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rust: list: add tracking for ListArc

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Add the ability to track whether a ListArc exists for a given value,
allowing for the creation of ListArcs without going through UniqueArc.

The `impl_list_arc_safe!` macro is extended with a `tracked_by` strategy
that defers the tracking of ListArcs to a field of the struct.
Additionally, the AtomicListArcTracker type is introduced, which can
track whether a ListArc exists using an atomic. By deferring the
tracking to a field of type AtomicListArcTracker, structs gain the
ability to create ListArcs without going through a UniqueArc.

Rust Binder uses this for some objects where we want to be able to
insert them into a linked list at any time. Using the
AtomicListArcTracker, we are able to check whether an item is already in
the list, and if not, we can create a `ListArc` and push it.

The macro has the ability to defer the tracking of ListArcs to a field,
using whatever strategy that field has. Since we don't add any
strategies other than AtomicListArcTracker, another similar option would
be to hard-code that the field should be an AtomicListArcTracker.
However, Rust Binder has a case where the AtomicListArcTracker is not
stored directly in the struct, but in a sub-struct. Furthermore, the
outer struct is generic:

struct Wrapper<T: ?Sized> {
    links: ListLinks,
    inner: T,
}

Here, the Wrapper struct implements ListArcSafe with `tracked_by inner`,
and then the various types used with `inner` also uses the macro to
implement ListArcSafe. Some of them use the untracked strategy, and some
of them use tracked_by with an AtomicListArcTracker. This way, Wrapper
just inherits whichever choice `inner` has made.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240814-linked-list-v5-3-f5f5e8075da0@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Alice Ryhl 2024-08-14 08:05:22 +00:00 committed by Miguel Ojeda
parent 6cd3417155
commit a48026315c
2 changed files with 170 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
rust/kernel/list.rs
View File

@ -5,4 +5,4 @@
//! A linked list implementation.
mod arc;
pub use self::arc::{impl_list_arc_safe, ListArc, ListArcSafe};
pub use self::arc::{impl_list_arc_safe, AtomicTracker, ListArc, ListArcSafe, TryNewListArc};

171
rust/kernel/list/arc.rs
View File

@ -7,9 +7,10 @@
use crate::alloc::{AllocError, Flags};
use crate::prelude::*;
use crate::sync::{Arc, ArcBorrow, UniqueArc};
use core::marker::Unsize;
use core::marker::{PhantomPinned, Unsize};
use core::ops::Deref;
use core::pin::Pin;
use core::sync::atomic::{AtomicBool, Ordering};
/// Declares that this type has some way to ensure that there is exactly one `ListArc` instance for
/// this id.
@ -48,9 +49,38 @@ pub trait ListArcSafe<const ID: u64 = 0> {
unsafe fn on_drop_list_arc(&self);
}
/// Declares that this type is able to safely attempt to create `ListArc`s at any time.
///
/// # Safety
///
/// The guarantees of `try_new_list_arc` must be upheld.
pub unsafe trait TryNewListArc<const ID: u64 = 0>: ListArcSafe<ID> {
/// Attempts to convert an `Arc<Self>` into an `ListArc<Self>`. Returns `true` if the
/// conversion was successful.
///
/// This method should not be called directly. Use [`ListArc::try_from_arc`] instead.
///
/// # Guarantees
///
/// If this call returns `true`, then there is no [`ListArc`] pointing to this value.
/// Additionally, this call will have transitioned the tracking inside `Self` from not thinking
/// that a [`ListArc`] exists, to thinking that a [`ListArc`] exists.
fn try_new_list_arc(&self) -> bool;
}
/// Declares that this type supports [`ListArc`].
///
/// When using this macro, it will only be possible to create a [`ListArc`] from a [`UniqueArc`].
/// This macro supports a few different strategies for implementing the tracking inside the type:
///
/// * The `untracked` strategy does not actually keep track of whether a [`ListArc`] exists. When
/// using this strategy, the only way to create a [`ListArc`] is using a [`UniqueArc`].
/// * The `tracked_by` strategy defers the tracking to a field of the struct. The user much specify
/// which field to defer the tracking to. The field must implement [`ListArcSafe`]. If the field
/// implements [`TryNewListArc`], then the type will also implement [`TryNewListArc`].
///
/// The `tracked_by` strategy is usually used by deferring to a field of type
/// [`AtomicTracker`]. However, it is also possible to defer the tracking to another struct
/// using also using this macro.
#[macro_export]
macro_rules! impl_list_arc_safe {
(impl$({$($generics:tt)*})? ListArcSafe<$num:tt> for $t:ty { untracked; } $($rest:tt)*) => {
@ -61,6 +91,39 @@ macro_rules! impl_list_arc_safe {
$crate::list::impl_list_arc_safe! { $($rest)* }
};
(impl$({$($generics:tt)*})? ListArcSafe<$num:tt> for $t:ty {
tracked_by $field:ident : $fty:ty;
} $($rest:tt)*) => {
impl$(<$($generics)*>)? $crate::list::ListArcSafe<$num> for $t {
unsafe fn on_create_list_arc_from_unique(self: ::core::pin::Pin<&mut Self>) {
$crate::assert_pinned!($t, $field, $fty, inline);
// SAFETY: This field is structurally pinned as per the above assertion.
let field = unsafe {
::core::pin::Pin::map_unchecked_mut(self, |me| &mut me.$field)
};
// SAFETY: The caller promises that there is no `ListArc`.
unsafe {
<$fty as $crate::list::ListArcSafe<$num>>::on_create_list_arc_from_unique(field)
};
}
unsafe fn on_drop_list_arc(&self) {
// SAFETY: The caller promises that there is no `ListArc` reference, and also
// promises that the tracking thinks there is a `ListArc` reference.
unsafe { <$fty as $crate::list::ListArcSafe<$num>>::on_drop_list_arc(&self.$field) };
}
}
unsafe impl$(<$($generics)*>)? $crate::list::TryNewListArc<$num> for $t
where
$fty: TryNewListArc<$num>,
{
fn try_new_list_arc(&self) -> bool {
<$fty as $crate::list::TryNewListArc<$num>>::try_new_list_arc(&self.$field)
}
}
$crate::list::impl_list_arc_safe! { $($rest)* }
};
() => {};
}
pub use impl_list_arc_safe;
@ -205,6 +268,52 @@ where
}
}
/// Try to create a new `ListArc`.
///
/// This fails if this value already has a `ListArc`.
pub fn try_from_arc(arc: Arc<T>) -> Result<Self, Arc<T>>
where
T: TryNewListArc<ID>,
{
if arc.try_new_list_arc() {
// SAFETY: The `try_new_list_arc` method returned true, so we made the tracking think
// that a `ListArc` exists. This lets us create a `ListArc`.
Ok(unsafe { Self::transmute_from_arc(arc) })
} else {
Err(arc)
}
}
/// Try to create a new `ListArc`.
///
/// This fails if this value already has a `ListArc`.
pub fn try_from_arc_borrow(arc: ArcBorrow<'_, T>) -> Option<Self>
where
T: TryNewListArc<ID>,
{
if arc.try_new_list_arc() {
// SAFETY: The `try_new_list_arc` method returned true, so we made the tracking think
// that a `ListArc` exists. This lets us create a `ListArc`.
Some(unsafe { Self::transmute_from_arc(Arc::from(arc)) })
} else {
None
}
}
/// Try to create a new `ListArc`.
///
/// If it's not possible to create a new `ListArc`, then the `Arc` is dropped. This will never
/// run the destructor of the value.
pub fn try_from_arc_or_drop(arc: Arc<T>) -> Option<Self>
where
T: TryNewListArc<ID>,
{
match Self::try_from_arc(arc) {
Ok(list_arc) => Some(list_arc),
Err(arc) => Arc::into_unique_or_drop(arc).map(Self::from),
}
}
/// Transmutes an [`Arc`] into a `ListArc` without updating the tracking inside `T`.
///
/// # Safety
@ -350,3 +459,61 @@ where
U: ListArcSafe<ID> + ?Sized,
{
}
/// A utility for tracking whether a [`ListArc`] exists using an atomic.
///
/// # Invariant
///
/// If the boolean is `false`, then there is no [`ListArc`] for this value.
#[repr(transparent)]
pub struct AtomicTracker<const ID: u64 = 0> {
inner: AtomicBool,
// This value needs to be pinned to justify the INVARIANT: comment in `AtomicTracker::new`.
_pin: PhantomPinned,
}
impl<const ID: u64> AtomicTracker<ID> {
/// Creates a new initializer for this type.
pub fn new() -> impl PinInit<Self> {
// INVARIANT: Pin-init initializers can't be used on an existing `Arc`, so this value will
// not be constructed in an `Arc` that already has a `ListArc`.
Self {
inner: AtomicBool::new(false),
_pin: PhantomPinned,
}
}
fn project_inner(self: Pin<&mut Self>) -> &mut AtomicBool {
// SAFETY: The `inner` field is not structurally pinned, so we may obtain a mutable
// reference to it even if we only have a pinned reference to `self`.
unsafe { &mut Pin::into_inner_unchecked(self).inner }
}
}
impl<const ID: u64> ListArcSafe<ID> for AtomicTracker<ID> {
unsafe fn on_create_list_arc_from_unique(self: Pin<&mut Self>) {
// INVARIANT: We just created a ListArc, so the boolean should be true.
*self.project_inner().get_mut() = true;
}
unsafe fn on_drop_list_arc(&self) {
// INVARIANT: We just dropped a ListArc, so the boolean should be false.
self.inner.store(false, Ordering::Release);
}
}
// SAFETY: If this method returns `true`, then by the type invariant there is no `ListArc` before
// this call, so it is okay to create a new `ListArc`.
//
// The acquire ordering will synchronize with the release store from the destruction of any
// previous `ListArc`, so if there was a previous `ListArc`, then the destruction of the previous
// `ListArc` happens-before the creation of the new `ListArc`.
unsafe impl<const ID: u64> TryNewListArc<ID> for AtomicTracker<ID> {
fn try_new_list_arc(&self) -> bool {
// INVARIANT: If this method returns true, then the boolean used to be false, and is no
// longer false, so it is okay for the caller to create a new [`ListArc`].
self.inner
.compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed)
.is_ok()
}
}