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rust: list: support heterogeneous lists

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Support linked lists that can hold many different structs at once. This
is generally done using trait objects. The main challenge is figuring
what the struct is given only a pointer to the ListLinks.

We do this by storing a pointer to the struct next to the ListLinks
field. The container_of operation will then just read that pointer. When
the type is a trait object, that pointer will be a fat pointer whose
metadata is a vtable that tells you what kind of struct it is.

Heterogeneous lists are heavily used by Rust Binder. There are a lot of
so-called todo lists containing various events that need to be delivered
to userspace next time userspace calls into the driver. And there are
quite a few different todo item types: incoming transaction, changes to
refcounts, death notifications, and more.

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-v5-9-f5f5e8075da0@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Alice Ryhl 2024-08-14 08:05:28 +00:00 committed by Miguel Ojeda
parent 9078a4f956
commit 2003c04b05
2 changed files with 177 additions and 1 deletions
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47
rust/kernel/list.rs
View File

@ -12,7 +12,9 @@ use core::marker::PhantomData;
use core::ptr;
mod impl_list_item_mod;
pub use self::impl_list_item_mod::{impl_has_list_links, impl_list_item, HasListLinks};
pub use self::impl_list_item_mod::{
impl_has_list_links, impl_has_list_links_self_ptr, impl_list_item, HasListLinks, HasSelfPtr,
};
mod arc;
pub use self::arc::{impl_list_arc_safe, AtomicTracker, ListArc, ListArcSafe, TryNewListArc};
@ -183,6 +185,49 @@ impl<const ID: u64> ListLinks<ID> {
}
}
/// Similar to [`ListLinks`], but also contains a pointer to the full value.
///
/// This type can be used instead of [`ListLinks`] to support lists with trait objects.
#[repr(C)]
pub struct ListLinksSelfPtr<T: ?Sized, const ID: u64 = 0> {
/// The `ListLinks` field inside this value.
///
/// This is public so that it can be used with `impl_has_list_links!`.
pub inner: ListLinks<ID>,
// UnsafeCell is not enough here because we use `Opaque::uninit` as a dummy value, and
// `ptr::null()` doesn't work for `T: ?Sized`.
self_ptr: Opaque<*const T>,
}
// SAFETY: The fields of a ListLinksSelfPtr can be moved across thread boundaries.
unsafe impl<T: ?Sized + Send, const ID: u64> Send for ListLinksSelfPtr<T, ID> {}
// SAFETY: The type is opaque so immutable references to a ListLinksSelfPtr are useless. Therefore,
// it's okay to have immutable access to a ListLinks from several threads at once.
//
// Note that `inner` being a public field does not prevent this type from being opaque, since
// `inner` is a opaque type.
unsafe impl<T: ?Sized + Sync, const ID: u64> Sync for ListLinksSelfPtr<T, ID> {}
impl<T: ?Sized, const ID: u64> ListLinksSelfPtr<T, ID> {
/// The offset from the [`ListLinks`] to the self pointer field.
pub const LIST_LINKS_SELF_PTR_OFFSET: usize = core::mem::offset_of!(Self, self_ptr);
/// 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: ListLinks {
inner: Opaque::new(ListLinksFields {
prev: ptr::null_mut(),
next: ptr::null_mut(),
}),
},
self_ptr: Opaque::uninit(),
}
}
}
impl<T: ?Sized + ListItem<ID>, const ID: u64> List<T, ID> {
/// Creates a new empty list.
pub const fn new() -> Self {

131
rust/kernel/list/impl_list_item_mod.rs
View File

@ -68,6 +68,49 @@ macro_rules! impl_has_list_links {
}
pub use impl_has_list_links;
/// Declares that the `ListLinks<ID>` field in this struct is inside a `ListLinksSelfPtr<T, ID>`.
///
/// # Safety
///
/// The `ListLinks<ID>` field of this struct at the offset `HasListLinks<ID>::OFFSET` must be
/// inside a `ListLinksSelfPtr<T, ID>`.
pub unsafe trait HasSelfPtr<T: ?Sized, const ID: u64 = 0>
where
Self: HasListLinks<ID>,
{
}
/// Implements the [`HasListLinks`] and [`HasSelfPtr`] traits for the given type.
#[macro_export]
macro_rules! impl_has_list_links_self_ptr {
($(impl$({$($implarg:tt)*})?
HasSelfPtr<$item_type:ty $(, $id:tt)?>
for $self:ident $(<$($selfarg:ty),*>)?
{ self.$field:ident }
)*) => {$(
// SAFETY: The implementation of `raw_get_list_links` only compiles if the field has the
// right type.
unsafe impl$(<$($implarg)*>)? $crate::list::HasSelfPtr<$item_type $(, $id)?> for
$self $(<$($selfarg),*>)?
{}
unsafe impl$(<$($implarg)*>)? $crate::list::HasListLinks$(<$id>)? for
$self $(<$($selfarg),*>)?
{
const OFFSET: usize = ::core::mem::offset_of!(Self, $field) as usize;
#[inline]
unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut $crate::list::ListLinks$(<$id>)? {
// SAFETY: The caller promises that the pointer is not dangling.
let ptr: *mut $crate::list::ListLinksSelfPtr<$item_type $(, $id)?> =
unsafe { ::core::ptr::addr_of_mut!((*ptr).$field) };
ptr.cast()
}
}
)*};
}
pub use impl_has_list_links_self_ptr;
/// Implements the [`ListItem`] trait for the given type.
///
/// Requires that the type implements [`HasListLinks`]. Use the [`impl_has_list_links!`] macro to
@ -139,5 +182,93 @@ macro_rules! impl_list_item {
}
}
)*};
(
$(impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
using ListLinksSelfPtr;
})*
) => {$(
// SAFETY: See GUARANTEES comment on each method.
unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t {
// GUARANTEES:
// This implementation of `ListItem` will not give out exclusive access to the same
// `ListLinks` several times because calls to `prepare_to_insert` and `post_remove`
// must alternate and exclusive access is given up when `post_remove` is called.
//
// Other invocations of `impl_list_item!` also cannot give out exclusive access to the
// same `ListLinks` because you can only implement `ListItem` once for each value of
// `ID`, and the `ListLinks` fields only work with the specified `ID`.
unsafe fn prepare_to_insert(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
// SAFETY: The caller promises that `me` points at a valid value of type `Self`.
let links_field = unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) };
let spoff = $crate::list::ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
// Goes via the offset as the field is private.
//
// SAFETY: The constant is equal to `offset_of!(ListLinksSelfPtr, self_ptr)`, so
// the pointer stays in bounds of the allocation.
let self_ptr = unsafe { (links_field as *const u8).add(spoff) }
as *const $crate::types::Opaque<*const Self>;
let cell_inner = $crate::types::Opaque::raw_get(self_ptr);
// SAFETY: This value is not accessed in any other places than `prepare_to_insert`,
// `post_remove`, or `view_value`. By the safety requirements of those methods,
// none of these three methods may be called in parallel with this call to
// `prepare_to_insert`, so this write will not race with any other access to the
// value.
unsafe { ::core::ptr::write(cell_inner, me) };
links_field
}
// GUARANTEES:
// * This returns the same pointer as `prepare_to_insert` because `prepare_to_insert`
// returns the return value of `view_links`.
// * By the type invariants of `ListLinks`, the `ListLinks` has two null pointers when
// this value is not in a list.
unsafe fn view_links(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
// SAFETY: The caller promises that `me` points at a valid value of type `Self`.
unsafe { <Self as HasListLinks<$num>>::raw_get_list_links(me.cast_mut()) }
}
// This function is also used as the implementation of `post_remove`, so the caller
// may choose to satisfy the safety requirements of `post_remove` instead of the safety
// requirements for `view_value`.
//
// GUARANTEES: (always)
// * This returns the same pointer as the one passed to the most recent call to
// `prepare_to_insert` since that call wrote that pointer to this location. The value
// is only modified in `prepare_to_insert`, so it has not been modified since the
// most recent call.
//
// GUARANTEES: (only when using the `view_value` safety requirements)
// * The pointer remains valid until the next call to `post_remove` because the caller
// of the most recent call to `prepare_to_insert` promised to retain ownership of the
// `ListArc` containing `Self` until the next call to `post_remove`. The value cannot
// be destroyed while a `ListArc` reference exists.
unsafe fn view_value(links_field: *mut $crate::list::ListLinks<$num>) -> *const Self {
let spoff = $crate::list::ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
// SAFETY: The constant is equal to `offset_of!(ListLinksSelfPtr, self_ptr)`, so
// the pointer stays in bounds of the allocation.
let self_ptr = unsafe { (links_field as *const u8).add(spoff) }
as *const ::core::cell::UnsafeCell<*const Self>;
let cell_inner = ::core::cell::UnsafeCell::raw_get(self_ptr);
// SAFETY: This is not a data race, because the only function that writes to this
// value is `prepare_to_insert`, but by the safety requirements the
// `prepare_to_insert` method may not be called in parallel with `view_value` or
// `post_remove`.
unsafe { ::core::ptr::read(cell_inner) }
}
// GUARANTEES:
// The first guarantee of `view_value` is exactly what `post_remove` guarantees.
unsafe fn post_remove(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
// SAFETY: This specific implementation of `view_value` allows the caller to
// promise the safety requirements of `post_remove` instead of the safety
// requirements for `view_value`.
unsafe { <Self as $crate::list::ListItem<$num>>::view_value(me) }
}
}
)*};
}
pub use impl_list_item;