Messages in this thread | | | From | Andreas Hindborg <> | Subject | Re: [PATCH v3 12/13] rust: sync: reduce stack usage of `UniqueArc::try_new_uninit` | Date | Fri, 31 Mar 2023 15:03:13 +0200 |
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y86-dev@protonmail.com writes:
> From: Benno Lossin <y86-dev@protonmail.com> > > `UniqueArc::try_new_uninit` calls `Arc::try_new(MaybeUninit::uninit())`. > This results in the uninitialized memory being placed on the stack, > which may be arbitrarily large due to the generic `T` and thus could > cause a stack overflow for large types. > > Change the implementation to use the pin-init API which enables in-place > initialization. In particular it avoids having to first construct and > then move the uninitialized memory from the stack into the final location. > > Signed-off-by: Benno Lossin <y86-dev@protonmail.com> > ---
Reviewed-by Andreas Hindborg <a.hindborg@samsung.com>
> rust/kernel/lib.rs | 1 - > rust/kernel/sync/arc.rs | 14 ++++++++++++-- > 2 files changed, 12 insertions(+), 3 deletions(-) > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > index 3e2777d26ff5..d9df77132fa2 100644 > --- a/rust/kernel/lib.rs > +++ b/rust/kernel/lib.rs > @@ -27,7 +27,6 @@ > #[cfg(not(CONFIG_RUST))] > compile_error!("Missing kernel configuration for conditional compilation"); > > -#[allow(unused_extern_crates)] > // Allow proc-macros to refer to `::kernel` inside the `kernel` crate (this crate). > extern crate self as kernel; > > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs > index 77a3833cc265..4ed6329a5e5f 100644 > --- a/rust/kernel/sync/arc.rs > +++ b/rust/kernel/sync/arc.rs > @@ -18,6 +18,7 @@ > use crate::{ > bindings, > error::{Error, Result}, > + init, > init::{InPlaceInit, Init, PinInit}, > types::{ForeignOwnable, Opaque}, > }; > @@ -29,6 +30,7 @@ use core::{ > pin::Pin, > ptr::NonNull, > }; > +use macros::pin_data; > > /// A reference-counted pointer to an instance of `T`. > /// > @@ -121,6 +123,7 @@ pub struct Arc<T: ?Sized> { > _p: PhantomData<ArcInner<T>>, > } > > +#[pin_data] > #[repr(C)] > struct ArcInner<T: ?Sized> { > refcount: Opaque<bindings::refcount_t>, > @@ -501,9 +504,16 @@ impl<T> UniqueArc<T> { > > /// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet. > pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>> { > - Ok(UniqueArc::<MaybeUninit<T>> { > + // INVARIANT: The refcount is initialised to a non-zero value. > + let inner = Box::init(init!(ArcInner { > + // SAFETY: There are no safety requirements for this FFI call. > + refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }), > + data <- init::uninit(), > + }))?; > + Ok(UniqueArc { > // INVARIANT: The newly-created object has a ref-count of 1. > - inner: Arc::try_new(MaybeUninit::uninit())?, > + // SAFETY: The pointer from the `Box` is valid. > + inner: unsafe { Arc::from_inner(Box::leak(inner).into()) }, > }) > } > }
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