From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-4316.protonmail.ch (mail-4316.protonmail.ch [185.70.43.16]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BDFBE50276 for ; Tue, 6 Aug 2024 17:22:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.70.43.16 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722964952; cv=none; b=NhsTVx6XBp87uJKO3D6QJt1EqfZCoe6ZTeQPE3K9/00UWAjzUqWDhGFgXa2yWDTCE2Oa1AzR0ZRr5gfkiVHBSfV11PV5B1gA0mWp4I3TicVkgbDHdbLvlVSu/eTP9wICMXHLIribXMZa7kVQuyKOraxenlL/glCsr5u4RhLxxj8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722964952; c=relaxed/simple; bh=m5kxkaXfCyTE8Wd83ongz8t5b5x/+elgqIVQ+v9kOtw=; h=Date:To:From:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=nSlke0LTNv2JF0xQzO4ny3ExvJE+dQhxsWe+HQn2sDg7m6tnC/z/SYnvCHuc7LXKkHjwg2Z4DfSIMTfj6lApvYJarNhpuABSjN+HOrPQNkP2lE6/cfYm0QjdUpHhu/6rLMVTfjB5Eh23/F+SmfOtbTHV0BFDQARjI4a/xgxGYO4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=proton.me; spf=pass smtp.mailfrom=proton.me; dkim=pass (2048-bit key) header.d=proton.me header.i=@proton.me header.b=Jp6B52kx; arc=none smtp.client-ip=185.70.43.16 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=proton.me Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=proton.me Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=proton.me header.i=@proton.me header.b="Jp6B52kx" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=proton.me; s=protonmail; t=1722964947; x=1723224147; bh=KwacUaqJ5YYWRVk8ykzjM7Y6SD6C+YG7cpGeuP2M4TE=; h=Date:To:From:Cc:Subject:Message-ID:In-Reply-To:References: Feedback-ID:From:To:Cc:Date:Subject:Reply-To:Feedback-ID: Message-ID:BIMI-Selector; b=Jp6B52kxI5n1YaZZZWjw0rHdJWSv4GmOv6fBaO0THOOH61gXplkoVp2llxoc3hsAW XHx86PKgIdakNV8LE1KWhD4P7CIROJEfCsqEtZNUeLhpbC9Sjw8ejVzIamyN9flQK8 qsew6EwgExhFV8vlQ84jALYyX2fB3Lp1BA3GXZMRJyvdfgAYKIBHANofTd66djbI3Z GVRwFlLBR2FOv7le9k0rqsXWRukKbZInu/lIElIR4ynUPeBPSduf7OyhIhdPKA6Z4I CtIf8KsbImIUfv+yNqQ4uUpHJwzjvsePw6YhzrRiu7tasfUAWzFrx5eIfScNrWr8+8 Az1jBtENXUMSQ== Date: Tue, 06 Aug 2024 17:22:21 +0000 To: Danilo Krummrich , ojeda@kernel.org, alex.gaynor@gmail.com, wedsonaf@gmail.com, boqun.feng@gmail.com, gary@garyguo.net, bjorn3_gh@protonmail.com, a.hindborg@samsung.com, aliceryhl@google.com, akpm@linux-foundation.org From: Benno Lossin Cc: daniel.almeida@collabora.com, faith.ekstrand@collabora.com, boris.brezillon@collabora.com, lina@asahilina.net, mcanal@igalia.com, zhiw@nvidia.com, acurrid@nvidia.com, cjia@nvidia.com, jhubbard@nvidia.com, airlied@redhat.com, ajanulgu@redhat.com, lyude@redhat.com, linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v4 08/28] rust: types: implement `Unique` Message-ID: In-Reply-To: <20240805152004.5039-9-dakr@kernel.org> References: <20240805152004.5039-1-dakr@kernel.org> <20240805152004.5039-9-dakr@kernel.org> Feedback-ID: 71780778:user:proton X-Pm-Message-ID: 716c158e9305903de2f6bf86a219c931834c4384 Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On 05.08.24 17:19, Danilo Krummrich wrote: > Implement the `Unique` type as a prerequisite for `Box` and `Vec` > introduced in subsequent patches. >=20 > `Unique` serves as wrapper around a `NonNull`, but indicates that the > possessor of this wrapper owns the referent. >=20 > This type already exists in Rust's core library, but, unfortunately, is > exposed as unstable API and hence shouldn't be used in the kernel. >=20 > This implementation of `Unique` is almost identical, but mostly stripped > down to the functionality we need for `Box` and `Vec`. Additionally, all > unstable features are removed and / or replaced by stable ones. >=20 > Reviewed-by: Alice Ryhl > Signed-off-by: Danilo Krummrich > --- > rust/kernel/types.rs | 183 +++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 183 insertions(+) >=20 > diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs > index bd189d646adb..7cf89067b5fc 100644 > --- a/rust/kernel/types.rs > +++ b/rust/kernel/types.rs > @@ -473,3 +473,186 @@ unsafe impl AsBytes for str {} > // does not have any uninitialized portions either. > unsafe impl AsBytes for [T] {} > unsafe impl AsBytes for [T; N] {} > + > +/// A wrapper around a raw non-null `*mut T` that indicates that the pos= sessor > +/// of this wrapper owns the referent. Useful for building abstractions = like > +/// `Box`, `Vec`, `String`, and `HashMap`. > +/// > +/// Unlike `*mut T`, `Unique` behaves "as if" it were an instance of = `T`. > +/// It implements `Send`/`Sync` if `T` is `Send`/`Sync`. It also implies > +/// the kind of strong aliasing guarantees an instance of `T` can expect= : > +/// the referent of the pointer should not be modified without a unique = path to > +/// its owning Unique. > +/// > +/// If you're uncertain of whether it's correct to use `Unique` for your= purposes, > +/// consider using `NonNull`, which has weaker semantics. > +/// > +/// Unlike `*mut T`, the pointer must always be non-null, even if the po= inter > +/// is never dereferenced. This is so that enums may use this forbidden = value > +/// as a discriminant -- `Option>` has the same size as `Uniqu= e`. > +/// However the pointer may still dangle if it isn't dereferenced. > +/// > +/// Unlike `*mut T`, `Unique` is covariant over `T`. This should alwa= ys be correct > +/// for any type which upholds Unique's aliasing requirements. > +#[repr(transparent)] > +pub struct Unique { > + pointer: NonNull, > + // NOTE: this marker has no consequences for variance, but is necess= ary > + // for dropck to understand that we logically own a `T`. > + // > + // For details, see: > + // https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-gen= eric-drop.md#phantom-data > + _marker: PhantomData, > +} > + > +/// `Unique` pointers are `Send` if `T` is `Send` because the data they > +/// reference is unaliased. Note that this aliasing invariant is > +/// unenforced by the type system; the abstraction using the > +/// `Unique` must enforce it. > +unsafe impl Send for Unique {} > + > +/// `Unique` pointers are `Sync` if `T` is `Sync` because the data they > +/// reference is unaliased. Note that this aliasing invariant is > +/// unenforced by the type system; the abstraction using the > +/// `Unique` must enforce it. > +unsafe impl Sync for Unique {} > + > +impl Unique { > + /// Creates a new `Unique` that is dangling, but well-aligned. > + /// > + /// This is useful for initializing types which lazily allocate, lik= e > + /// `Vec::new` does. > + /// > + /// Note that the pointer value may potentially represent a valid po= inter to > + /// a `T`, which means this must not be used as a "not yet initializ= ed" > + /// sentinel value. Types that lazily allocate must track initializa= tion by > + /// some other means. > + #[must_use] > + #[inline] > + pub const fn dangling() -> Self { > + Unique { > + pointer: NonNull::dangling(), > + _marker: PhantomData, > + } > + } I think I already asked this, but the code until this point is copied from the rust stdlib and nowhere cited, does that work with the licensing? I also think that the code above could use some improvements: - add an `# Invariants` section with appropriate invariants (what are they supposed to be?) - Do we really want this type to be public and exported from the kernel crate? I think it would be better if it were crate-private. - What do we gain from having this type? As I learned recently, the `Unique` type from `core` doesn't actually put the `noalias` onto `Box` and `Vec`. The functions are mostly delegations to `NonNull`, so if the only advantages are that `Send` and `Sync` are already implemented, then I think we should drop this. > +} > + > +impl Unique { > + /// Creates a new `Unique`. > + /// > + /// # Safety > + /// > + /// `ptr` must be non-null. > + #[inline] > + pub const unsafe fn new_unchecked(ptr: *mut T) -> Self { > + // SAFETY: the caller must guarantee that `ptr` is non-null. > + unsafe { The only unsafe operation in the body is `new_unchecked` only that one should be wrapped in `unsafe {}`. > + Unique { > + pointer: NonNull::new_unchecked(ptr), > + _marker: PhantomData, > + } > + } > + } > + > + /// Creates a new `Unique` if `ptr` is non-null. > + #[allow(clippy::manual_map)] > + #[inline] > + pub fn new(ptr: *mut T) -> Option { > + if let Some(pointer) =3D NonNull::new(ptr) { > + Some(Unique { > + pointer, > + _marker: PhantomData, > + }) > + } else { > + None > + } Why is this so verbose? You even needed to disable the clippy lint! Can't this just be?: Some(Unique { pointer: NonNull::new(ptr)?, _marker: PhantomData, }) or maybe even NonNull::new(ptr).map(Unique::from) > + } > + > + /// Acquires the underlying `*mut` pointer. > + #[must_use =3D "`self` will be dropped if the result is not used"] This seems like an odd thing, there is no `Drop` impl that drops the pointee... --- Cheers, Benno