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rust: init: update expanded macro explanation
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The previous patches changed the internals of the macros resulting in
the example expanded code being outdated. This patch updates the example
and only changes documentation.

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230814084602.25699-14-benno.lossin@proton.me
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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@ojeda
Benno Lossin authored and ojeda committed Aug 21, 2023
1 parent 66e7f77 commit 37152d4
Showing 1 changed file with 69 additions and 57 deletions.
126 changes: 69 additions & 57 deletions rust/kernel/init/macros.rs
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Expand Up @@ -45,7 +45,7 @@
//! #[pinned_drop]
//! impl PinnedDrop for Foo {
//! fn drop(self: Pin<&mut Self>) {
//! println!("{self:p} is getting dropped.");
//! pr_info!("{self:p} is getting dropped.");
//! }
//! }
//!
Expand Down Expand Up @@ -170,8 +170,10 @@
//! t: T,
//! }
//! #[doc(hidden)]
//! impl<'__pin, T>
//! ::core::marker::Unpin for Bar<T> where __Unpin<'__pin, T>: ::core::marker::Unpin {}
//! impl<'__pin, T> ::core::marker::Unpin for Bar<T>
//! where
//! __Unpin<'__pin, T>: ::core::marker::Unpin,
//! {}
//! // Now we need to ensure that `Bar` does not implement `Drop`, since that would give users
//! // access to `&mut self` inside of `drop` even if the struct was pinned. This could lead to
//! // UB with only safe code, so we disallow this by giving a trait implementation error using
Expand All @@ -188,8 +190,9 @@
//! // for safety, but a good sanity check, since no normal code calls `PinnedDrop::drop`.
//! #[allow(non_camel_case_types)]
//! trait UselessPinnedDropImpl_you_need_to_specify_PinnedDrop {}
//! impl<T: ::kernel::init::PinnedDrop>
//! UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for T {}
//! impl<
//! T: ::kernel::init::PinnedDrop,
//! > UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for T {}
//! impl<T> UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for Bar<T> {}
//! };
//! ```
Expand Down Expand Up @@ -219,88 +222,91 @@
//! // return type and shadow it later when we insert the arbitrary user code. That way
//! // there will be no possibility of returning without `unsafe`.
//! struct __InitOk;
//! // Get the pin-data type from the initialized type.
//! // Get the data about fields from the supplied type.
//! // - the function is unsafe, hence the unsafe block
//! // - we `use` the `HasPinData` trait in the block, it is only available in that
//! // scope.
//! let data = unsafe {
//! use ::kernel::init::__internal::HasPinData;
//! Self::__pin_data()
//! };
//! // Use `data` to help with type inference, the closure supplied will have the type
//! // `FnOnce(*mut Self) -> Result<__InitOk, Infallible>`.
//! // Ensure that `data` really is of type `PinData` and help with type inference:
//! let init = ::kernel::init::__internal::PinData::make_closure::<
//! _,
//! __InitOk,
//! ::core::convert::Infallible,
//! >(data, move |slot| {
//! {
//! // Shadow the structure so it cannot be used to return early. If a user
//! // tries to write `return Ok(__InitOk)`, then they get a type error, since
//! // that will refer to this struct instead of the one defined above.
//! // tries to write `return Ok(__InitOk)`, then they get a type error,
//! // since that will refer to this struct instead of the one defined
//! // above.
//! struct __InitOk;
//! // This is the expansion of `t,`, which is syntactic sugar for `t: t,`.
//! unsafe { ::core::ptr::write(::core::addr_of_mut!((*slot).t), t) };
//! // Since initialization could fail later (not in this case, since the error
//! // type is `Infallible`) we will need to drop this field if there is an
//! // error later. This `DropGuard` will drop the field when it gets dropped
//! // and has not yet been forgotten. We make a reference to it, so users
//! // cannot `mem::forget` it from the initializer, since the name is the same
//! // as the field (including hygiene).
//! let t = &unsafe {
//! ::kernel::init::__internal::DropGuard::new(
//! ::core::addr_of_mut!((*slot).t),
//! )
//! {
//! unsafe { ::core::ptr::write(::core::addr_of_mut!((*slot).t), t) };
//! }
//! // Since initialization could fail later (not in this case, since the
//! // error type is `Infallible`) we will need to drop this field if there
//! // is an error later. This `DropGuard` will drop the field when it gets
//! // dropped and has not yet been forgotten.
//! let t = unsafe {
//! ::pinned_init::__internal::DropGuard::new(::core::addr_of_mut!((*slot).t))
//! };
//! // Expansion of `x: 0,`:
//! // Since this can be an arbitrary expression we cannot place it inside of
//! // the `unsafe` block, so we bind it here.
//! let x = 0;
//! unsafe { ::core::ptr::write(::core::addr_of_mut!((*slot).x), x) };
//! // Since this can be an arbitrary expression we cannot place it inside
//! // of the `unsafe` block, so we bind it here.
//! {
//! let x = 0;
//! unsafe { ::core::ptr::write(::core::addr_of_mut!((*slot).x), x) };
//! }
//! // We again create a `DropGuard`.
//! let x = &unsafe {
//! ::kernel::init::__internal::DropGuard::new(
//! ::core::addr_of_mut!((*slot).x),
//! )
//! let x = unsafe {
//! ::kernel::init::__internal::DropGuard::new(::core::addr_of_mut!((*slot).x))
//! };
//!
//! // Since initialization has successfully completed, we can now forget
//! // the guards. This is not `mem::forget`, since we only have
//! // `&DropGuard`.
//! ::core::mem::forget(x);
//! ::core::mem::forget(t);
//! // Here we use the type checker to ensure that every field has been
//! // initialized exactly once, since this is `if false` it will never get
//! // executed, but still type-checked.
//! // Additionally we abuse `slot` to automatically infer the correct type for
//! // the struct. This is also another check that every field is accessible
//! // from this scope.
//! // Additionally we abuse `slot` to automatically infer the correct type
//! // for the struct. This is also another check that every field is
//! // accessible from this scope.
//! #[allow(unreachable_code, clippy::diverging_sub_expression)]
//! if false {
//! let _ = || {
//! unsafe {
//! ::core::ptr::write(
//! slot,
//! Self {
//! // We only care about typecheck finding every field here,
//! // the expression does not matter, just conjure one using
//! // `panic!()`:
//! // We only care about typecheck finding every field
//! // here, the expression does not matter, just conjure
//! // one using `panic!()`:
//! t: ::core::panic!(),
//! x: ::core::panic!(),
//! },
//! );
//! };
//! }
//! // Since initialization has successfully completed, we can now forget the
//! // guards. This is not `mem::forget`, since we only have `&DropGuard`.
//! unsafe { ::kernel::init::__internal::DropGuard::forget(t) };
//! unsafe { ::kernel::init::__internal::DropGuard::forget(x) };
//! };
//! }
//! // We leave the scope above and gain access to the previously shadowed
//! // `__InitOk` that we need to return.
//! Ok(__InitOk)
//! });
//! // Change the return type from `__InitOk` to `()`.
//! let init = move |slot| -> ::core::result::Result<(), ::core::convert::Infallible> {
//! let init = move |
//! slot,
//! | -> ::core::result::Result<(), ::core::convert::Infallible> {
//! init(slot).map(|__InitOk| ())
//! };
//! // Construct the initializer.
//! let init = unsafe {
//! ::kernel::init::pin_init_from_closure::<_, ::core::convert::Infallible>(init)
//! ::kernel::init::pin_init_from_closure::<
//! _,
//! ::core::convert::Infallible,
//! >(init)
//! };
//! init
//! }
Expand Down Expand Up @@ -374,7 +380,10 @@
//! b: Bar<u32>,
//! }
//! #[doc(hidden)]
//! impl<'__pin> ::core::marker::Unpin for Foo where __Unpin<'__pin>: ::core::marker::Unpin {}
//! impl<'__pin> ::core::marker::Unpin for Foo
//! where
//! __Unpin<'__pin>: ::core::marker::Unpin,
//! {}
//! // Since we specified `PinnedDrop` as the argument to `#[pin_data]`, we expect `Foo` to
//! // implement `PinnedDrop`. Thus we do not need to prevent `Drop` implementations like
//! // before, instead we implement `Drop` here and delegate to `PinnedDrop`.
Expand Down Expand Up @@ -403,7 +412,7 @@
//! #[pinned_drop]
//! impl PinnedDrop for Foo {
//! fn drop(self: Pin<&mut Self>) {
//! println!("{self:p} is getting dropped.");
//! pr_info!("{self:p} is getting dropped.");
//! }
//! }
//! ```
Expand All @@ -414,7 +423,7 @@
//! // `unsafe`, full path and the token parameter are added, everything else stays the same.
//! unsafe impl ::kernel::init::PinnedDrop for Foo {
//! fn drop(self: Pin<&mut Self>, _: ::kernel::init::__internal::OnlyCallFromDrop) {
//! println!("{self:p} is getting dropped.");
//! pr_info!("{self:p} is getting dropped.");
//! }
//! }
//! ```
Expand Down Expand Up @@ -449,18 +458,21 @@
//! >(data, move |slot| {
//! {
//! struct __InitOk;
//! unsafe { ::core::ptr::write(::core::addr_of_mut!((*slot).a), a) };
//! let a = &unsafe {
//! {
//! unsafe { ::core::ptr::write(::core::addr_of_mut!((*slot).a), a) };
//! }
//! let a = unsafe {
//! ::kernel::init::__internal::DropGuard::new(::core::addr_of_mut!((*slot).a))
//! };
//! let b = Bar::new(36);
//! let init = Bar::new(36);
//! unsafe { data.b(::core::addr_of_mut!((*slot).b), b)? };
//! let b = &unsafe {
//! let b = unsafe {
//! ::kernel::init::__internal::DropGuard::new(::core::addr_of_mut!((*slot).b))
//! };
//!
//! ::core::mem::forget(b);
//! ::core::mem::forget(a);
//! #[allow(unreachable_code, clippy::diverging_sub_expression)]
//! if false {
//! let _ = || {
//! unsafe {
//! ::core::ptr::write(
//! slot,
Expand All @@ -470,13 +482,13 @@
//! },
//! );
//! };
//! }
//! unsafe { ::kernel::init::__internal::DropGuard::forget(a) };
//! unsafe { ::kernel::init::__internal::DropGuard::forget(b) };
//! };
//! }
//! Ok(__InitOk)
//! });
//! let init = move |slot| -> ::core::result::Result<(), ::core::convert::Infallible> {
//! let init = move |
//! slot,
//! | -> ::core::result::Result<(), ::core::convert::Infallible> {
//! init(slot).map(|__InitOk| ())
//! };
//! let init = unsafe {
Expand Down

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