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The term "receiver" means that a type can be used as the type of `self`,
and thus enables method call syntax `foo.bar()` instead of
`Foo::bar(foo)`. Stable Rust as of today (1.81) enables a limited
selection of types (primitives and types in std, e.g. `Box` and `Arc`)
to be used as receivers, while custom types cannot.
We want the kernel `Arc` type to have the same functionality as the Rust
std `Arc`, so we use the `Receiver` trait (gated behind `receiver_trait`
unstable feature) to gain the functionality.
The `arbitrary_self_types` RFC [1] (tracking issue [2]) is accepted and
it will allow all types that implement a new `Receiver` trait (different
from today's unstable trait) to be used as receivers. This trait will be
automatically implemented for all `Deref` types, which include our `Arc`
type, so we no longer have to opt-in to be used as receiver. To prepare
us for the change, remove the `Receiver` implementation and the
associated feature. To still allow `Arc` and others to be used as method
receivers, turn on `arbitrary_self_types` feature instead.
This feature gate is introduced in 1.23.0. It used to enable both
`Deref` types and raw pointer types to be used as receivers, but the
latter is now split into a different feature gate in Rust 1.83 nightly.
We do not need receivers on raw pointers so this change would not affect
us and usage of `arbitrary_self_types` feature would work for all Rust
versions that we support (>=1.78).
Cc: Adrian Taylor <ade@hohum.me.uk>
Link: https://github.com/rust-lang/rfcs/pull/3519 [1]
Link: https://github.com/rust-lang/rust/issues/44874 [2]
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240915132734.1653004-1-gary@garyguo.net
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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In Rust, it is possible to `allow` particular warnings (diagnostics,
lints) locally, making the compiler ignore instances of a given warning
within a given function, module, block, etc.
It is similar to `#pragma GCC diagnostic push` + `ignored` + `pop` in C:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
static void f(void) {}
#pragma GCC diagnostic pop
But way less verbose:
#[allow(dead_code)]
fn f() {}
By that virtue, it makes it possible to comfortably enable more
diagnostics by default (i.e. outside `W=` levels) that may have some
false positives but that are otherwise quite useful to keep enabled to
catch potential mistakes.
The `#[expect(...)]` attribute [1] takes this further, and makes the
compiler warn if the diagnostic was _not_ produced. For instance, the
following will ensure that, when `f()` is called somewhere, we will have
to remove the attribute:
#[expect(dead_code)]
fn f() {}
If we do not, we get a warning from the compiler:
warning: this lint expectation is unfulfilled
--> x.rs:3:10
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3 | #[expect(dead_code)]
| ^^^^^^^^^
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= note: `#[warn(unfulfilled_lint_expectations)]` on by default
This means that `expect`s do not get forgotten when they are not needed.
See the next commit for more details, nuances on its usage and
documentation on the feature.
The attribute requires the `lint_reasons` [2] unstable feature, but it
is becoming stable in 1.81.0 (to be released on 2024-09-05) and it has
already been useful to clean things up in this patch series, finding
cases where the `allow`s should not have been there.
Thus, enable `lint_reasons` and convert some of our `allow`s to `expect`s
where possible.
This feature was also an example of the ongoing collaboration between
Rust and the kernel -- we tested it in the kernel early on and found an
issue that was quickly resolved [3].
Cc: Fridtjof Stoldt <xfrednet@gmail.com>
Cc: Urgau <urgau@numericable.fr>
Link: https://rust-lang.github.io/rfcs/2383-lint-reasons.html#expect-lint-attribute [1]
Link: https://github.com/rust-lang/rust/issues/54503 [2]
Link: https://github.com/rust-lang/rust/issues/114557 [3]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-18-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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One way to explain what `ListArc` does is that it controls exclusive
access to the prev/next pointer field in a refcounted object. The
feature of having a special reference to a refcounted object with
exclusive access to specific fields is useful for other things, so
provide a general utility for that.
This is used by Rust Binder to keep track of which processes have a
reference to a given node. This involves an object for each process/node
pair, that is referenced by both the process and the node. For some
fields in this object, only the process's reference needs to access
them (and it needs mutable access), so Binder uses a ListArc to give the
process's reference exclusive access.
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-v5-10-f5f5e8075da0@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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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>
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Add the actual linked list itself.
The linked list uses the following design: The List type itself just has
a single pointer to the first element of the list. And the actual list
items then form a cycle. So the last item is `first->prev`.
This is slightly different from the usual kernel linked list. Matching
that exactly would amount to giving List two pointers, and having it be
part of the cycle of items. This alternate design has the advantage that
the cycle is never completely empty, which can reduce the number of
branches in some cases. However, it also has the disadvantage that List
must be pinned, which this design is trying to avoid.
Having the list items form a cycle rather than having null pointers at
the beginning/end is convenient for several reasons. For one, it lets us
store only one pointer in List, and it simplifies the implementation of
several functions.
Unfortunately, the `remove` function that removes an arbitrary element
from the list has to be unsafe. This is needed because there is no way
to handle the case where you pass an element from the wrong list. For
example, if it is the first element of some other list, then that other
list's `first` pointer would not be updated. Similarly, it could be a
data race if you try to remove it from two different lists in parallel.
(There's no problem with passing `remove` an item that's not in any
list. Additionally, other removal methods such as `pop_front` need not
be unsafe, as they can't be used to remove items from another list.)
A future patch in this series will introduce support for cursors that
can be used to remove arbitrary items without unsafe code.
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-v5-6-f5f5e8075da0@google.com
[ Fixed a few typos. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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Adds a macro for safely implementing the ListItem trait. As part of the
implementation of the macro, we also provide a HasListLinks trait
similar to the workqueue's HasWorkItem trait.
The HasListLinks trait is only necessary if you are implementing
ListItem using the impl_list_item macro.
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-v5-5-f5f5e8075da0@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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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>
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The `ListArc` type can be thought of as a special reference to a
refcounted object that owns the permission to manipulate the
`next`/`prev` pointers stored in the refcounted object. By ensuring that
each object has only one `ListArc` reference, the owner of that
reference is assured exclusive access to the `next`/`prev` pointers.
When a `ListArc` is inserted into a `List`, the `List` takes ownership
of the `ListArc` reference.
There are various strategies for ensuring that a value has only one
`ListArc` reference. The simplest is to convert a `UniqueArc` into a
`ListArc`. However, the refcounted object could also keep track of
whether a `ListArc` exists using a boolean, which could allow for the
creation of new `ListArc` references from an `Arc` reference. Whatever
strategy is used, the relevant tracking is referred to as "the tracking
inside `T`", and the `ListArcSafe` trait (and its subtraits) are used to
update the tracking when a `ListArc` is created or destroyed.
Note that we allow the case where the tracking inside `T` thinks that a
`ListArc` exists, but actually, there isn't a `ListArc`. However, we do
not allow the opposite situation where a `ListArc` exists, but the
tracking thinks it doesn't. This is because the former can at most
result in us failing to create a `ListArc` when the operation could
succeed, whereas the latter can result in the creation of two `ListArc`
references. Only the latter situation can lead to memory safety issues.
This patch introduces the `impl_list_arc_safe!` macro that allows you to
implement `ListArcSafe` for types using the strategy where a `ListArc`
can only be created from a `UniqueArc`. Other strategies are introduced
in later patches.
This is part of the linked list that Rust Binder will use for many
different things. The strategy where a `ListArc` can only be created
from a `UniqueArc` is actually sufficient for most of the objects that
Rust Binder needs to insert into linked lists. Usually, these are todo
items that are created and then immediately inserted into a queue.
The const generic ID allows objects to have several prev/next pointer
pairs so that the same object can be inserted into several different
lists. You are able to have several `ListArc` references as long as they
correspond to different pointer pairs. The ID itself is purely a
compile-time concept and will not be present in the final binary. Both
the `List` and the `ListArc` will need to agree on the ID for them to
work together. Rust Binder uses this in a few places (e.g. death
recipients) where the same object can be inserted into both generic todo
lists and some other lists for tracking the status of the object.
The ID is a const generic rather than a type parameter because the
`pair_from_unique` method needs to be able to assert that the two ids
are different. There's no easy way to assert that when using types
instead of integers.
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-v5-2-f5f5e8075da0@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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