[PATCH v1 0/2] Add atomic bool support

[PATCH v1 0/2] Add atomic bool support

[FUJITA Tomonori]

This adds `bool` support to the Rust LKMM atomics.

Rust specifies that `bool` has size 1 and alignment 1 [1], so it can
be represented using an `i8` backing type.

Since `bool` only permits the bit patterns 0x00 and 0x01, the first
patch also documents an additional safety preconditions for unsafe
`Atomic::<T>::from_ptr`.

`from_ptr()` exists to operate on C-side storage so I don't think it
makes sense for bool. We could restrict from_ptr() via a marker trait.

[1] https://doc.rust-lang.org/reference/types/boolean.html


FUJITA Tomonori (2):
  rust: sync: atomic: Add atomic bool support via i8 representation
  rust: sync: atomic: Add atomic bool tests

 rust/kernel/sync/atomic.rs           |  1 +
 rust/kernel/sync/atomic/internal.rs  |  8 ++++++++
 rust/kernel/sync/atomic/predefine.rs | 27 +++++++++++++++++++++++++++
 3 files changed, 36 insertions(+)

base-commit: 13ade169e801a423bc1a5a5c3c6ac680a144a608
-- 
2.43.0

[PATCH v1 1/2] rust: sync: atomic: Add atomic bool support via i8 representation

[FUJITA Tomonori]

Add `bool` support, `Atomic<bool>` by using `i8` as its underlying
representation.

Rust specifies that `bool` has size 1 and alignment 1 [1], so it
matches `i8` on layout; keep `static_assert!()` checks to enforce this
assumption at build time.

Implement `AtomicImpl` for `bool` under
`CONFIG_ARCH_SUPPORTS_ATOMIC_RMW`, consistent with the existing
`i8/i16` gating.

Document the additional safety requirement for
`Atomic::<bool>::from_ptr`: only bit patterns 0 (false) and 1 (true)
are valid.

Link: https://doc.rust-lang.org/reference/types/boolean.html [1]
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/kernel/sync/atomic.rs           |  1 +
 rust/kernel/sync/atomic/internal.rs  |  8 ++++++++
 rust/kernel/sync/atomic/predefine.rs | 11 +++++++++++
 3 files changed, 20 insertions(+)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index 4aebeacb961a..2c998cbd300e 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -158,6 +158,7 @@ pub const fn new(v: T) -> Self {
     ///
     /// - `ptr` is aligned to `align_of::<T>()`.
     /// - `ptr` is valid for reads and writes for `'a`.
+    /// - If `T` is `bool`, only the bit patterns 0 (`false`) and 1 (`true`) are valid.
     /// - For the duration of `'a`, other accesses to `*ptr` must not cause data races (defined
     ///   by [`LKMM`]) against atomic operations on the returned reference. Note that if all other
     ///   accesses are atomic, then this safety requirement is trivially fulfilled.
diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
index 0dac58bca2b3..0e12955082e5 100644
--- a/rust/kernel/sync/atomic/internal.rs
+++ b/rust/kernel/sync/atomic/internal.rs
@@ -16,6 +16,7 @@ pub trait Sealed {}
 // The C side supports atomic primitives only for `i32` and `i64` (`atomic_t` and `atomic64_t`),
 // while the Rust side also layers provides atomic support for `i8` and `i16`
 // on top of lower-level C primitives.
+impl private::Sealed for bool {}
 impl private::Sealed for i8 {}
 impl private::Sealed for i16 {}
 impl private::Sealed for i32 {}
@@ -37,6 +38,13 @@ pub trait AtomicImpl: Sized + Send + Copy + private::Sealed {
     type Delta;
 }
 
+// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
+// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
+#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
+impl AtomicImpl for bool {
+    type Delta = Self;
+}
+
 // The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
 // guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
 #[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
index 248d26555ccf..3fc99174b086 100644
--- a/rust/kernel/sync/atomic/predefine.rs
+++ b/rust/kernel/sync/atomic/predefine.rs
@@ -5,6 +5,17 @@
 use crate::static_assert;
 use core::mem::{align_of, size_of};
 
+// Ensure size and alignment requirements are checked.
+static_assert!(size_of::<bool>() == size_of::<i8>());
+static_assert!(align_of::<bool>() == align_of::<i8>());
+
+// SAFETY: `bool` has the same size and alignment as `i8`, and Rust guarantees that `bool` has
+// only two valid bit patterns: 0 (false) and 1 (true). Those are valid `i8` values, so `bool` is
+// round-trip transmutable to `i8`.
+unsafe impl super::AtomicType for bool {
+    type Repr = i8;
+}
+
 // SAFETY: `i8` has the same size and alignment with itself, and is round-trip transmutable to
 // itself.
 unsafe impl super::AtomicType for i8 {
-- 
2.43.0

[PATCH v1 2/2] rust: sync: atomic: Add atomic bool tests

[FUJITA Tomonori]

Add tests for Atomic<bool> operations.

Atomic<bool> does not fit into the existing u8/16/32/64 tests so
introduce a dedicated tests for it.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/kernel/sync/atomic/predefine.rs | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
index 3fc99174b086..42067c6a266c 100644
--- a/rust/kernel/sync/atomic/predefine.rs
+++ b/rust/kernel/sync/atomic/predefine.rs
@@ -199,4 +199,20 @@ fn atomic_arithmetic_tests() {
             assert_eq!(v + 25, x.load(Relaxed));
         });
     }
+
+    #[test]
+    fn atomic_bool_tests() {
+        let x = Atomic::new(false);
+
+        assert_eq!(false, x.load(Relaxed));
+        x.store(true, Relaxed);
+        assert_eq!(true, x.load(Relaxed));
+
+        assert_eq!(true, x.xchg(false, Relaxed));
+        assert_eq!(false, x.load(Relaxed));
+
+        assert_eq!(Err(false), x.cmpxchg(true, true, Relaxed));
+        assert_eq!(false, x.load(Relaxed));
+        assert_eq!(Ok(false), x.cmpxchg(false, true, Full));
+    }
 }
-- 
2.43.0

Re: [PATCH v1 1/2] rust: sync: atomic: Add atomic bool support via i8 representation

[Boqun Feng]

On Tue, Dec 30, 2025 at 01:50:27PM +0900, FUJITA Tomonori wrote:
> Add `bool` support, `Atomic<bool>` by using `i8` as its underlying
> representation.
> 
> Rust specifies that `bool` has size 1 and alignment 1 [1], so it
> matches `i8` on layout; keep `static_assert!()` checks to enforce this
> assumption at build time.
> 
> Implement `AtomicImpl` for `bool` under
> `CONFIG_ARCH_SUPPORTS_ATOMIC_RMW`, consistent with the existing
> `i8/i16` gating.
> 
> Document the additional safety requirement for
> `Atomic::<bool>::from_ptr`: only bit patterns 0 (false) and 1 (true)
> are valid.
> 
> Link: https://doc.rust-lang.org/reference/types/boolean.html [1]
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
>  rust/kernel/sync/atomic.rs           |  1 +
>  rust/kernel/sync/atomic/internal.rs  |  8 ++++++++
>  rust/kernel/sync/atomic/predefine.rs | 11 +++++++++++
>  3 files changed, 20 insertions(+)
> 
> diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
> index 4aebeacb961a..2c998cbd300e 100644
> --- a/rust/kernel/sync/atomic.rs
> +++ b/rust/kernel/sync/atomic.rs
> @@ -158,6 +158,7 @@ pub const fn new(v: T) -> Self {
>      ///
>      /// - `ptr` is aligned to `align_of::<T>()`.
>      /// - `ptr` is valid for reads and writes for `'a`.
> +    /// - If `T` is `bool`, only the bit patterns 0 (`false`) and 1 (`true`) are valid.

This line is unnecessary, since "`ptr` is valid for ..." means `*ptr`
has to have the valid binary representive of `T`.

>      /// - For the duration of `'a`, other accesses to `*ptr` must not cause data races (defined
>      ///   by [`LKMM`]) against atomic operations on the returned reference. Note that if all other
>      ///   accesses are atomic, then this safety requirement is trivially fulfilled.
> diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
> index 0dac58bca2b3..0e12955082e5 100644
> --- a/rust/kernel/sync/atomic/internal.rs
> +++ b/rust/kernel/sync/atomic/internal.rs
> @@ -16,6 +16,7 @@ pub trait Sealed {}
>  // The C side supports atomic primitives only for `i32` and `i64` (`atomic_t` and `atomic64_t`),
>  // while the Rust side also layers provides atomic support for `i8` and `i16`
>  // on top of lower-level C primitives.
> +impl private::Sealed for bool {}
>  impl private::Sealed for i8 {}
>  impl private::Sealed for i16 {}
>  impl private::Sealed for i32 {}
> @@ -37,6 +38,13 @@ pub trait AtomicImpl: Sized + Send + Copy + private::Sealed {
>      type Delta;
>  }
>  
> +// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
> +// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
> +#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
> +impl AtomicImpl for bool {
> +    type Delta = Self;
> +}

I don't think you need this impl block.

Regards,
Boqun

> +
>  // The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
>  // guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
>  #[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
> diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
> index 248d26555ccf..3fc99174b086 100644
> --- a/rust/kernel/sync/atomic/predefine.rs
> +++ b/rust/kernel/sync/atomic/predefine.rs
> @@ -5,6 +5,17 @@
>  use crate::static_assert;
>  use core::mem::{align_of, size_of};
>  
> +// Ensure size and alignment requirements are checked.
> +static_assert!(size_of::<bool>() == size_of::<i8>());
> +static_assert!(align_of::<bool>() == align_of::<i8>());
> +
> +// SAFETY: `bool` has the same size and alignment as `i8`, and Rust guarantees that `bool` has
> +// only two valid bit patterns: 0 (false) and 1 (true). Those are valid `i8` values, so `bool` is
> +// round-trip transmutable to `i8`.
> +unsafe impl super::AtomicType for bool {
> +    type Repr = i8;
> +}
> +
>  // SAFETY: `i8` has the same size and alignment with itself, and is round-trip transmutable to
>  // itself.
>  unsafe impl super::AtomicType for i8 {
> -- 
> 2.43.0
> 

Re: [PATCH v1 1/2] rust: sync: atomic: Add atomic bool support via i8 representation

[FUJITA Tomonori]

On Wed, 31 Dec 2025 06:45:34 +0800
Boqun Feng <boqun.feng@gmail.com> wrote:

> On Tue, Dec 30, 2025 at 01:50:27PM +0900, FUJITA Tomonori wrote:
>> Add `bool` support, `Atomic<bool>` by using `i8` as its underlying
>> representation.
>> 
>> Rust specifies that `bool` has size 1 and alignment 1 [1], so it
>> matches `i8` on layout; keep `static_assert!()` checks to enforce this
>> assumption at build time.
>> 
>> Implement `AtomicImpl` for `bool` under
>> `CONFIG_ARCH_SUPPORTS_ATOMIC_RMW`, consistent with the existing
>> `i8/i16` gating.
>> 
>> Document the additional safety requirement for
>> `Atomic::<bool>::from_ptr`: only bit patterns 0 (false) and 1 (true)
>> are valid.
>> 
>> Link: https://doc.rust-lang.org/reference/types/boolean.html [1]
>> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
>> ---
>>  rust/kernel/sync/atomic.rs           |  1 +
>>  rust/kernel/sync/atomic/internal.rs  |  8 ++++++++
>>  rust/kernel/sync/atomic/predefine.rs | 11 +++++++++++
>>  3 files changed, 20 insertions(+)
>> 
>> diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
>> index 4aebeacb961a..2c998cbd300e 100644
>> --- a/rust/kernel/sync/atomic.rs
>> +++ b/rust/kernel/sync/atomic.rs
>> @@ -158,6 +158,7 @@ pub const fn new(v: T) -> Self {
>>      ///
>>      /// - `ptr` is aligned to `align_of::<T>()`.
>>      /// - `ptr` is valid for reads and writes for `'a`.
>> +    /// - If `T` is `bool`, only the bit patterns 0 (`false`) and 1 (`true`) are valid.
> 
> This line is unnecessary, since "`ptr` is valid for ..." means `*ptr`
> has to have the valid binary representive of `T`.

Understood, I will drop this in v2.


>>      /// - For the duration of `'a`, other accesses to `*ptr` must not cause data races (defined
>>      ///   by [`LKMM`]) against atomic operations on the returned reference. Note that if all other
>>      ///   accesses are atomic, then this safety requirement is trivially fulfilled.
>> diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
>> index 0dac58bca2b3..0e12955082e5 100644
>> --- a/rust/kernel/sync/atomic/internal.rs
>> +++ b/rust/kernel/sync/atomic/internal.rs
>> @@ -16,6 +16,7 @@ pub trait Sealed {}
>>  // The C side supports atomic primitives only for `i32` and `i64` (`atomic_t` and `atomic64_t`),
>>  // while the Rust side also layers provides atomic support for `i8` and `i16`
>>  // on top of lower-level C primitives.
>> +impl private::Sealed for bool {}
>>  impl private::Sealed for i8 {}
>>  impl private::Sealed for i16 {}
>>  impl private::Sealed for i32 {}
>> @@ -37,6 +38,13 @@ pub trait AtomicImpl: Sized + Send + Copy + private::Sealed {
>>      type Delta;
>>  }
>>  
>> +// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
>> +// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
>> +#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
>> +impl AtomicImpl for bool {
>> +    type Delta = Self;
>> +}
> 
> I don't think you need this impl block.

You are right. Only the backing atomic types need to implement this. I
will drop in v2.


Thanks!

Re: [PATCH v1 0/2] Add atomic bool support

[Gary Guo]

On Tue, 30 Dec 2025 13:50:26 +0900
FUJITA Tomonori <fujita.tomonori@gmail.com> wrote:

> This adds `bool` support to the Rust LKMM atomics.
> 
> Rust specifies that `bool` has size 1 and alignment 1 [1], so it can
> be represented using an `i8` backing type.
> 
> Since `bool` only permits the bit patterns 0x00 and 0x01, the first
> patch also documents an additional safety preconditions for unsafe
> `Atomic::<T>::from_ptr`.
> 
> `from_ptr()` exists to operate on C-side storage so I don't think it
> makes sense for bool. We could restrict from_ptr() via a marker trait.

The C-side does have `bool` type, too (`_Bool` has been there since C99 and
we have typedef'd it to `bool`). User might still want to perform relaxed
load/store on these C-side bool storage.

I think it's fine to leave out this line too (given Boqun also points out
that the additional safety precondition is not needed for `from_ptr).

Best,
Gary

> 
> [1] https://doc.rust-lang.org/reference/types/boolean.html
> 
> 
> FUJITA Tomonori (2):
>   rust: sync: atomic: Add atomic bool support via i8 representation
>   rust: sync: atomic: Add atomic bool tests
> 
>  rust/kernel/sync/atomic.rs           |  1 +
>  rust/kernel/sync/atomic/internal.rs  |  8 ++++++++
>  rust/kernel/sync/atomic/predefine.rs | 27 +++++++++++++++++++++++++++
>  3 files changed, 36 insertions(+)
> 
> base-commit: 13ade169e801a423bc1a5a5c3c6ac680a144a608

Re: [PATCH v1 0/2] Add atomic bool support

[FUJITA Tomonori]

On Wed, 31 Dec 2025 15:34:31 +0000
Gary Guo <gary@garyguo.net> wrote:

> On Tue, 30 Dec 2025 13:50:26 +0900
> FUJITA Tomonori <fujita.tomonori@gmail.com> wrote:
> 
>> This adds `bool` support to the Rust LKMM atomics.
>> 
>> Rust specifies that `bool` has size 1 and alignment 1 [1], so it can
>> be represented using an `i8` backing type.
>> 
>> Since `bool` only permits the bit patterns 0x00 and 0x01, the first
>> patch also documents an additional safety preconditions for unsafe
>> `Atomic::<T>::from_ptr`.
>> 
>> `from_ptr()` exists to operate on C-side storage so I don't think it
>> makes sense for bool. We could restrict from_ptr() via a marker trait.
> 
> The C-side does have `bool` type, too (`_Bool` has been there since C99 and
> we have typedef'd it to `bool`). User might still want to perform relaxed
> load/store on these C-side bool storage.
> 
> I think it's fine to leave out this line too (given Boqun also points out
> that the additional safety precondition is not needed for `from_ptr).

I see. I'll drop the description in v2.

Thanks!