: usize,
children: Vec<Arc<TreeNode>>,
is_cancelled: bool,
num_handles: usize,
}
/// Returns whether or not the node is cancelled
pub(crate) fn is_cancelled(node: &Arc<TreeNode>) -> bool {
node.inner.lock().unwrap().is_cancelled
}
/// Creates a child node
pub(crate) fn child_node(parent: &Arc<TreeNode>) -> Arc<TreeNode> {
let mut locked_parent = parent.inner.lock().unwrap();
// Do not register as child if we are already cancelled.
// Cancelled trees can never be uncancelled and therefore
// need no connection to parents or children any more.
if locked_parent.is_cancelled {
return Arc::new(TreeNode {
inner: Mutex::new(Inner {
parent: None,
parent_idx: 0,
children: vec![],
is_cancelled: true,
num_handles: 1,
}),
waker: tokio::sync::Notify::new(),
});
}
let child = Arc::new(TreeNode {
inner: Mutex::new(Inner {
parent: Some(parent.clone()),
parent_idx: locked_parent.children.len(),
children: vec![],
is_cancelled: false,
num_handles: 1,
}),
waker: tokio::sync::Notify::new(),
});
locked_parent.children.push(child.clone());
child
}
/// Disconnects the given parent from all of its children.
///
/// Takes a reference to [Inner] to make sure the parent is already locked.
fn disconnect_children(node: &mut Inner) {
for child in std::mem::take(&mut node.children) {
let mut locked_child = child.inner.lock().unwrap();
locked_child.parent_idx = 0;
locked_child.parent = None;
}
}
/// Figures out the parent of the node and locks the node and its parent atomically.
///
/// The basic principle of preventing deadlocks in the tree is
/// that we always lock the parent first, and then the child.
/// For more info look at *deadlock safety* and *invariant #2*.
///
/// Sadly, it's impossible to figure out the parent of a node without
/// locking it. To then achieve locking order consistency, the node
/// has to be unlocked before the parent gets locked.
/// This leaves a small window where we already assume that we know the parent,
/// but neither the parent nor the node is locked. Therefore, the parent could change.
///
/// To prevent that this problem leaks into the rest of the code, it is abstracted
/// in this function.
///
/// The locked child and optionally its locked parent, if a parent exists, get passed
/// to the `func` argument via (node, None) or (node, Some(parent)).
fn with_locked_node_and_parent<F, Ret>(node: &Arc<TreeNode>, func: F) -> Ret
where
F: FnOnce(MutexGuard<'_, Inner>, Option<MutexGuard<'_, Inner>>) -> Ret,
{
let mut potential_parent = {
let locked_node = node.inner.lock().unwrap();
match locked_node.parent.clone() {
Some(parent) => parent,
// If we locked the node and its parent is `None`, we are in a valid state
// and can return.
None => return func(locked_node, None),
}
};
loop {
// Deadlock safety:
//
// Due to invariant #2, we know that we have to lock the parent first, and then the child.
// This is true even if the potential_parent is no longer the current parent or even its
// sibling, as the invariant still holds.
let locked_parent = potential_parent.inner.lock().unwrap();
let locked_node = node.inner.lock().unwrap();
let actual_parent = match locked_node.parent.clone() {
Some(parent) => parent,
// If we locked the node and its parent is `None`, we are in a valid state
// and can return.
None => {
// Was the wrong parent, so unlock it before calling `func`
drop(locked_parent);
return func(locked_node, None);
}
};
// Loop until we managed to lock both the node and its parent
if Arc::ptr_eq(&actual_parent, &potential_parent) {
return func(locked_node, Some(locked_parent));
}
// Drop locked_parent before reassigning to potential_parent,
// as potential_parent is borrowed in it
drop(locked_node);
drop(locked_parent);
potential_parent = actual_parent;
}
}
/// Moves all children from `node` to `parent`.
///
/// `parent` MUST have been a parent of the node when they both got locked,
/// otherwise there is a potential for a deadlock as invariant #2 would be violated.
///
/// To aquire the locks for node and parent, use [with_locked_node_and_parent].
fn move_children_to_parent(node: &mut Inner, parent: &mut Inner) {
// Pre-allocate in the parent, for performance
parent.children.reserve(node.children.len());
for child in std::mem::take(&mut node.children) {
{
let mut child_locked = child.inner.lock().unwrap();
child_locked.parent = node.parent.clone();
child_locked.parent_idx = parent.children.len();
}
parent.children.push(child);
}
}
/// Removes a child from the parent.
///
/// `parent` MUST be the parent of `node`.
/// To aquire the locks for node and parent, use [with_locked_node_and_parent].
fn remove_child(parent: &mut Inner, mut node: MutexGuard<'_, Inner>) {
// Query the position from where to remove a node
let pos = node.parent_idx;
node.parent = None;
node.parent_idx = 0;
// Unlock node, so that only one child at a time is locked.
// Otherwise we would violate the lock order (see 'deadlock safety') as we
// don't know the creation order of the child nodes
drop(node);
// If `node` is the last element in the list, we don't need any swapping
if parent.children.len() == pos + 1 {
parent.children.pop().unwrap();
} else {
// If `node` is not the last element in the list, we need to
// replace it with the last element
let replacement_child = parent.children.pop().unwrap();
replacement_child.inner.lock().unwrap().parent_idx = pos;
parent.children[pos] = replacement_child;
}
let len = parent.children.len();
if 4 * len <= parent.children.capacity() {
// equal to:
// parent.children.shrink_to(2 * len);
// but shrink_to was not yet stabilized in our minimal compatible version
let old_children = std::mem::replace(&mut parent.children, Vec::with_capacity(2 * len));
parent.children.extend(old_children);
}
}
/// Increases the reference count of handles.
pub(crate) fn increase_handle_refcount(node: &Arc<TreeNode>) {
let mut locked_node = node.inner.lock().unwrap();
// Once no handles are left over, the node gets detached from the tree.
// There should never be a new handle once all handles are dropped.
assert!(locked_node.num_handles > 0);
locked_node.num_handles += 1;
}
/// Decreases the reference count of handles.
///
/// Once no handle is left, we can remove the node from the
/// tree and connect its parent directly to its children.
pub(crate) fn decrease_handle_refcount(node: &Arc<TreeNode>) {
let num_handles = {
let mut locked_node = node.inner.lock().unwrap();
locked_node.num_handles -= 1;
locked_node.num_handles
};
if num_handles == 0 {
with_locked_node_and_parent(node, |mut node, parent| {
// Remove the node from the tree
match parent {
Some(mut parent) => {
// As we want to remove ourselves from the tree,
// we have to move the children to the parent, so that
// they still receive the cancellation event without us.
// Moving them does not violate invariant #1.
move_children_to_parent(&mut node, &mut parent);
// Remove the node from the parent
remove_child(&mut parent, node);
}
None => {
// Due to invariant #1, we can assume that our
// children can no longer be cancelled through us.
// (as we now have neither a parent nor handles)
// Therefore we can disconnect them.
disconnect_children(&mut node);
}
}
});
}
}
/// Cancels a node and its children.
pub(crate) fn cancel(node: &Arc<TreeNode>) {
let mut locked_node = node.inner.lock().unwrap();
if locked_node.is_cancelled {
return;
}
// One by one, adopt grandchildren and then cancel and detach the child
while let Some(child) = locked_node.children.pop() {
// This can't deadlock because the mutex we are already
// holding is the parent of child.
let mut locked_child = child.inner.lock().unwrap();
// Detach the child from node
// No need to modify node.children, as the child already got removed with `.pop`
locked_child.parent = None;
locked_child.parent_idx = 0;
// If child is already cancelled, detaching is enough
if locked_child.is_cancelled {
continue;
}
// Cancel or adopt grandchildren
while let Some(grandchild) = locked_child.children.pop() {
// This can't deadlock because the two mutexes we are already
// holding is the parent and grandparent of grandchild.
let mut locked_grandchild = grandchild.inner.lock().unwrap();
// Detach the grandchild
locked_grandchild.parent = None;
locked_grandchild.parent_idx = 0;
// If grandchild is already cancelled, detaching is enough
if locked_grandchild.is_cancelled {
continue;
}
// For performance reasons, only adopt grandchildren that have children.
// Otherwise, just cancel them right away, no need for another iteration.
if locked_grandchild.children.is_empty() {
// Cancel the grandchild
locked_grandchild.is_cancelled = true;
locked_grandchild.children = Vec::new();
drop(locked_grandchild);
grandchild.waker.notify_waiters();
} else {
// Otherwise, adopt grandchild
locked_grandchild.parent = Some(node.clone());
locked_grandchild.parent_idx = locked_node.children.len();
drop(locked_grandchild);
locked_node.children.push(grandchild);
}
}
// Cancel the child
locked_child.is_cancelled = true;
locked_child.children = Vec::new();
drop(locked_child);
child.waker.notify_waiters();
// Now the child is cancelled and detached and all its children are adopted.
// Just continue until all (including adopted) children are cancelled and detached.
}
// Cancel the node itself.
locked_node.is_cancelled = true;
locked_node.children = Vec::new();
drop(locked_node);
node.waker.notify_waiters();
}