rust: list: support heterogeneous lists

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>

1 files changed, 131 insertions, 0 deletions

diff --git a/rust/kernel/list/impl_list_item_mod.rs b/rust/kernel/list/impl_list_item_mod.rs
index 1bcb14774aeb..a0438537cee1 100644
--- a/rust/kernel/list/impl_list_item_mod.rs
+++ b/rust/kernel/list/impl_list_item_mod.rs
@@ -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;