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This struct is = 4 bytes >>>>>> on x86 when lockdep is not enabled. The structure is not padded to f= it a >>>>>> cache line. The effect of this for `SpinLock` is that the lock varia= ble and >>>>>> the value protected by the lock might share a cache line, depending = on the >>>>>> alignment requirements of the protected value. Wrapping the value in >>>>>> `CacheAligned` to get a `SpinLock>` solves this prob= lem. >>>>>> >>>>>> Signed-off-by: Andreas Hindborg >>>>>> --- >>>>>> Signed-off-by: Andreas Hindborg >>>>>> --- >>>>>> rust/kernel/cache_aligned.rs | 59 +++++++++++++++++++++++++++++++++= +++++++++++ >>>>>> rust/kernel/lib.rs | 2 ++ >>>>>> 2 files changed, 61 insertions(+) >>>>>> >>>>>> diff --git a/rust/kernel/cache_aligned.rs b/rust/kernel/cache_aligne= d.rs >>>>>> new file mode 100644 >>>>>> index 0000000000000..9c33b8613c077 >>>>>> --- /dev/null >>>>>> +++ b/rust/kernel/cache_aligned.rs >>>>>> @@ -0,0 +1,59 @@ >>>>>> +// SPDX-License-Identifier: GPL-2.0 >>>>>> + >>>>>> +use kernel::try_pin_init; >>>>>> +use pin_init::{ >>>>>> + pin_data, >>>>>> + pin_init, >>>>>> + PinInit, // >>>>>> +}; >>>>>> + >>>>>> +/// Wrapper type that alings content to a 64 byte cache line. >>>>> >>>>> nit: s/alings/aligns >>>>> >>>>>> +#[repr(align(64))] >>>>> >>>>> While 64 bytes is the most common cache line size, AFAIK this is not >>>>> a universal value? Can we expose and use `L1_CACHE_BYTES` here? >>>> >>>> Unfortunately `repr(align())` does not accept expression or macro invo= cations. >>>> It's still possible with code-generation, but it'll be more tricky. >>>> >>>> On all archs that we do support today, I think the value is always 64.= However >>>> it'd worth putting a FIXME or TODO (or assertion, maybe?) in case new = archs gets >>>> addded where this isn't true. >>> >>> I was looking into how to implement this properly. Apparently, we don't >>> have a config item that specifies the L1 cache line size for all >>> architectures. Each architectures defines the cache line size as C >>> define in a header. For x86 we have >>> >>> #define L1_CACHE_SHIFT (CONFIG_X86_L1_CACHE_SHIFT) >>> >>> and for arm64 >>> >>> #define L1_CACHE_SHIFT (6) >>> >>> and so on. >>> >>> One thing we could do is run the C preprocessor on a small snippet to >>> expand the `L1_CACHE_SHIFT` symbol at some point before invoking >>> `rustc`. Then we can pass the value to `rustc` via environment variable >>> when building the `macros` crate. This is similar to how we pass >>> `RUST_MODFILE` to `rustc`, sans the cpp invocation. >>> >>> Otherwise we have to convince all architectures that support Rust to >>> emit a config that we can rely on, like `CONFIG_L1_CACHE_SHIFT`. >>> >>> The latter options is probably the better one, what do you all think? >>> >>> Best regards, >>> Andreas Hindborg >> >> You can implement this with generics alone using associative types. >> >> mod sealed { >> pub trait Sealed { >> type Repr; >> } >> } >> >> trait Alignment: sealed::Sealed {} >> >> #[repr(transparent)] >> struct Align([::Repr; 0]) >> where >> Self: Alignment; >> >> impl Align >> where >> Self: Alignment, >> { >> #[inline] >> pub fn new() -> Self { >> Align([]) >> } >> } >> >> macro_rules! impl_align { >> () =3D> {}; >> ($a:literal $($rest:literal)*) =3D> { >> const _: () =3D { >> #[repr(align($a))] >> struct Repr; >> >> impl sealed::Sealed for Align<$a> { >> type Repr =3D Repr; >> } >> =20 >> impl Alignment for Align<$a> {} >> }; >> =20 >> impl_align!($($rest)*); >> } >> } >> >> impl_align!(32 64 128 256); > > Right, that is great for avoiding code duplication in case we want to > have `Align<16>`, `Align<32>`, etc. > > My discussion was about having just `CacheAligned` and having the > alignment of that type automatically be the `1 << L1_CACHE_SHIFT`. #[repr(transparent)] struct CacheAligned(Align<{ 1 << L1_CACHE_SHIFT }>); Best, Gary