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more algorithm-independent GC infra #7194
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GC-infra independent to EBR
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,4 +1,17 @@ | ||
| use crate::atomic::{Ordering, PyAtomic, Radium}; | ||
|
|
||
| // State layout (usize): | ||
| // [1 bit: destructed] [1 bit: reserved] [1 bit: leaked] [N bits: weak_count] [M bits: strong_count] | ||
| // 64-bit: N=30, M=31. 32-bit: N=14, M=15. | ||
| const FLAG_BITS: u32 = 3; | ||
| const DESTRUCTED: usize = 1 << (usize::BITS - 1); | ||
| const LEAKED: usize = 1 << (usize::BITS - 3); | ||
| const TOTAL_COUNT_WIDTH: u32 = usize::BITS - FLAG_BITS; | ||
| const WEAK_WIDTH: u32 = TOTAL_COUNT_WIDTH / 2; | ||
| const STRONG_WIDTH: u32 = TOTAL_COUNT_WIDTH - WEAK_WIDTH; | ||
| const STRONG: usize = (1 << STRONG_WIDTH) - 1; | ||
| const COUNT: usize = 1; | ||
| const WEAK_COUNT: usize = 1 << STRONG_WIDTH; | ||
|
|
||
| #[inline(never)] | ||
| #[cold] | ||
|
|
@@ -9,15 +22,56 @@ fn refcount_overflow() -> ! { | |
| core::panic!("refcount overflow"); | ||
| } | ||
|
|
||
| /// State wraps reference count + flags in a single word (platform usize) | ||
| #[derive(Clone, Copy)] | ||
| struct State { | ||
| inner: usize, | ||
| } | ||
|
|
||
| impl State { | ||
| #[inline] | ||
| fn from_raw(inner: usize) -> Self { | ||
| Self { inner } | ||
| } | ||
|
|
||
| #[inline] | ||
| fn as_raw(self) -> usize { | ||
| self.inner | ||
| } | ||
|
|
||
| #[inline] | ||
| fn strong(self) -> u32 { | ||
| ((self.inner & STRONG) / COUNT) as u32 | ||
| } | ||
|
|
||
| #[inline] | ||
| fn destructed(self) -> bool { | ||
| (self.inner & DESTRUCTED) != 0 | ||
| } | ||
|
|
||
| #[inline] | ||
| fn leaked(self) -> bool { | ||
| (self.inner & LEAKED) != 0 | ||
| } | ||
|
|
||
| #[inline] | ||
| fn add_strong(self, val: u32) -> Self { | ||
| Self::from_raw(self.inner + (val as usize) * COUNT) | ||
| } | ||
|
|
||
| #[inline] | ||
| fn with_leaked(self, leaked: bool) -> Self { | ||
| Self::from_raw((self.inner & !LEAKED) | if leaked { LEAKED } else { 0 }) | ||
| } | ||
| } | ||
|
|
||
| /// Reference count using state layout with LEAKED support. | ||
| /// | ||
| /// State layout (usize): | ||
| /// 64-bit: [1 bit: destructed] [1 bit: reserved] [1 bit: leaked] [30 bits: weak_count] [31 bits: strong_count] | ||
| /// 32-bit: [1 bit: destructed] [1 bit: reserved] [1 bit: leaked] [14 bits: weak_count] [15 bits: strong_count] | ||
| pub struct RefCount { | ||
| state: PyAtomic<usize>, | ||
| } | ||
|
|
||
| impl Default for RefCount { | ||
|
|
@@ -27,71 +81,191 @@ impl Default for RefCount { | |
| } | ||
|
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||
| impl RefCount { | ||
| /// Create a new RefCount with strong count = 1 | ||
| pub fn new() -> Self { | ||
| // Initial state: strong=1, weak=1 (implicit weak for strong refs) | ||
| Self { | ||
| state: Radium::new(COUNT + WEAK_COUNT), | ||
| } | ||
| } | ||
|
|
||
| /// Get current strong count | ||
| #[inline] | ||
| pub fn get(&self) -> usize { | ||
| State::from_raw(self.state.load(Ordering::SeqCst)).strong() as usize | ||
| } | ||
|
|
||
| /// Increment strong count | ||
| #[inline] | ||
| pub fn inc(&self) { | ||
| let val = State::from_raw(self.state.fetch_add(COUNT, Ordering::SeqCst)); | ||
| if val.destructed() { | ||
| refcount_overflow(); | ||
| } | ||
| if val.strong() == 0 { | ||
| // The previous fetch_add created a permission to run decrement again | ||
| self.state.fetch_add(COUNT, Ordering::SeqCst); | ||
| } | ||
| } | ||
|
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||
|
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| #[inline] | ||
| pub fn inc_by(&self, n: usize) { | ||
| debug_assert!(n <= STRONG); | ||
| let val = State::from_raw(self.state.fetch_add(n * COUNT, Ordering::SeqCst)); | ||
| if val.destructed() || (val.strong() as usize) > STRONG - n { | ||
| refcount_overflow(); | ||
| } | ||
| } | ||
|
|
||
| /// Returns true if successful | ||
| #[inline] | ||
| #[must_use] | ||
| pub fn safe_inc(&self) -> bool { | ||
| let mut old = State::from_raw(self.state.load(Ordering::SeqCst)); | ||
| loop { | ||
| if old.destructed() { | ||
| return false; | ||
| } | ||
| if (old.strong() as usize) >= STRONG { | ||
| refcount_overflow(); | ||
| } | ||
| let new_state = old.add_strong(1); | ||
| match self.state.compare_exchange( | ||
| old.as_raw(), | ||
| new_state.as_raw(), | ||
| Ordering::SeqCst, | ||
| Ordering::SeqCst, | ||
| ) { | ||
| Ok(_) => return true, | ||
| Err(curr) => old = State::from_raw(curr), | ||
| } | ||
| } | ||
|
Comment on lines
120
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+142
Contributor
There was a problem hiding this comment. Guard
♻️ Suggested fix pub fn safe_inc(&self) -> bool {
let mut old = State::from_raw(self.state.load(Ordering::SeqCst));
loop {
if old.destructed() {
return false;
}
+ if (old.strong() as usize) >= STRONG {
+ refcount_overflow();
+ }
let new_state = old.add_strong(1);
match self.state.compare_exchange(
old.as_raw(),
new_state.as_raw(),
Ordering::SeqCst,
Ordering::SeqCst,
) {🤖 Prompt for AI Agents |
||
| } | ||
|
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||
| /// Decrement strong count. Returns true when count drops to 0. | ||
| #[inline] | ||
| #[must_use] | ||
| pub fn dec(&self) -> bool { | ||
| let old = State::from_raw(self.state.fetch_sub(COUNT, Ordering::SeqCst)); | ||
|
|
||
| // LEAKED objects never reach 0 | ||
| if old.leaked() { | ||
| return false; | ||
| } | ||
|
|
||
| old.strong() == 1 | ||
| } | ||
|
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||
|
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| /// Mark this object as leaked (interned). It will never be deallocated. | ||
| pub fn leak(&self) { | ||
| debug_assert!(!self.is_leaked()); | ||
| let mut old = State::from_raw(self.state.load(Ordering::SeqCst)); | ||
| loop { | ||
| let new_state = old.with_leaked(true); | ||
| match self.state.compare_exchange( | ||
| old.as_raw(), | ||
| new_state.as_raw(), | ||
| Ordering::SeqCst, | ||
| Ordering::SeqCst, | ||
| ) { | ||
| Ok(_) => return, | ||
| Err(curr) => old = State::from_raw(curr), | ||
| } | ||
| } | ||
| } | ||
|
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| /// Check if this object is leaked (interned). | ||
| pub fn is_leaked(&self) -> bool { | ||
| State::from_raw(self.state.load(Ordering::Acquire)).leaked() | ||
| } | ||
| } | ||
|
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||
| // Deferred Drop Infrastructure | ||
| // | ||
| // This mechanism allows untrack_object() calls to be deferred until after | ||
| // the GC collection phase completes, preventing deadlocks that occur when | ||
| // clear (pop_edges) triggers object destruction while holding the tracked_objects lock. | ||
|
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| #[cfg(feature = "std")] | ||
| use core::cell::{Cell, RefCell}; | ||
|
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| #[cfg(feature = "std")] | ||
| thread_local! { | ||
| /// Flag indicating if we're inside a deferred drop context. | ||
| /// When true, drop operations should defer untrack calls. | ||
| static IN_DEFERRED_CONTEXT: Cell<bool> = const { Cell::new(false) }; | ||
|
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| /// Queue of deferred untrack operations. | ||
| /// No Send bound needed - this is thread-local and only accessed from the same thread. | ||
| static DEFERRED_QUEUE: RefCell<Vec<Box<dyn FnOnce()>>> = const { RefCell::new(Vec::new()) }; | ||
| } | ||
|
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|
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| #[cfg(feature = "std")] | ||
| struct DeferredDropGuard { | ||
| was_in_context: bool, | ||
| } | ||
|
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| #[cfg(feature = "std")] | ||
| impl Drop for DeferredDropGuard { | ||
| fn drop(&mut self) { | ||
| IN_DEFERRED_CONTEXT.with(|in_ctx| { | ||
| in_ctx.set(self.was_in_context); | ||
| }); | ||
| // Only flush if we're the outermost context | ||
| if !self.was_in_context { | ||
| flush_deferred_drops(); | ||
| } | ||
| } | ||
| } | ||
|
Comment on lines
+208
to
+220
Contributor
There was a problem hiding this comment.
If the closure passed to 🛡️ Proposed fix — skip flush during unwinding impl Drop for DeferredDropGuard {
fn drop(&mut self) {
IN_DEFERRED_CONTEXT.with(|in_ctx| {
in_ctx.set(self.was_in_context);
});
// Only flush if we're the outermost context
- if !self.was_in_context {
+ if !self.was_in_context && !std::thread::panicking() {
flush_deferred_drops();
}
}
}Skipping the flush during unwinding means deferred untrack operations will be lost on panic, which is acceptable given GC state is already inconsistent in a panic scenario. 🤖 Prompt for AI Agents |
||
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| /// Execute a function within a deferred drop context. | ||
| /// Any calls to `try_defer_drop` within this context will be queued | ||
| /// and executed when the context exits (even on panic). | ||
| #[cfg(feature = "std")] | ||
| #[inline] | ||
| pub fn with_deferred_drops<F, R>(f: F) -> R | ||
| where | ||
| F: FnOnce() -> R, | ||
| { | ||
| let _guard = IN_DEFERRED_CONTEXT.with(|in_ctx| { | ||
| let was_in_context = in_ctx.get(); | ||
| in_ctx.set(true); | ||
| DeferredDropGuard { was_in_context } | ||
| }); | ||
| f() | ||
| } | ||
|
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| /// Try to defer a drop-related operation. | ||
| /// If inside a deferred context, the operation is queued. | ||
| /// Otherwise, it executes immediately. | ||
| #[cfg(feature = "std")] | ||
| #[inline] | ||
| pub fn try_defer_drop<F>(f: F) | ||
| where | ||
| F: FnOnce() + 'static, | ||
| { | ||
| let should_defer = IN_DEFERRED_CONTEXT.with(|in_ctx| in_ctx.get()); | ||
|
|
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| if should_defer { | ||
| DEFERRED_QUEUE.with(|q| { | ||
| q.borrow_mut().push(Box::new(f)); | ||
| }); | ||
| } else { | ||
| f(); | ||
| } | ||
| } | ||
|
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| /// Flush all deferred drop operations. | ||
| /// This is automatically called when exiting a deferred context. | ||
| #[cfg(feature = "std")] | ||
| #[inline] | ||
| pub fn flush_deferred_drops() { | ||
| DEFERRED_QUEUE.with(|q| { | ||
| // Take all queued operations | ||
| let ops: Vec<_> = q.borrow_mut().drain(..).collect(); | ||
| // Execute them outside the borrow | ||
| for op in ops { | ||
| op(); | ||
| } | ||
| }); | ||
|
Comment on lines
+263
to
+271
Contributor
There was a problem hiding this comment.
Two concerns:
🛡️ Suggested hardening pub fn flush_deferred_drops() {
+ debug_assert!(
+ !IN_DEFERRED_CONTEXT.with(|c| c.get()),
+ "flush_deferred_drops called from within an active deferred context"
+ );
DEFERRED_QUEUE.with(|q| {
// Take all queued operations
let ops: Vec<_> = q.borrow_mut().drain(..).collect();
// Execute them outside the borrow
for op in ops {
op();
}
});
}For the panic-completeness concern, consider iterating with a RAII sentinel that re-pushes un-executed ops back to the queue on drop, though this adds complexity that may not be warranted for a GC untrack path. 🤖 Prompt for AI Agents |
||
| } | ||
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