From: Wedson Almeida Filho <[email protected]>
These methods can be used to turn an `Arc` into a raw pointer and back,
in a way that preserves the metadata for fat pointers.
This is done using the unstable ptr_metadata feature [1]. However, it
could also be done using the unstable pointer_byte_offsets feature [2],
which is likely to have a shorter path to stabilization than
ptr_metadata.
Link: https://github.com/rust-lang/rust/issues/81513 [1]
Link: https://github.com/rust-lang/rust/issues/96283 [2]
Signed-off-by: Wedson Almeida Filho <[email protected]>
Co-developed-by: Alice Ryhl <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
---
rust/kernel/lib.rs | 1 +
rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
2 files changed, 42 insertions(+), 1 deletion(-)
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 7ea777b731e6..ad9142928fb1 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -17,6 +17,7 @@
#![feature(const_refs_to_cell)]
#![feature(dispatch_from_dyn)]
#![feature(new_uninit)]
+#![feature(ptr_metadata)]
#![feature(receiver_trait)]
#![feature(unsize)]
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index a89843cacaad..684be9f73aca 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -24,7 +24,7 @@
};
use alloc::boxed::Box;
use core::{
- alloc::AllocError,
+ alloc::{AllocError, Layout},
fmt,
marker::{PhantomData, Unsize},
mem::{ManuallyDrop, MaybeUninit},
@@ -212,6 +212,46 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
}
}
+ /// Convert the [`Arc`] into a raw pointer.
+ ///
+ /// The raw pointer has ownership of the refcount that this Arc object owned.
+ pub fn into_raw(self) -> *const T {
+ let ptr = self.ptr.as_ptr();
+ core::mem::forget(self);
+ // SAFETY: The pointer is valid.
+ unsafe { core::ptr::addr_of!((*ptr).data) }
+ }
+
+ /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
+ ///
+ /// This code relies on the `repr(C)` layout of structs as described in
+ /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
+ ///
+ /// # Safety
+ ///
+ /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
+ /// can only be called once for each previous call to [`Arc::into_raw`].
+ pub unsafe fn from_raw(ptr: *const T) -> Self {
+ let refcount_layout = Layout::new::<bindings::refcount_t>();
+ // SAFETY: The caller guarantees that the pointer is valid.
+ let val_layout = unsafe { Layout::for_value(&*ptr) };
+ // SAFETY: We're computing the layout of a real struct that existed when compiling this
+ // binary, so its layout is not so large that it can trigger arithmetic overflow.
+ let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
+
+ // This preserves the metadata in the pointer, if any.
+ //
+ // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at
+ // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
+ let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
+ let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
+ let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
+
+ // SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
+ // reference count held then will be owned by the new `Arc` object.
+ unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
+ }
+
/// Returns an [`ArcBorrow`] from the given [`Arc`].
///
/// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
--
2.41.0.rc0.172.g3f132b7071-goog
On Thu, 1 Jun 2023 13:49:41 +0000
Alice Ryhl <[email protected]> wrote:
> From: Wedson Almeida Filho <[email protected]>
>
> These methods can be used to turn an `Arc` into a raw pointer and back,
> in a way that preserves the metadata for fat pointers.
>
> This is done using the unstable ptr_metadata feature [1]. However, it
> could also be done using the unstable pointer_byte_offsets feature [2],
> which is likely to have a shorter path to stabilization than
> ptr_metadata.
>
> Link: https://github.com/rust-lang/rust/issues/81513 [1]
> Link: https://github.com/rust-lang/rust/issues/96283 [2]
> Signed-off-by: Wedson Almeida Filho <[email protected]>
> Co-developed-by: Alice Ryhl <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Reviewed-by: Gary Guo <[email protected]>
> ---
> rust/kernel/lib.rs | 1 +
> rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 42 insertions(+), 1 deletion(-)
Alice Ryhl <[email protected]> writes:
> From: Wedson Almeida Filho <[email protected]>
>
> These methods can be used to turn an `Arc` into a raw pointer and back,
> in a way that preserves the metadata for fat pointers.
>
> This is done using the unstable ptr_metadata feature [1]. However, it
> could also be done using the unstable pointer_byte_offsets feature [2],
> which is likely to have a shorter path to stabilization than
> ptr_metadata.
>
> Link: https://github.com/rust-lang/rust/issues/81513 [1]
> Link: https://github.com/rust-lang/rust/issues/96283 [2]
> Signed-off-by: Wedson Almeida Filho <[email protected]>
> Co-developed-by: Alice Ryhl <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
> ---
> rust/kernel/lib.rs | 1 +
> rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 42 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 7ea777b731e6..ad9142928fb1 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -17,6 +17,7 @@
> #![feature(const_refs_to_cell)]
> #![feature(dispatch_from_dyn)]
> #![feature(new_uninit)]
> +#![feature(ptr_metadata)]
> #![feature(receiver_trait)]
> #![feature(unsize)]
>
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a89843cacaad..684be9f73aca 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -24,7 +24,7 @@
> };
> use alloc::boxed::Box;
> use core::{
> - alloc::AllocError,
> + alloc::{AllocError, Layout},
> fmt,
> marker::{PhantomData, Unsize},
> mem::{ManuallyDrop, MaybeUninit},
> @@ -212,6 +212,46 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
> }
> }
>
> + /// Convert the [`Arc`] into a raw pointer.
> + ///
> + /// The raw pointer has ownership of the refcount that this Arc object owned.
> + pub fn into_raw(self) -> *const T {
> + let ptr = self.ptr.as_ptr();
> + core::mem::forget(self);
> + // SAFETY: The pointer is valid.
> + unsafe { core::ptr::addr_of!((*ptr).data) }
> + }
> +
> + /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
> + ///
> + /// This code relies on the `repr(C)` layout of structs as described in
> + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
> + ///
> + /// # Safety
> + ///
> + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
> + /// can only be called once for each previous call to [`Arc::into_raw`].
> + pub unsafe fn from_raw(ptr: *const T) -> Self {
> + let refcount_layout = Layout::new::<bindings::refcount_t>();
> + // SAFETY: The caller guarantees that the pointer is valid.
> + let val_layout = unsafe { Layout::for_value(&*ptr) };
> + // SAFETY: We're computing the layout of a real struct that existed when compiling this
> + // binary, so its layout is not so large that it can trigger arithmetic overflow.
> + let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
> +
> + // This preserves the metadata in the pointer, if any.
> + //
> + // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at
> + // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
> + let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
Thanks for updating the comment with the link. I looked into this and I
find that what we are doing here, even though it works, does not feel
right at all. We should be able to do this:
let metadata = core::ptr::metadata(ptr);
let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
but the way `Pointee::Metadata` is defined will not allow this, even
though we know it is valid. I would suggest the following instead:
let metadata = core::ptr::metadata(ptr);
// Convert <T as Pointee>::Metadata to <ArcInner<T> as
// Pointee>::Metadata. We know they have identical representation and thus this is OK.
let metadata: <ArcInner<T> as Pointee>::Metadata = *unsafe {
&*((&metadata as *const <T as Pointee>::Metadata as *const ())
as *const <ArcInner<T> as Pointee>::Metadata)
};
let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
Even though it is a bit more complex, it captures what we are trying to
do better.
Best regards,
Andreas
> + let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> + let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> +
> + // SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
> + // reference count held then will be owned by the new `Arc` object.
> + unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
> + }
> +
> /// Returns an [`ArcBorrow`] from the given [`Arc`].
> ///
> /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
On Fri, 02 Jun 2023 12:51:08 +0200
"Andreas Hindborg (Samsung)" <[email protected]> wrote:
> Alice Ryhl <[email protected]> writes:
>
> > From: Wedson Almeida Filho <[email protected]>
> >
> > These methods can be used to turn an `Arc` into a raw pointer and back,
> > in a way that preserves the metadata for fat pointers.
> >
> > This is done using the unstable ptr_metadata feature [1]. However, it
> > could also be done using the unstable pointer_byte_offsets feature [2],
> > which is likely to have a shorter path to stabilization than
> > ptr_metadata.
> >
> > Link: https://github.com/rust-lang/rust/issues/81513 [1]
> > Link: https://github.com/rust-lang/rust/issues/96283 [2]
> > Signed-off-by: Wedson Almeida Filho <[email protected]>
> > Co-developed-by: Alice Ryhl <[email protected]>
> > Signed-off-by: Alice Ryhl <[email protected]>
> > Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
> > ---
> > rust/kernel/lib.rs | 1 +
> > rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
> > 2 files changed, 42 insertions(+), 1 deletion(-)
> >
> > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> > index 7ea777b731e6..ad9142928fb1 100644
> > --- a/rust/kernel/lib.rs
> > +++ b/rust/kernel/lib.rs
> > @@ -17,6 +17,7 @@
> > #![feature(const_refs_to_cell)]
> > #![feature(dispatch_from_dyn)]
> > #![feature(new_uninit)]
> > +#![feature(ptr_metadata)]
> > #![feature(receiver_trait)]
> > #![feature(unsize)]
> >
> > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> > index a89843cacaad..684be9f73aca 100644
> > --- a/rust/kernel/sync/arc.rs
> > +++ b/rust/kernel/sync/arc.rs
> > @@ -24,7 +24,7 @@
> > };
> > use alloc::boxed::Box;
> > use core::{
> > - alloc::AllocError,
> > + alloc::{AllocError, Layout},
> > fmt,
> > marker::{PhantomData, Unsize},
> > mem::{ManuallyDrop, MaybeUninit},
> > @@ -212,6 +212,46 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
> > }
> > }
> >
> > + /// Convert the [`Arc`] into a raw pointer.
> > + ///
> > + /// The raw pointer has ownership of the refcount that this Arc object owned.
> > + pub fn into_raw(self) -> *const T {
> > + let ptr = self.ptr.as_ptr();
> > + core::mem::forget(self);
> > + // SAFETY: The pointer is valid.
> > + unsafe { core::ptr::addr_of!((*ptr).data) }
> > + }
> > +
> > + /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
> > + ///
> > + /// This code relies on the `repr(C)` layout of structs as described in
> > + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
> > + ///
> > + /// # Safety
> > + ///
> > + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
> > + /// can only be called once for each previous call to [`Arc::into_raw`].
> > + pub unsafe fn from_raw(ptr: *const T) -> Self {
> > + let refcount_layout = Layout::new::<bindings::refcount_t>();
> > + // SAFETY: The caller guarantees that the pointer is valid.
> > + let val_layout = unsafe { Layout::for_value(&*ptr) };
> > + // SAFETY: We're computing the layout of a real struct that existed when compiling this
> > + // binary, so its layout is not so large that it can trigger arithmetic overflow.
> > + let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
> > +
> > + // This preserves the metadata in the pointer, if any.
> > + //
> > + // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at
> > + // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
> > + let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
>
> Thanks for updating the comment with the link. I looked into this and I
> find that what we are doing here, even though it works, does not feel
> right at all. We should be able to do this:
>
> let metadata = core::ptr::metadata(ptr);
> let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
>
> but the way `Pointee::Metadata` is defined will not allow this, even
> though we know it is valid. I would suggest the following instead:
>
> let metadata = core::ptr::metadata(ptr);
> // Convert <T as Pointee>::Metadata to <ArcInner<T> as
> // Pointee>::Metadata. We know they have identical representation and thus this is OK.
> let metadata: <ArcInner<T> as Pointee>::Metadata = *unsafe {
> &*((&metadata as *const <T as Pointee>::Metadata as *const ())
> as *const <ArcInner<T> as Pointee>::Metadata)
> };
This could just be a `transmute_copy`.
> let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
>
> Even though it is a bit more complex, it captures what we are trying to
> do better.
I agree this captures the semantics better.
Best,
Gary
Gary Guo <[email protected]> writes:
> On Fri, 02 Jun 2023 12:51:08 +0200
> "Andreas Hindborg (Samsung)" <[email protected]> wrote:
>
>> Alice Ryhl <[email protected]> writes:
>>
>> > From: Wedson Almeida Filho <[email protected]>
>> >
>> > These methods can be used to turn an `Arc` into a raw pointer and back,
>> > in a way that preserves the metadata for fat pointers.
>> >
>> > This is done using the unstable ptr_metadata feature [1]. However, it
>> > could also be done using the unstable pointer_byte_offsets feature [2],
>> > which is likely to have a shorter path to stabilization than
>> > ptr_metadata.
>> >
>> > Link: https://github.com/rust-lang/rust/issues/81513 [1]
>> > Link: https://github.com/rust-lang/rust/issues/96283 [2]
>> > Signed-off-by: Wedson Almeida Filho <[email protected]>
>> > Co-developed-by: Alice Ryhl <[email protected]>
>> > Signed-off-by: Alice Ryhl <[email protected]>
>> > Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
>> > ---
>> > rust/kernel/lib.rs | 1 +
>> > rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
>> > 2 files changed, 42 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
>> > index 7ea777b731e6..ad9142928fb1 100644
>> > --- a/rust/kernel/lib.rs
>> > +++ b/rust/kernel/lib.rs
>> > @@ -17,6 +17,7 @@
>> > #![feature(const_refs_to_cell)]
>> > #![feature(dispatch_from_dyn)]
>> > #![feature(new_uninit)]
>> > +#![feature(ptr_metadata)]
>> > #![feature(receiver_trait)]
>> > #![feature(unsize)]
>> >
>> > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
>> > index a89843cacaad..684be9f73aca 100644
>> > --- a/rust/kernel/sync/arc.rs
>> > +++ b/rust/kernel/sync/arc.rs
>> > @@ -24,7 +24,7 @@
>> > };
>> > use alloc::boxed::Box;
>> > use core::{
>> > - alloc::AllocError,
>> > + alloc::{AllocError, Layout},
>> > fmt,
>> > marker::{PhantomData, Unsize},
>> > mem::{ManuallyDrop, MaybeUninit},
>> > @@ -212,6 +212,46 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
>> > }
>> > }
>> >
>> > + /// Convert the [`Arc`] into a raw pointer.
>> > + ///
>> > + /// The raw pointer has ownership of the refcount that this Arc object owned.
>> > + pub fn into_raw(self) -> *const T {
>> > + let ptr = self.ptr.as_ptr();
>> > + core::mem::forget(self);
>> > + // SAFETY: The pointer is valid.
>> > + unsafe { core::ptr::addr_of!((*ptr).data) }
>> > + }
>> > +
>> > + /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
>> > + ///
>> > + /// This code relies on the `repr(C)` layout of structs as described in
>> > + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
>> > + ///
>> > + /// # Safety
>> > + ///
>> > + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
>> > + /// can only be called once for each previous call to [`Arc::into_raw`].
>> > + pub unsafe fn from_raw(ptr: *const T) -> Self {
>> > + let refcount_layout = Layout::new::<bindings::refcount_t>();
>> > + // SAFETY: The caller guarantees that the pointer is valid.
>> > + let val_layout = unsafe { Layout::for_value(&*ptr) };
>> > + // SAFETY: We're computing the layout of a real struct that existed when compiling this
>> > + // binary, so its layout is not so large that it can trigger arithmetic overflow.
>> > + let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
>> > +
>> > + // This preserves the metadata in the pointer, if any.
>> > + //
>> > + // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at
>> > + // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
>> > + let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
>>
>> Thanks for updating the comment with the link. I looked into this and I
>> find that what we are doing here, even though it works, does not feel
>> right at all. We should be able to do this:
>>
>> let metadata = core::ptr::metadata(ptr);
>> let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
>> let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
>>
>> but the way `Pointee::Metadata` is defined will not allow this, even
>> though we know it is valid. I would suggest the following instead:
>>
>> let metadata = core::ptr::metadata(ptr);
>> // Convert <T as Pointee>::Metadata to <ArcInner<T> as
>> // Pointee>::Metadata. We know they have identical representation and thus this is OK.
>> let metadata: <ArcInner<T> as Pointee>::Metadata = *unsafe {
>> &*((&metadata as *const <T as Pointee>::Metadata as *const ())
>> as *const <ArcInner<T> as Pointee>::Metadata)
>> };
>
> This could just be a `transmute_copy`.
Even better ????
BR Andreas
>
>> let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
>> let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
>>
>> Even though it is a bit more complex, it captures what we are trying to
>> do better.
>
> I agree this captures the semantics better.
>
> Best,
> Gary
On Mon, Jun 05, 2023 at 03:31:42PM +0100, Gary Guo wrote:
> On Fri, 02 Jun 2023 12:51:08 +0200
> "Andreas Hindborg (Samsung)" <[email protected]> wrote:
>
> > Alice Ryhl <[email protected]> writes:
> >
> > > From: Wedson Almeida Filho <[email protected]>
> > >
> > > These methods can be used to turn an `Arc` into a raw pointer and back,
> > > in a way that preserves the metadata for fat pointers.
> > >
> > > This is done using the unstable ptr_metadata feature [1]. However, it
> > > could also be done using the unstable pointer_byte_offsets feature [2],
> > > which is likely to have a shorter path to stabilization than
> > > ptr_metadata.
> > >
> > > Link: https://github.com/rust-lang/rust/issues/81513 [1]
> > > Link: https://github.com/rust-lang/rust/issues/96283 [2]
> > > Signed-off-by: Wedson Almeida Filho <[email protected]>
> > > Co-developed-by: Alice Ryhl <[email protected]>
> > > Signed-off-by: Alice Ryhl <[email protected]>
> > > Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
> > > ---
> > > rust/kernel/lib.rs | 1 +
> > > rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
> > > 2 files changed, 42 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> > > index 7ea777b731e6..ad9142928fb1 100644
> > > --- a/rust/kernel/lib.rs
> > > +++ b/rust/kernel/lib.rs
> > > @@ -17,6 +17,7 @@
> > > #![feature(const_refs_to_cell)]
> > > #![feature(dispatch_from_dyn)]
> > > #![feature(new_uninit)]
> > > +#![feature(ptr_metadata)]
> > > #![feature(receiver_trait)]
> > > #![feature(unsize)]
> > >
> > > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> > > index a89843cacaad..684be9f73aca 100644
> > > --- a/rust/kernel/sync/arc.rs
> > > +++ b/rust/kernel/sync/arc.rs
> > > @@ -24,7 +24,7 @@
> > > };
> > > use alloc::boxed::Box;
> > > use core::{
> > > - alloc::AllocError,
> > > + alloc::{AllocError, Layout},
> > > fmt,
> > > marker::{PhantomData, Unsize},
> > > mem::{ManuallyDrop, MaybeUninit},
> > > @@ -212,6 +212,46 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
> > > }
> > > }
> > >
> > > + /// Convert the [`Arc`] into a raw pointer.
> > > + ///
> > > + /// The raw pointer has ownership of the refcount that this Arc object owned.
> > > + pub fn into_raw(self) -> *const T {
> > > + let ptr = self.ptr.as_ptr();
> > > + core::mem::forget(self);
> > > + // SAFETY: The pointer is valid.
> > > + unsafe { core::ptr::addr_of!((*ptr).data) }
> > > + }
> > > +
> > > + /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
> > > + ///
> > > + /// This code relies on the `repr(C)` layout of structs as described in
> > > + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
> > > + ///
> > > + /// # Safety
> > > + ///
> > > + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
> > > + /// can only be called once for each previous call to [`Arc::into_raw`].
> > > + pub unsafe fn from_raw(ptr: *const T) -> Self {
> > > + let refcount_layout = Layout::new::<bindings::refcount_t>();
> > > + // SAFETY: The caller guarantees that the pointer is valid.
> > > + let val_layout = unsafe { Layout::for_value(&*ptr) };
> > > + // SAFETY: We're computing the layout of a real struct that existed when compiling this
> > > + // binary, so its layout is not so large that it can trigger arithmetic overflow.
> > > + let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
> > > +
> > > + // This preserves the metadata in the pointer, if any.
> > > + //
> > > + // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at
> > > + // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
> > > + let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
> >
> > Thanks for updating the comment with the link. I looked into this and I
> > find that what we are doing here, even though it works, does not feel
> > right at all. We should be able to do this:
> >
> > let metadata = core::ptr::metadata(ptr);
> > let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> > let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> >
> > but the way `Pointee::Metadata` is defined will not allow this, even
> > though we know it is valid. I would suggest the following instead:
> >
> > let metadata = core::ptr::metadata(ptr);
> > // Convert <T as Pointee>::Metadata to <ArcInner<T> as
> > // Pointee>::Metadata. We know they have identical representation and thus this is OK.
> > let metadata: <ArcInner<T> as Pointee>::Metadata = *unsafe {
> > &*((&metadata as *const <T as Pointee>::Metadata as *const ())
> > as *const <ArcInner<T> as Pointee>::Metadata)
> > };
>
> This could just be a `transmute_copy`.
>
Or just `transmute`:
let metadata = unsafe {
core::mem::transmute<_, <ArcInner<T> as
Pointee>>::Metadata>(metadata)
};
? Since `Pointee::Metadata` is `Copy`.
Regards,
Boqun
> > let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> > let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> >
> > Even though it is a bit more complex, it captures what we are trying to
> > do better.
>
> I agree this captures the semantics better.
>
I actually wish that we could use `wrapping_byte_offset`[1], and just
// `*const T` and `*const ArcInner<T>` should have the same
// metdata, so convert the pointer type first.
let ptr = ptr as *const ArcInner<T>;
// .. and then adjust the byte offset.
let ptr = ptr.wrapping_byte_offset(-val_offset);
This may be the opposite direction as Andreas proposed ;-), but the
result is less code.
Regards,
Boqun
> Best,
> Gary
On Mon, Jun 05, 2023 at 08:00:57AM -0700, Boqun Feng wrote:
> On Mon, Jun 05, 2023 at 03:31:42PM +0100, Gary Guo wrote:
> > On Fri, 02 Jun 2023 12:51:08 +0200
> > "Andreas Hindborg (Samsung)" <[email protected]> wrote:
> >
> > > Alice Ryhl <[email protected]> writes:
> > >
> > > > From: Wedson Almeida Filho <[email protected]>
> > > >
> > > > These methods can be used to turn an `Arc` into a raw pointer and back,
> > > > in a way that preserves the metadata for fat pointers.
> > > >
> > > > This is done using the unstable ptr_metadata feature [1]. However, it
> > > > could also be done using the unstable pointer_byte_offsets feature [2],
> > > > which is likely to have a shorter path to stabilization than
> > > > ptr_metadata.
> > > >
> > > > Link: https://github.com/rust-lang/rust/issues/81513 [1]
> > > > Link: https://github.com/rust-lang/rust/issues/96283 [2]
> > > > Signed-off-by: Wedson Almeida Filho <[email protected]>
> > > > Co-developed-by: Alice Ryhl <[email protected]>
> > > > Signed-off-by: Alice Ryhl <[email protected]>
> > > > Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
> > > > ---
> > > > rust/kernel/lib.rs | 1 +
> > > > rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
> > > > 2 files changed, 42 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> > > > index 7ea777b731e6..ad9142928fb1 100644
> > > > --- a/rust/kernel/lib.rs
> > > > +++ b/rust/kernel/lib.rs
> > > > @@ -17,6 +17,7 @@
> > > > #![feature(const_refs_to_cell)]
> > > > #![feature(dispatch_from_dyn)]
> > > > #![feature(new_uninit)]
> > > > +#![feature(ptr_metadata)]
> > > > #![feature(receiver_trait)]
> > > > #![feature(unsize)]
> > > >
> > > > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> > > > index a89843cacaad..684be9f73aca 100644
> > > > --- a/rust/kernel/sync/arc.rs
> > > > +++ b/rust/kernel/sync/arc.rs
> > > > @@ -24,7 +24,7 @@
> > > > };
> > > > use alloc::boxed::Box;
> > > > use core::{
> > > > - alloc::AllocError,
> > > > + alloc::{AllocError, Layout},
> > > > fmt,
> > > > marker::{PhantomData, Unsize},
> > > > mem::{ManuallyDrop, MaybeUninit},
> > > > @@ -212,6 +212,46 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
> > > > }
> > > > }
> > > >
> > > > + /// Convert the [`Arc`] into a raw pointer.
> > > > + ///
> > > > + /// The raw pointer has ownership of the refcount that this Arc object owned.
> > > > + pub fn into_raw(self) -> *const T {
> > > > + let ptr = self.ptr.as_ptr();
> > > > + core::mem::forget(self);
> > > > + // SAFETY: The pointer is valid.
> > > > + unsafe { core::ptr::addr_of!((*ptr).data) }
> > > > + }
> > > > +
> > > > + /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
> > > > + ///
> > > > + /// This code relies on the `repr(C)` layout of structs as described in
> > > > + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
> > > > + ///
> > > > + /// # Safety
> > > > + ///
> > > > + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
> > > > + /// can only be called once for each previous call to [`Arc::into_raw`].
> > > > + pub unsafe fn from_raw(ptr: *const T) -> Self {
> > > > + let refcount_layout = Layout::new::<bindings::refcount_t>();
> > > > + // SAFETY: The caller guarantees that the pointer is valid.
> > > > + let val_layout = unsafe { Layout::for_value(&*ptr) };
> > > > + // SAFETY: We're computing the layout of a real struct that existed when compiling this
> > > > + // binary, so its layout is not so large that it can trigger arithmetic overflow.
> > > > + let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
> > > > +
> > > > + // This preserves the metadata in the pointer, if any.
> > > > + //
> > > > + // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at
> > > > + // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
> > > > + let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
> > >
> > > Thanks for updating the comment with the link. I looked into this and I
> > > find that what we are doing here, even though it works, does not feel
> > > right at all. We should be able to do this:
> > >
> > > let metadata = core::ptr::metadata(ptr);
> > > let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> > > let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> > >
> > > but the way `Pointee::Metadata` is defined will not allow this, even
> > > though we know it is valid. I would suggest the following instead:
> > >
> > > let metadata = core::ptr::metadata(ptr);
> > > // Convert <T as Pointee>::Metadata to <ArcInner<T> as
> > > // Pointee>::Metadata. We know they have identical representation and thus this is OK.
> > > let metadata: <ArcInner<T> as Pointee>::Metadata = *unsafe {
> > > &*((&metadata as *const <T as Pointee>::Metadata as *const ())
> > > as *const <ArcInner<T> as Pointee>::Metadata)
> > > };
> >
> > This could just be a `transmute_copy`.
> >
>
> Or just `transmute`:
>
> let metadata = unsafe {
> core::mem::transmute<_, <ArcInner<T> as
> Pointee>>::Metadata>(metadata)
> };
>
> ? Since `Pointee::Metadata` is `Copy`.
>
> Regards,
> Boqun
>
> > > let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> > > let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> > >
> > > Even though it is a bit more complex, it captures what we are trying to
> > > do better.
> >
> > I agree this captures the semantics better.
> >
>
> I actually wish that we could use `wrapping_byte_offset`[1], and just
>
> // `*const T` and `*const ArcInner<T>` should have the same
> // metdata, so convert the pointer type first.
> let ptr = ptr as *const ArcInner<T>;
>
> // .. and then adjust the byte offset.
> let ptr = ptr.wrapping_byte_offset(-val_offset);
>
> This may be the opposite direction as Andreas proposed ;-), but the
> result is less code.
>
(Forget the link to the function)
[1]: https://doc.rust-lang.org/std/primitive.pointer.html#method.wrapping_byte_offset
> Regards,
> Boqun
>
> > Best,
> > Gary
Boqun Feng <[email protected]> writes:
> On Mon, Jun 05, 2023 at 03:31:42PM +0100, Gary Guo wrote:
>> On Fri, 02 Jun 2023 12:51:08 +0200
>> "Andreas Hindborg (Samsung)" <[email protected]> wrote:
>>
>> > Alice Ryhl <[email protected]> writes:
>> >
>> > > From: Wedson Almeida Filho <[email protected]>
>> > >
>> > > These methods can be used to turn an `Arc` into a raw pointer and back,
>> > > in a way that preserves the metadata for fat pointers.
>> > >
>> > > This is done using the unstable ptr_metadata feature [1]. However, it
>> > > could also be done using the unstable pointer_byte_offsets feature [2],
>> > > which is likely to have a shorter path to stabilization than
>> > > ptr_metadata.
>> > >
>> > > Link: https://github.com/rust-lang/rust/issues/81513 [1]
>> > > Link: https://github.com/rust-lang/rust/issues/96283 [2]
>> > > Signed-off-by: Wedson Almeida Filho <[email protected]>
>> > > Co-developed-by: Alice Ryhl <[email protected]>
>> > > Signed-off-by: Alice Ryhl <[email protected]>
>> > > Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
>> > > ---
>> > > rust/kernel/lib.rs | 1 +
>> > > rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
>> > > 2 files changed, 42 insertions(+), 1 deletion(-)
>> > >
>> > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
>> > > index 7ea777b731e6..ad9142928fb1 100644
>> > > --- a/rust/kernel/lib.rs
>> > > +++ b/rust/kernel/lib.rs
>> > > @@ -17,6 +17,7 @@
>> > > #![feature(const_refs_to_cell)]
>> > > #![feature(dispatch_from_dyn)]
>> > > #![feature(new_uninit)]
>> > > +#![feature(ptr_metadata)]
>> > > #![feature(receiver_trait)]
>> > > #![feature(unsize)]
>> > >
>> > > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
>> > > index a89843cacaad..684be9f73aca 100644
>> > > --- a/rust/kernel/sync/arc.rs
>> > > +++ b/rust/kernel/sync/arc.rs
>> > > @@ -24,7 +24,7 @@
>> > > };
>> > > use alloc::boxed::Box;
>> > > use core::{
>> > > - alloc::AllocError,
>> > > + alloc::{AllocError, Layout},
>> > > fmt,
>> > > marker::{PhantomData, Unsize},
>> > > mem::{ManuallyDrop, MaybeUninit},
>> > > @@ -212,6 +212,46 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
>> > > }
>> > > }
>> > >
>> > > + /// Convert the [`Arc`] into a raw pointer.
>> > > + ///
>> > > + /// The raw pointer has ownership of the refcount that this Arc object owned.
>> > > + pub fn into_raw(self) -> *const T {
>> > > + let ptr = self.ptr.as_ptr();
>> > > + core::mem::forget(self);
>> > > + // SAFETY: The pointer is valid.
>> > > + unsafe { core::ptr::addr_of!((*ptr).data) }
>> > > + }
>> > > +
>> > > + /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
>> > > + ///
>> > > + /// This code relies on the `repr(C)` layout of structs as described in
>> > > + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
>> > > + ///
>> > > + /// # Safety
>> > > + ///
>> > > + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
>> > > + /// can only be called once for each previous call to [`Arc::into_raw`].
>> > > + pub unsafe fn from_raw(ptr: *const T) -> Self {
>> > > + let refcount_layout = Layout::new::<bindings::refcount_t>();
>> > > + // SAFETY: The caller guarantees that the pointer is valid.
>> > > + let val_layout = unsafe { Layout::for_value(&*ptr) };
>> > > + // SAFETY: We're computing the layout of a real struct that existed when compiling this
>> > > + // binary, so its layout is not so large that it can trigger arithmetic overflow.
>> > > + let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
>> > > +
>> > > + // This preserves the metadata in the pointer, if any.
>> > > + //
>> > > + // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at
>> > > + // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
>> > > + let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
>> >
>> > Thanks for updating the comment with the link. I looked into this and I
>> > find that what we are doing here, even though it works, does not feel
>> > right at all. We should be able to do this:
>> >
>> > let metadata = core::ptr::metadata(ptr);
>> > let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
>> > let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
>> >
>> > but the way `Pointee::Metadata` is defined will not allow this, even
>> > though we know it is valid. I would suggest the following instead:
>> >
>> > let metadata = core::ptr::metadata(ptr);
>> > // Convert <T as Pointee>::Metadata to <ArcInner<T> as
>> > // Pointee>::Metadata. We know they have identical representation and thus this is OK.
>> > let metadata: <ArcInner<T> as Pointee>::Metadata = *unsafe {
>> > &*((&metadata as *const <T as Pointee>::Metadata as *const ())
>> > as *const <ArcInner<T> as Pointee>::Metadata)
>> > };
>>
>> This could just be a `transmute_copy`.
>>
>
> Or just `transmute`:
>
> let metadata = unsafe {
> core::mem::transmute<_, <ArcInner<T> as
> Pointee>>::Metadata>(metadata)
> };
>
> ? Since `Pointee::Metadata` is `Copy`.
I like `transmute_copy()` better for being more explicit.
>
> Regards,
> Boqun
>
>> > let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
>> > let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
>> >
>> > Even though it is a bit more complex, it captures what we are trying to
>> > do better.
>>
>> I agree this captures the semantics better.
>>
>
> I actually wish that we could use `wrapping_byte_offset`[1], and just
>
> // `*const T` and `*const ArcInner<T>` should have the same
> // metdata, so convert the pointer type first.
> let ptr = ptr as *const ArcInner<T>;
>
> // .. and then adjust the byte offset.
> let ptr = ptr.wrapping_byte_offset(-val_offset);
>
> This may be the opposite direction as Andreas proposed ;-), but the
> result is less code.
Idk, it is sort of similar to the original approach. I think it is a
good idea to be a bit explicit about what we are doing here.
BR Andreas
On 01.06.23 15:49, Alice Ryhl wrote:
> From: Wedson Almeida Filho <[email protected]>
>
> These methods can be used to turn an `Arc` into a raw pointer and back,
> in a way that preserves the metadata for fat pointers.
>
> This is done using the unstable ptr_metadata feature [1]. However, it
> could also be done using the unstable pointer_byte_offsets feature [2],
> which is likely to have a shorter path to stabilization than
> ptr_metadata.
>
> Link: https://github.com/rust-lang/rust/issues/81513 [1]
> Link: https://github.com/rust-lang/rust/issues/96283 [2]
> Signed-off-by: Wedson Almeida Filho <[email protected]>
> Co-developed-by: Alice Ryhl <[email protected]>
> Signed-off-by: Alice Ryhl <[email protected]>
> Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
--
Cheers,
Benno
> ---
> rust/kernel/lib.rs | 1 +
> rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 42 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 7ea777b731e6..ad9142928fb1 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -17,6 +17,7 @@
> #![feature(const_refs_to_cell)]
> #![feature(dispatch_from_dyn)]
> #![feature(new_uninit)]
> +#![feature(ptr_metadata)]
> #![feature(receiver_trait)]
> #![feature(unsize)]
>
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a89843cacaad..684be9f73aca 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -24,7 +24,7 @@
> };
> use alloc::boxed::Box;
> use core::{
> - alloc::AllocError,
> + alloc::{AllocError, Layout},
> fmt,
> marker::{PhantomData, Unsize},
> mem::{ManuallyDrop, MaybeUninit},
> @@ -212,6 +212,46 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
> }
> }
>
> + /// Convert the [`Arc`] into a raw pointer.
> + ///
> + /// The raw pointer has ownership of the refcount that this Arc object owned.
> + pub fn into_raw(self) -> *const T {
> + let ptr = self.ptr.as_ptr();
> + core::mem::forget(self);
> + // SAFETY: The pointer is valid.
> + unsafe { core::ptr::addr_of!((*ptr).data) }
> + }
> +
> + /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
> + ///
> + /// This code relies on the `repr(C)` layout of structs as described in
> + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
> + ///
> + /// # Safety
> + ///
> + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
> + /// can only be called once for each previous call to [`Arc::into_raw`].
> + pub unsafe fn from_raw(ptr: *const T) -> Self {
> + let refcount_layout = Layout::new::<bindings::refcount_t>();
> + // SAFETY: The caller guarantees that the pointer is valid.
> + let val_layout = unsafe { Layout::for_value(&*ptr) };
> + // SAFETY: We're computing the layout of a real struct that existed when compiling this
> + // binary, so its layout is not so large that it can trigger arithmetic overflow.
> + let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
> +
> + // This preserves the metadata in the pointer, if any.
> + //
> + // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at
> + // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
> + let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
> + let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> + let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> +
> + // SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
> + // reference count held then will be owned by the new `Arc` object.
> + unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
> + }
> +
> /// Returns an [`ArcBorrow`] from the given [`Arc`].
> ///
> /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
> --
> 2.41.0.rc0.172.g3f132b7071-goog
>