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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
#![allow(unsafe_code)]
use crate::applicable_declarations::CascadePriority;
use crate::shared_lock::StylesheetGuards;
use crate::stylesheets::layer_rule::LayerOrder;
use malloc_size_of::{MallocShallowSizeOf, MallocSizeOf, MallocSizeOfOps};
use parking_lot::RwLock;
use smallvec::SmallVec;
use std::fmt;
use std::hash;
use std::io::Write;
use std::mem;
use std::ptr;
use std::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
use super::map::{Entry, Map};
use super::unsafe_box::UnsafeBox;
use super::{CascadeLevel, StyleSource};
/// The rule tree, the structure servo uses to preserve the results of selector
/// matching.
///
/// This is organized as a tree of rules. When a node matches a set of rules,
/// they're inserted in order in the tree, starting with the less specific one.
///
/// When a rule is inserted in the tree, other elements may share the path up to
/// a given rule. If that's the case, we don't duplicate child nodes, but share
/// them.
///
/// When the rule node refcount drops to zero, it doesn't get freed. It gets
/// instead put into a free list, and it is potentially GC'd after a while.
///
/// That way, a rule node that represents a likely-to-match-again rule (like a
/// :hover rule) can be reused if we haven't GC'd it yet.
#[derive(Debug)]
pub struct RuleTree {
root: StrongRuleNode,
}
impl Drop for RuleTree {
fn drop(&mut self) {
unsafe { self.swap_free_list_and_gc(ptr::null_mut()) }
}
}
impl MallocSizeOf for RuleTree {
fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
let mut n = 0;
let mut stack = SmallVec::<[_; 32]>::new();
stack.push(self.root.clone());
while let Some(node) = stack.pop() {
n += unsafe { ops.malloc_size_of(&*node.p) };
let children = node.p.children.read();
children.shallow_size_of(ops);
for c in &*children {
stack.push(unsafe { c.upgrade() });
}
}
n
}
}
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
struct ChildKey(CascadePriority, ptr::NonNull<()>);
unsafe impl Send for ChildKey {}
unsafe impl Sync for ChildKey {}
impl RuleTree {
/// Construct a new rule tree.
pub fn new() -> Self {
RuleTree {
root: StrongRuleNode::new(Box::new(RuleNode::root())),
}
}
/// Get the root rule node.
pub fn root(&self) -> &StrongRuleNode {
&self.root
}
/// This can only be called when no other threads is accessing this tree.
pub fn gc(&self) {
unsafe { self.swap_free_list_and_gc(RuleNode::DANGLING_PTR) }
}
/// This can only be called when no other threads is accessing this tree.
pub fn maybe_gc(&self) {
#[cfg(debug_assertions)]
self.maybe_dump_stats();
if self.root.p.approximate_free_count.load(Ordering::Relaxed) > RULE_TREE_GC_INTERVAL {
self.gc();
}
}
#[cfg(debug_assertions)]
fn maybe_dump_stats(&self) {
use itertools::Itertools;
use std::cell::Cell;
use std::time::{Duration, Instant};
if !log_enabled!(log::Level::Trace) {
return;
}
const RULE_TREE_STATS_INTERVAL: Duration = Duration::from_secs(2);
thread_local! {
pub static LAST_STATS: Cell<Instant> = Cell::new(Instant::now());
};
let should_dump = LAST_STATS.with(|s| {
let now = Instant::now();
if now.duration_since(s.get()) < RULE_TREE_STATS_INTERVAL {
return false;
}
s.set(now);
true
});
if !should_dump {
return;
}
let mut children_count = fxhash::FxHashMap::default();
let mut stack = SmallVec::<[_; 32]>::new();
stack.push(self.root.clone());
while let Some(node) = stack.pop() {
let children = node.p.children.read();
*children_count.entry(children.len()).or_insert(0) += 1;
for c in &*children {
stack.push(unsafe { c.upgrade() });
}
}
trace!("Rule tree stats:");
let counts = children_count.keys().sorted();
for count in counts {
trace!(" {} - {}", count, children_count[count]);
}
}
/// Steals the free list and drops its contents.
unsafe fn swap_free_list_and_gc(&self, ptr: *mut RuleNode) {
let root = &self.root.p;
debug_assert!(!root.next_free.load(Ordering::Relaxed).is_null());
// Reset the approximate free count to zero, as we are going to steal
// the free list.
root.approximate_free_count.store(0, Ordering::Relaxed);
// Steal the free list head. Memory loads on nodes while iterating it
// must observe any prior changes that occured so this requires
// acquire ordering, but there are no writes that need to be kept
// before this swap so there is no need for release.
let mut head = root.next_free.swap(ptr, Ordering::Acquire);
while head != RuleNode::DANGLING_PTR {
debug_assert!(!head.is_null());
let mut node = UnsafeBox::from_raw(head);
// The root node cannot go on the free list.
debug_assert!(node.root.is_some());
// The refcount of nodes on the free list never goes below 1.
debug_assert!(node.refcount.load(Ordering::Relaxed) > 0);
// No one else is currently writing to that field. Get the address
// of the next node in the free list and replace it with null,
// other threads will now consider that this node is not on the
// free list.
head = node.next_free.swap(ptr::null_mut(), Ordering::Relaxed);
// This release write synchronises with the acquire fence in
// `WeakRuleNode::upgrade`, making sure that if `upgrade` observes
// decrements the refcount to 0, it will also observe the
// `node.next_free` swap to null above.
if node.refcount.fetch_sub(1, Ordering::Release) == 1 {
// And given it observed the null swap above, it will need
// `pretend_to_be_on_free_list` to finish its job, writing
// `RuleNode::DANGLING_PTR` in `node.next_free`.
RuleNode::pretend_to_be_on_free_list(&node);
// Drop this node now that we just observed its refcount going
// down to zero.
RuleNode::drop_without_free_list(&mut node);
}
}
}
}
/// The number of RuleNodes added to the free list before we will consider
/// doing a GC when calling maybe_gc(). (The value is copied from Gecko,
/// where it likely did not result from a rigorous performance analysis.)
const RULE_TREE_GC_INTERVAL: usize = 300;
/// A node in the rule tree.
struct RuleNode {
/// The root node. Only the root has no root pointer, for obvious reasons.
root: Option<WeakRuleNode>,
/// The parent rule node. Only the root has no parent.
parent: Option<StrongRuleNode>,
/// The actual style source, either coming from a selector in a StyleRule,
/// or a raw property declaration block (like the style attribute).
///
/// None for the root node.
source: Option<StyleSource>,
/// The cascade level + layer order this rule is positioned at.
cascade_priority: CascadePriority,
/// The refcount of this node.
///
/// Starts at one. Incremented in `StrongRuleNode::clone` and
/// `WeakRuleNode::upgrade`. Decremented in `StrongRuleNode::drop`
/// and `RuleTree::swap_free_list_and_gc`.
///
/// If a non-root node's refcount reaches zero, it is incremented back to at
/// least one in `RuleNode::pretend_to_be_on_free_list` until the caller who
/// observed it dropping to zero had a chance to try to remove it from its
/// parent's children list.
///
/// The refcount should never be decremented to zero if the value in
/// `next_free` is not null.
refcount: AtomicUsize,
/// Only used for the root, stores the number of free rule nodes that are
/// around.
approximate_free_count: AtomicUsize,
/// The children of a given rule node. Children remove themselves from here
/// when they go away.
children: RwLock<Map<ChildKey, WeakRuleNode>>,
/// This field has two different meanings depending on whether this is the
/// root node or not.
///
/// If it is the root, it represents the head of the free list. It may be
/// null, which means the free list is gone because the tree was dropped,
/// and it may be `RuleNode::DANGLING_PTR`, which means the free list is
/// empty.
///
/// If it is not the root node, this field is either null if the node is
/// not on the free list, `RuleNode::DANGLING_PTR` if it is the last item
/// on the free list or the node is pretending to be on the free list, or
/// any valid non-null pointer representing the next item on the free list
/// after this one.
///
/// See `RuleNode::push_on_free_list`, `swap_free_list_and_gc`, and
/// `WeakRuleNode::upgrade`.
///
/// Two threads should never attempt to put the same node on the free list
/// both at the same time.
next_free: AtomicPtr<RuleNode>,
}
// On Gecko builds, hook into the leak checking machinery.
#[cfg(feature = "gecko_refcount_logging")]
mod gecko_leak_checking {
use super::RuleNode;
use std::mem::size_of;
use std::os::raw::{c_char, c_void};
extern "C" {
fn NS_LogCtor(aPtr: *mut c_void, aTypeName: *const c_char, aSize: u32);
fn NS_LogDtor(aPtr: *mut c_void, aTypeName: *const c_char, aSize: u32);
}
static NAME: &'static [u8] = b"RuleNode\0";
/// Logs the creation of a heap-allocated object to Gecko's leak-checking machinery.
pub(super) fn log_ctor(ptr: *const RuleNode) {
let s = NAME as *const [u8] as *const u8 as *const c_char;
unsafe {
NS_LogCtor(ptr as *mut c_void, s, size_of::<RuleNode>() as u32);
}
}
/// Logs the destruction of a heap-allocated object to Gecko's leak-checking machinery.
pub(super) fn log_dtor(ptr: *const RuleNode) {
let s = NAME as *const [u8] as *const u8 as *const c_char;
unsafe {
NS_LogDtor(ptr as *mut c_void, s, size_of::<RuleNode>() as u32);
}
}
}
#[inline(always)]
fn log_new(_ptr: *const RuleNode) {
#[cfg(feature = "gecko_refcount_logging")]
gecko_leak_checking::log_ctor(_ptr);
}
#[inline(always)]
fn log_drop(_ptr: *const RuleNode) {
#[cfg(feature = "gecko_refcount_logging")]
gecko_leak_checking::log_dtor(_ptr);
}
impl RuleNode {
const DANGLING_PTR: *mut Self = ptr::NonNull::dangling().as_ptr();
unsafe fn new(
root: WeakRuleNode,
parent: StrongRuleNode,
source: StyleSource,
cascade_priority: CascadePriority,
) -> Self {
debug_assert!(root.p.parent.is_none());
RuleNode {
root: Some(root),
parent: Some(parent),
source: Some(source),
cascade_priority,
refcount: AtomicUsize::new(1),
children: Default::default(),
approximate_free_count: AtomicUsize::new(0),
next_free: AtomicPtr::new(ptr::null_mut()),
}
}
fn root() -> Self {
RuleNode {
root: None,
parent: None,
source: None,
cascade_priority: CascadePriority::new(CascadeLevel::UANormal, LayerOrder::root()),
refcount: AtomicUsize::new(1),
approximate_free_count: AtomicUsize::new(0),
children: Default::default(),
next_free: AtomicPtr::new(RuleNode::DANGLING_PTR),
}
}
fn key(&self) -> ChildKey {
ChildKey(
self.cascade_priority,
self.source
.as_ref()
.expect("Called key() on the root node")
.key(),
)
}
/// Drops a node without ever putting it on the free list.
///
/// Note that the node may not be dropped if we observe that its refcount
/// isn't zero anymore when we write-lock its parent's children map to
/// remove it.
///
/// This loops over parents of dropped nodes if their own refcount reaches
/// zero to avoid recursion when dropping deep hierarchies of nodes.
///
/// For non-root nodes, this should always be preceded by a call of
/// `RuleNode::pretend_to_be_on_free_list`.
unsafe fn drop_without_free_list(this: &mut UnsafeBox<Self>) {
// We clone the box and shadow the original one to be able to loop
// over its ancestors if they also need to be dropped.
let mut this = UnsafeBox::clone(this);
loop {
// If the node has a parent, we need to remove it from its parent's
// children list.
if let Some(parent) = this.parent.as_ref() {
debug_assert!(!this.next_free.load(Ordering::Relaxed).is_null());
// We lock the parent's children list, which means no other
// thread will have any more opportunity to resurrect the node
// anymore.
let mut children = parent.p.children.write();
this.next_free.store(ptr::null_mut(), Ordering::Relaxed);
// We decrement the counter to remove the "pretend to be
// on the free list" reference.
let old_refcount = this.refcount.fetch_sub(1, Ordering::Release);
debug_assert!(old_refcount != 0);
if old_refcount != 1 {
// Other threads resurrected this node and those references
// are still alive, we have nothing to do anymore.
return;
}
// We finally remove the node from its parent's children list,
// there are now no other references to it and it cannot
// be resurrected anymore even after we unlock the list.
debug!(
"Remove from child list: {:?}, parent: {:?}",
this.as_mut_ptr(),
this.parent.as_ref().map(|p| p.p.as_mut_ptr())
);
let weak = children.remove(&this.key(), |node| node.p.key()).unwrap();
assert_eq!(weak.p.as_mut_ptr(), this.as_mut_ptr());
} else {
debug_assert_eq!(this.next_free.load(Ordering::Relaxed), ptr::null_mut());
debug_assert_eq!(this.refcount.load(Ordering::Relaxed), 0);
}
// We are going to drop this node for good this time, as per the
// usual refcounting protocol we need an acquire fence here before
// we run the destructor.
//
// for why it doesn't matter whether this is a load or a fence.
atomic::fence(Ordering::Acquire);
// Remove the parent reference from the child to avoid
// recursively dropping it and putting it on the free list.
let parent = UnsafeBox::deref_mut(&mut this).parent.take();
// We now drop the actual box and its contents, no one should
// access the current value in `this` anymore.
log_drop(&*this);
UnsafeBox::drop(&mut this);
if let Some(parent) = parent {
// We will attempt to drop the node's parent without the free
// list, so we clone the inner unsafe box and forget the
// original parent to avoid running its `StrongRuleNode`
// destructor which would attempt to use the free list if it
// still exists.
this = UnsafeBox::clone(&parent.p);
mem::forget(parent);
if this.refcount.fetch_sub(1, Ordering::Release) == 1 {
debug_assert_eq!(this.next_free.load(Ordering::Relaxed), ptr::null_mut());
if this.root.is_some() {
RuleNode::pretend_to_be_on_free_list(&this);
}
// Parent also reached refcount zero, we loop to drop it.
continue;
}
}
return;
}
}
/// Pushes this node on the tree's free list. Returns false if the free list
/// is gone. Should only be called after we decremented a node's refcount
/// to zero and pretended to be on the free list.
unsafe fn push_on_free_list(this: &UnsafeBox<Self>) -> bool {
let root = &this.root.as_ref().unwrap().p;
debug_assert!(this.refcount.load(Ordering::Relaxed) > 0);
debug_assert_eq!(this.next_free.load(Ordering::Relaxed), Self::DANGLING_PTR);
// Increment the approximate free count by one.
root.approximate_free_count.fetch_add(1, Ordering::Relaxed);
// If the compare-exchange operation fails in the loop, we will retry
// with the new head value, so this can be a relaxed load.
let mut head = root.next_free.load(Ordering::Relaxed);
while !head.is_null() {
// Two threads can never attempt to push the same node on the free
// list both at the same time, so whoever else pushed a node on the
// free list cannot have done so with this node.
debug_assert_ne!(head, this.as_mut_ptr());
// Store the current head of the free list in this node.
this.next_free.store(head, Ordering::Relaxed);
// Any thread acquiring the free list must observe the previous
// next_free changes that occured, hence the release ordering
// on success.
match root.next_free.compare_exchange_weak(
head,
this.as_mut_ptr(),
Ordering::Release,
Ordering::Relaxed,
) {
Ok(_) => {
// This node is now on the free list, caller should not use
// the node anymore.
return true;
},
Err(new_head) => head = new_head,
}
}
// Tree was dropped and free list has been destroyed. We did not push
// this node on the free list but we still pretend to be on the free
// list to be ready to call `drop_without_free_list`.
false
}
/// Makes the node pretend to be on the free list. This will increment the
/// refcount by 1 and store `Self::DANGLING_PTR` in `next_free`. This
/// method should only be called after caller decremented the refcount to
/// zero, with the null pointer stored in `next_free`.
unsafe fn pretend_to_be_on_free_list(this: &UnsafeBox<Self>) {
debug_assert_eq!(this.next_free.load(Ordering::Relaxed), ptr::null_mut());
this.refcount.fetch_add(1, Ordering::Relaxed);
this.next_free.store(Self::DANGLING_PTR, Ordering::Release);
}
fn as_mut_ptr(&self) -> *mut RuleNode {
self as *const RuleNode as *mut RuleNode
}
}
pub(crate) struct WeakRuleNode {
p: UnsafeBox<RuleNode>,
}
/// A strong reference to a rule node.
pub struct StrongRuleNode {
p: UnsafeBox<RuleNode>,
}
#[cfg(feature = "servo")]
malloc_size_of_is_0!(StrongRuleNode);
impl StrongRuleNode {
fn new(n: Box<RuleNode>) -> Self {
debug_assert_eq!(n.parent.is_none(), !n.source.is_some());
log_new(&*n);
debug!("Creating rule node: {:p}", &*n);
Self {
p: UnsafeBox::from_box(n),
}
}
unsafe fn from_unsafe_box(p: UnsafeBox<RuleNode>) -> Self {
Self { p }
}
unsafe fn downgrade(&self) -> WeakRuleNode {
WeakRuleNode {
p: UnsafeBox::clone(&self.p),
}
}
/// Get the parent rule node of this rule node.
pub fn parent(&self) -> Option<&StrongRuleNode> {
self.p.parent.as_ref()
}
pub(super) fn ensure_child(
&self,
root: &StrongRuleNode,
source: StyleSource,
cascade_priority: CascadePriority,
) -> StrongRuleNode {
use parking_lot::RwLockUpgradableReadGuard;
debug_assert!(
self.p.cascade_priority <= cascade_priority,
"Should be ordered (instead {:?} > {:?}), from {:?} and {:?}",
self.p.cascade_priority,
cascade_priority,
self.p.source,
source,
);
let key = ChildKey(cascade_priority, source.key());
let children = self.p.children.upgradable_read();
if let Some(child) = children.get(&key, |node| node.p.key()) {
// Sound to call because we read-locked the parent's children.
return unsafe { child.upgrade() };
}
let mut children = RwLockUpgradableReadGuard::upgrade(children);
match children.entry(key, |node| node.p.key()) {
Entry::Occupied(child) => {
// Sound to call because we write-locked the parent's children.
unsafe { child.upgrade() }
},
Entry::Vacant(entry) => unsafe {
let node = StrongRuleNode::new(Box::new(RuleNode::new(
root.downgrade(),
self.clone(),
source,
cascade_priority,
)));
// Sound to call because we still own a strong reference to
// this node, through the `node` variable itself that we are
// going to return to the caller.
entry.insert(node.downgrade());
node
},
}
}
/// Get the style source corresponding to this rule node. May return `None`
/// if it's the root node, which means that the node hasn't matched any
/// rules.
pub fn style_source(&self) -> Option<&StyleSource> {
self.p.source.as_ref()
}
/// The cascade priority.
#[inline]
pub fn cascade_priority(&self) -> CascadePriority {
self.p.cascade_priority
}
/// The cascade level.
#[inline]
pub fn cascade_level(&self) -> CascadeLevel {
self.cascade_priority().cascade_level()
}
/// The importance.
#[inline]
pub fn importance(&self) -> crate::properties::Importance {
self.cascade_level().importance()
}
/// Returns whether this node has any child, only intended for testing
/// purposes.
pub unsafe fn has_children_for_testing(&self) -> bool {
!self.p.children.read().is_empty()
}
pub(super) fn dump<W: Write>(&self, guards: &StylesheetGuards, writer: &mut W, indent: usize) {
const INDENT_INCREMENT: usize = 4;
for _ in 0..indent {
let _ = write!(writer, " ");
}
let _ = writeln!(
writer,
" - {:p} (ref: {:?}, parent: {:?})",
&*self.p,
self.p.refcount.load(Ordering::Relaxed),
self.parent().map(|p| &*p.p as *const RuleNode)
);
for _ in 0..indent {
let _ = write!(writer, " ");
}
if let Some(source) = self.style_source() {
source.dump(self.cascade_level().guard(guards), writer);
} else {
if indent != 0 {
warn!("How has this happened?");
}
let _ = write!(writer, "(root)");
}
let _ = write!(writer, "\n");
for child in &*self.p.children.read() {
unsafe {
child
.upgrade()
.dump(guards, writer, indent + INDENT_INCREMENT);
}
}
}
}
impl Clone for StrongRuleNode {
fn clone(&self) -> Self {
debug!(
"{:p}: {:?}+",
&*self.p,
self.p.refcount.load(Ordering::Relaxed)
);
debug_assert!(self.p.refcount.load(Ordering::Relaxed) > 0);
self.p.refcount.fetch_add(1, Ordering::Relaxed);
unsafe { StrongRuleNode::from_unsafe_box(UnsafeBox::clone(&self.p)) }
}
}
impl Drop for StrongRuleNode {
#[cfg_attr(feature = "servo", allow(unused_mut))]
fn drop(&mut self) {
let node = &*self.p;
debug!("{:p}: {:?}-", node, node.refcount.load(Ordering::Relaxed));
debug!(
"Dropping node: {:p}, root: {:?}, parent: {:?}",
node,
node.root.as_ref().map(|r| &*r.p as *const RuleNode),
node.parent.as_ref().map(|p| &*p.p as *const RuleNode)
);
let should_drop = {
debug_assert!(node.refcount.load(Ordering::Relaxed) > 0);
node.refcount.fetch_sub(1, Ordering::Release) == 1
};
if !should_drop {
// The refcount didn't even drop zero yet, there is nothing for us
// to do anymore.
return;
}
unsafe {
if node.root.is_some() {
// This is a non-root node and we just observed the refcount
// dropping to zero, we need to pretend to be on the free list
// to unstuck any thread who tried to resurrect this node first
// through `WeakRuleNode::upgrade`.
RuleNode::pretend_to_be_on_free_list(&self.p);
// Attempt to push the node on the free list. This may fail
// if the free list is gone.
if RuleNode::push_on_free_list(&self.p) {
return;
}
}
// Either this was the last reference of the root node, or the
// tree rule is gone and there is no free list anymore. Drop the
// node.
RuleNode::drop_without_free_list(&mut self.p);
}
}
}
impl WeakRuleNode {
/// Upgrades this weak node reference, returning a strong one.
///
/// Must be called with items stored in a node's children list. The children
/// list must at least be read-locked when this is called.
unsafe fn upgrade(&self) -> StrongRuleNode {
debug!("Upgrading weak node: {:p}", &*self.p);
if self.p.refcount.fetch_add(1, Ordering::Relaxed) == 0 {
// We observed a refcount of 0, we need to wait for this node to
// be put on the free list. Resetting the `next_free` pointer to
// null is only done in `RuleNode::drop_without_free_list`, just
// before a release refcount decrement, so this acquire fence here
// makes sure that we observed the write to null before we loop
// until there is a non-null value.
atomic::fence(Ordering::Acquire);
while self.p.next_free.load(Ordering::Relaxed).is_null() {}
}
StrongRuleNode::from_unsafe_box(UnsafeBox::clone(&self.p))
}
}
impl fmt::Debug for StrongRuleNode {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
(&*self.p as *const RuleNode).fmt(f)
}
}
impl Eq for StrongRuleNode {}
impl PartialEq for StrongRuleNode {
fn eq(&self, other: &Self) -> bool {
&*self.p as *const RuleNode == &*other.p
}
}
impl hash::Hash for StrongRuleNode {
fn hash<H>(&self, state: &mut H)
where
H: hash::Hasher,
{
(&*self.p as *const RuleNode).hash(state)
}
}
// Large pages generate thousands of RuleNode objects.
size_of_test!(RuleNode, 80);
// StrongRuleNode should be pointer-sized even inside an option.
size_of_test!(Option<StrongRuleNode>, 8);