linux-kernel - Re: [PATCH] rust: kernel: Support more jump

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Message-ID: <aQoIliAbrn0Re7E3@google.com>
Date: Tue, 4 Nov 2025 14:07:18 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: chenmiao <chenmiao@...natom.club>
Cc: ojeda@...nel.org, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, "Steven Rostedt (Google)" <rostedt@...dmis.org>, 
	"open list:RUST" <rust-for-linux@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rust: kernel: Support more jump_label api

On Tue, Nov 04, 2025 at 02:04:17AM +0000, chenmiao wrote:
> The initial implementation of arch_static_branch was achieved by accessing
> the offset from the original type. However, this approach extended the
> path and introduced redundant calculations when dealing with types like
> `static_key_true/false`, as shown below:
> 
> ```
> static_brach_unlikely(tp, tracepoint, key)
>   => tracepoint->key->key
>   => &tracepoint->key(static_key_false) == &tracepoint->key.key(static_key)
>   => off: tracepoint->key - tracepoint
> ```
> 
> In practice, the implementation of `arch_static_branch` overlooked many
> detailed descriptions. To improve clarity, additional comments have been
> added to the original logic. The approach has been modified to directly
> locate the corresponding `static_key` instead of using offsets, thereby
> reducing  computational overhead.
> 
> If finding the offset from the primitive type is necessary for this
> implementation, I will abandon this change.
> 
> Additionally, support for the `static_branch_enable/disable` APIs has been
> introduced.
> 
> Signed-off-by: chenmiao <chenmiao@...natom.club>

> +    ($basety:ty, $branch:expr, $key:path, $keytyp:ty, $field:ident) => {{
>          let _key: *const $keytyp = ::core::ptr::addr_of!($key);
> -        let _key: *const $crate::bindings::static_key_false = ::core::ptr::addr_of!((*_key).$field);
> +        let _key: *const $basety = ::core::ptr::addr_of!((*_key).$field);
>          let _key: *const $crate::bindings::static_key = _key.cast();
>  
>          #[cfg(not(CONFIG_JUMP_LABEL))]
> @@ -30,7 +71,88 @@ macro_rules! static_branch_unlikely {
>          }
>  
>          #[cfg(CONFIG_JUMP_LABEL)]
> -        $crate::jump_label::arch_static_branch! { $key, $keytyp, $field, false }
> +        {
> +            $crate::jump_label::arch_static_branch! { _key, $branch }
> +        }

So ... this is changing from $key to _key. That's replacing the global
variable with a local variable holding a pointer to the global variable.
However, the arch_static_branch! macro uses the `sym` operand which
requires you to pass it the global directly.

Did you try this code? I don't believe it will work.

Alice