ForeignOwnable::Target defines the payload data of a ForeignOwnable. For
Arc<T> for instance, ForeignOwnable::Target would just be T.
This is useful for cases where a trait bound is required on the target
type of the ForeignOwnable. For instance:
fn example<P>(data: P)
where
P: ForeignOwnable,
P::Target: MyTrait,
{}
Suggested-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
rust/kernel/alloc/kbox.rs | 2 ++
rust/kernel/sync/arc.rs | 1 +
rust/kernel/types.rs | 4 ++++
3 files changed, 7 insertions(+)
diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index c386ff771d50..66fad9777567 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -403,6 +403,7 @@ unsafe impl<T: 'static, A> ForeignOwnable for Box<T, A>
where
A: Allocator,
{
+ type Target = T;
type PointedTo = T;
type Borrowed<'a> = &'a T;
type BorrowedMut<'a> = &'a mut T;
@@ -435,6 +436,7 @@ unsafe impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
where
A: Allocator,
{
+ type Target = T;
type PointedTo = T;
type Borrowed<'a> = Pin<&'a T>;
type BorrowedMut<'a> = Pin<&'a mut T>;
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index c7af0aa48a0a..24fb63597d35 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -374,6 +374,7 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
// SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
+ type Target = T;
type PointedTo = ArcInner<T>;
type Borrowed<'a> = ArcBorrow<'a, T>;
type BorrowedMut<'a> = Self::Borrowed<'a>;
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 3958a5f44d56..74c787b352a9 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -27,6 +27,9 @@
/// [`into_foreign`]: Self::into_foreign
/// [`PointedTo`]: Self::PointedTo
pub unsafe trait ForeignOwnable: Sized {
+ /// The payload type of the foreign-owned value.
+ type Target;
+
/// Type used when the value is foreign-owned. In practical terms only defines the alignment of
/// the pointer.
type PointedTo;
@@ -128,6 +131,7 @@ unsafe fn try_from_foreign(ptr: *mut Self::PointedTo) -> Option<Self> {
// SAFETY: The `into_foreign` function returns a pointer that is dangling, but well-aligned.
unsafe impl ForeignOwnable for () {
+ type Target = ();
type PointedTo = ();
type Borrowed<'a> = ();
type BorrowedMut<'a> = ();
--
2.49.0On Thu, Jun 26, 2025 at 10:01 PM Danilo Krummrich <dakr@kernel.org> wrote:
>
> ForeignOwnable::Target defines the payload data of a ForeignOwnable. For
> Arc<T> for instance, ForeignOwnable::Target would just be T.
>
> This is useful for cases where a trait bound is required on the target
> type of the ForeignOwnable. For instance:
>
> fn example<P>(data: P)
> where
> P: ForeignOwnable,
> P::Target: MyTrait,
> {}
>
> Suggested-by: Benno Lossin <lossin@kernel.org>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
I am skeptical about this patch. I think that most of the time, you
should just place the trait bound on `P` instead of having a Target
type like this.
Alice
On Thu, Jun 26, 2025 at 10:01 PM Danilo Krummrich <dakr@kernel.org> wrote:
>
> ForeignOwnable::Target defines the payload data of a ForeignOwnable. For
> Arc<T> for instance, ForeignOwnable::Target would just be T.
>
> This is useful for cases where a trait bound is required on the target
> type of the ForeignOwnable. For instance:
>
> fn example<P>(data: P)
> where
> P: ForeignOwnable,
> P::Target: MyTrait,
> {}
>
> Suggested-by: Benno Lossin <lossin@kernel.org>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Cheers,
Miguel
On Thu Jun 26, 2025 at 10:00 PM CEST, Danilo Krummrich wrote:
> ForeignOwnable::Target defines the payload data of a ForeignOwnable. For
> Arc<T> for instance, ForeignOwnable::Target would just be T.
>
> This is useful for cases where a trait bound is required on the target
> type of the ForeignOwnable. For instance:
>
> fn example<P>(data: P)
> where
> P: ForeignOwnable,
> P::Target: MyTrait,
> {}
>
> Suggested-by: Benno Lossin <lossin@kernel.org>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
We might also be able to add a `Deref<Target = Self::Target>` bound on
`Borrowed` and `BorrowedMut`, but we should only do that when necessary.
---
Cheers,
Benno
> ---
> rust/kernel/alloc/kbox.rs | 2 ++
> rust/kernel/sync/arc.rs | 1 +
> rust/kernel/types.rs | 4 ++++
> 3 files changed, 7 insertions(+)
On Thu, Jun 26, 2025 at 10:00:42PM +0200, Danilo Krummrich wrote:
> ForeignOwnable::Target defines the payload data of a ForeignOwnable. For
> Arc<T> for instance, ForeignOwnable::Target would just be T.
>
> This is useful for cases where a trait bound is required on the target
> type of the ForeignOwnable. For instance:
>
> fn example<P>(data: P)
> where
> P: ForeignOwnable,
> P::Target: MyTrait,
> {}
>
> Suggested-by: Benno Lossin <lossin@kernel.org>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
One nit below:
> ---
> rust/kernel/alloc/kbox.rs | 2 ++
> rust/kernel/sync/arc.rs | 1 +
> rust/kernel/types.rs | 4 ++++
> 3 files changed, 7 insertions(+)
>
> diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
> index c386ff771d50..66fad9777567 100644
> --- a/rust/kernel/alloc/kbox.rs
> +++ b/rust/kernel/alloc/kbox.rs
> @@ -403,6 +403,7 @@ unsafe impl<T: 'static, A> ForeignOwnable for Box<T, A>
> where
> A: Allocator,
> {
> + type Target = T;
> type PointedTo = T;
> type Borrowed<'a> = &'a T;
> type BorrowedMut<'a> = &'a mut T;
> @@ -435,6 +436,7 @@ unsafe impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
> where
> A: Allocator,
> {
> + type Target = T;
> type PointedTo = T;
> type Borrowed<'a> = Pin<&'a T>;
> type BorrowedMut<'a> = Pin<&'a mut T>;
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index c7af0aa48a0a..24fb63597d35 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -374,6 +374,7 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
>
> // SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
> unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
> + type Target = T;
> type PointedTo = ArcInner<T>;
> type Borrowed<'a> = ArcBorrow<'a, T>;
> type BorrowedMut<'a> = Self::Borrowed<'a>;
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 3958a5f44d56..74c787b352a9 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -27,6 +27,9 @@
> /// [`into_foreign`]: Self::into_foreign
> /// [`PointedTo`]: Self::PointedTo
> pub unsafe trait ForeignOwnable: Sized {
> + /// The payload type of the foreign-owned value.
> + type Target;
I think `ForeignOwnable` also implies a `T` maybe get dropped via a
pointer from `into_foreign()`. Not sure it's worth mentioning though.
Regards,
Boqun
> +
> /// Type used when the value is foreign-owned. In practical terms only defines the alignment of
> /// the pointer.
> type PointedTo;
> @@ -128,6 +131,7 @@ unsafe fn try_from_foreign(ptr: *mut Self::PointedTo) -> Option<Self> {
>
> // SAFETY: The `into_foreign` function returns a pointer that is dangling, but well-aligned.
> unsafe impl ForeignOwnable for () {
> + type Target = ();
> type PointedTo = ();
> type Borrowed<'a> = ();
> type BorrowedMut<'a> = ();
> --
> 2.49.0
>
On Thu Jun 26, 2025 at 10:20 PM CEST, Boqun Feng wrote:
> On Thu, Jun 26, 2025 at 10:00:42PM +0200, Danilo Krummrich wrote:
>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>> index 3958a5f44d56..74c787b352a9 100644
>> --- a/rust/kernel/types.rs
>> +++ b/rust/kernel/types.rs
>> @@ -27,6 +27,9 @@
>> /// [`into_foreign`]: Self::into_foreign
>> /// [`PointedTo`]: Self::PointedTo
>> pub unsafe trait ForeignOwnable: Sized {
>> + /// The payload type of the foreign-owned value.
>> + type Target;
>
> I think `ForeignOwnable` also implies a `T` maybe get dropped via a
> pointer from `into_foreign()`. Not sure it's worth mentioning though.
What? How would that happen?
---
Cheers,
Benno
On Thu, Jun 26, 2025, at 4:17 PM, Benno Lossin wrote:
> On Thu Jun 26, 2025 at 10:20 PM CEST, Boqun Feng wrote:
>> On Thu, Jun 26, 2025 at 10:00:42PM +0200, Danilo Krummrich wrote:
>>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>>> index 3958a5f44d56..74c787b352a9 100644
>>> --- a/rust/kernel/types.rs
>>> +++ b/rust/kernel/types.rs
>>> @@ -27,6 +27,9 @@
>>> /// [`into_foreign`]: Self::into_foreign
>>> /// [`PointedTo`]: Self::PointedTo
>>> pub unsafe trait ForeignOwnable: Sized {
>>> + /// The payload type of the foreign-owned value.
>>> + type Target;
>>
>> I think `ForeignOwnable` also implies a `T` maybe get dropped via a
>> pointer from `into_foreign()`. Not sure it's worth mentioning though.
>
> What? How would that happen?
>
The owner of the pointer can do from_foreign() and eventually drop
the ForeignOwnable, hence dropping T.
Regards,
Boqun
> ---
> Cheers,
> Benno
On Fri Jun 27, 2025 at 1:21 AM CEST, Boqun Feng wrote:
> On Thu, Jun 26, 2025, at 4:17 PM, Benno Lossin wrote:
>> On Thu Jun 26, 2025 at 10:20 PM CEST, Boqun Feng wrote:
>>> On Thu, Jun 26, 2025 at 10:00:42PM +0200, Danilo Krummrich wrote:
>>>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>>>> index 3958a5f44d56..74c787b352a9 100644
>>>> --- a/rust/kernel/types.rs
>>>> +++ b/rust/kernel/types.rs
>>>> @@ -27,6 +27,9 @@
>>>> /// [`into_foreign`]: Self::into_foreign
>>>> /// [`PointedTo`]: Self::PointedTo
>>>> pub unsafe trait ForeignOwnable: Sized {
>>>> + /// The payload type of the foreign-owned value.
>>>> + type Target;
>>>
>>> I think `ForeignOwnable` also implies a `T` maybe get dropped via a
>>> pointer from `into_foreign()`. Not sure it's worth mentioning though.
>>
>> What? How would that happen?
>
> The owner of the pointer can do from_foreign() and eventually drop
> the ForeignOwnable, hence dropping T.
I'm confused, you said `into_foreign` above. I don't think any sensible
ForeignOwnable implementation will drop a T in any of its functions.
---
Cheers,
Benno
On Thu, Jun 26, 2025, at 4:36 PM, Benno Lossin wrote:
> On Fri Jun 27, 2025 at 1:21 AM CEST, Boqun Feng wrote:
>> On Thu, Jun 26, 2025, at 4:17 PM, Benno Lossin wrote:
>>> On Thu Jun 26, 2025 at 10:20 PM CEST, Boqun Feng wrote:
>>>> On Thu, Jun 26, 2025 at 10:00:42PM +0200, Danilo Krummrich wrote:
>>>>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>>>>> index 3958a5f44d56..74c787b352a9 100644
>>>>> --- a/rust/kernel/types.rs
>>>>> +++ b/rust/kernel/types.rs
>>>>> @@ -27,6 +27,9 @@
>>>>> /// [`into_foreign`]: Self::into_foreign
>>>>> /// [`PointedTo`]: Self::PointedTo
>>>>> pub unsafe trait ForeignOwnable: Sized {
>>>>> + /// The payload type of the foreign-owned value.
>>>>> + type Target;
>>>>
>>>> I think `ForeignOwnable` also implies a `T` maybe get dropped via a
>>>> pointer from `into_foreign()`. Not sure it's worth mentioning though.
>>>
>>> What? How would that happen?
>>
>> The owner of the pointer can do from_foreign() and eventually drop
>> the ForeignOwnable, hence dropping T.
>
> I'm confused, you said `into_foreign` above. I don't think any sensible
> ForeignOwnable implementation will drop a T in any of its functions.
>
A KBox<T> would drop T when it gets dropped, no?
A Arc<T> would drop T when it gets dropped if it’s the last refcount.
I was trying to say “ForeignOwnable::drop() may implies Target::drop()”,
that’s what a “payload” means. Maybe that I used “T” instead of “Target”
in the original message caused confusion?
Regards,
Boqun
> ---
> Cheers,
> Benno
On Thu, Jun 26, 2025, at 4:45 PM, Boqun Feng wrote:
> On Thu, Jun 26, 2025, at 4:36 PM, Benno Lossin wrote:
>> On Fri Jun 27, 2025 at 1:21 AM CEST, Boqun Feng wrote:
>>> On Thu, Jun 26, 2025, at 4:17 PM, Benno Lossin wrote:
>>>> On Thu Jun 26, 2025 at 10:20 PM CEST, Boqun Feng wrote:
>>>>> On Thu, Jun 26, 2025 at 10:00:42PM +0200, Danilo Krummrich wrote:
>>>>>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>>>>>> index 3958a5f44d56..74c787b352a9 100644
>>>>>> --- a/rust/kernel/types.rs
>>>>>> +++ b/rust/kernel/types.rs
>>>>>> @@ -27,6 +27,9 @@
>>>>>> /// [`into_foreign`]: Self::into_foreign
>>>>>> /// [`PointedTo`]: Self::PointedTo
>>>>>> pub unsafe trait ForeignOwnable: Sized {
>>>>>> + /// The payload type of the foreign-owned value.
>>>>>> + type Target;
>>>>>
>>>>> I think `ForeignOwnable` also implies a `T` maybe get dropped via a
>>>>> pointer from `into_foreign()`. Not sure it's worth mentioning though.
>>>>
>>>> What? How would that happen?
>>>
>>> The owner of the pointer can do from_foreign() and eventually drop
>>> the ForeignOwnable, hence dropping T.
>>
>> I'm confused, you said `into_foreign` above. I don't think any sensible
>> ForeignOwnable implementation will drop a T in any of its functions.
>>
>
> A KBox<T> would drop T when it gets dropped, no?
> A Arc<T> would drop T when it gets dropped if it’s the last refcount.
>
> I was trying to say “ForeignOwnable::drop() may implies Target::drop()”,
> that’s what a “payload” means. Maybe that I used “T” instead of “Target”
> in the original message caused confusion?
>
The point is whichever receives the pointer from a into_foreign()
would owns the Target, because it can from_foreign() and
drop the ForeignOwnable. So for example, if the pointer can
be passed across threads, that means Target needs to be Send.
Regards,
Boqun
> Regards,
> Boqun
>
>> ---
>> Cheers,
>> Benno
On Fri Jun 27, 2025 at 1:55 AM CEST, Boqun Feng wrote:
> On Thu, Jun 26, 2025, at 4:45 PM, Boqun Feng wrote:
>> On Thu, Jun 26, 2025, at 4:36 PM, Benno Lossin wrote:
>>> On Fri Jun 27, 2025 at 1:21 AM CEST, Boqun Feng wrote:
>>>> On Thu, Jun 26, 2025, at 4:17 PM, Benno Lossin wrote:
>>>>> On Thu Jun 26, 2025 at 10:20 PM CEST, Boqun Feng wrote:
>>>>>> On Thu, Jun 26, 2025 at 10:00:42PM +0200, Danilo Krummrich wrote:
>>>>>>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>>>>>>> index 3958a5f44d56..74c787b352a9 100644
>>>>>>> --- a/rust/kernel/types.rs
>>>>>>> +++ b/rust/kernel/types.rs
>>>>>>> @@ -27,6 +27,9 @@
>>>>>>> /// [`into_foreign`]: Self::into_foreign
>>>>>>> /// [`PointedTo`]: Self::PointedTo
>>>>>>> pub unsafe trait ForeignOwnable: Sized {
>>>>>>> + /// The payload type of the foreign-owned value.
>>>>>>> + type Target;
>>>>>>
>>>>>> I think `ForeignOwnable` also implies a `T` maybe get dropped via a
>>>>>> pointer from `into_foreign()`. Not sure it's worth mentioning though.
>>>>>
>>>>> What? How would that happen?
>>>>
>>>> The owner of the pointer can do from_foreign() and eventually drop
>>>> the ForeignOwnable, hence dropping T.
>>>
>>> I'm confused, you said `into_foreign` above. I don't think any sensible
>>> ForeignOwnable implementation will drop a T in any of its functions.
>>>
>>
>> A KBox<T> would drop T when it gets dropped, no?
>> A Arc<T> would drop T when it gets dropped if it’s the last refcount.
>>
>> I was trying to say “ForeignOwnable::drop() may implies Target::drop()”,
>> that’s what a “payload” means. Maybe that I used “T” instead of “Target”
>> in the original message caused confusion?
Ah now I understand what you are saying. Your mentioning of
`into_foreign` and `from_foreign` confused me. Yes a `ForeignOwnable`
may drop a `T`/`Target` in its own drop function. But I don't think we
need to document that.
> The point is whichever receives the pointer from a into_foreign()
> would owns the Target, because it can from_foreign() and
> drop the ForeignOwnable. So for example, if the pointer can
> be passed across threads, that means Target needs to be Send.
We should solve this in a different manner. Document the `Send` & `Sync`
requirements on `into_foreign`. So when you turn a `P: ForeignOwnable`
that is `!Send` into a raw pointer, you are not allowed to call
`from_foreign` on a different thread.
If `P: !Sync` then you're not allowed to call `borrow[_mut]` on the
pointer from two different threads (ie only the one that is currently
owning the value is allowed to call that).
---
Cheers,
Benno
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