Re: [PATCH v10 1/5] rust: add static_branch_unlikely for static_key_false

[Gary Guo <gary@garyguo.net>]

On Fri, 11 Oct 2024 10:13:34 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> Add just enough support for static key so that we can use it from
> tracepoints. Tracepoints rely on `static_branch_unlikely` with a `struct
> static_key_false`, so we add the same functionality to Rust.
> 
> This patch only provides a generic implementation without code patching
> (matching the one used when CONFIG_JUMP_LABEL is disabled). Later
> patches add support for inline asm implementations that use runtime
> patching.
> 
> When CONFIG_JUMP_LABEL is unset, `static_key_count` is a static inline
> function, so a Rust helper is defined for `static_key_count` in this
> case. If Rust is compiled with LTO, this call should get inlined. The
> helper can be eliminated once we have the necessary inline asm to make
> atomic operations from Rust.
> 
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/bindings/bindings_helper.h |  1 +
>  rust/helpers/helpers.c          |  1 +
>  rust/helpers/jump_label.c       | 15 +++++++++++++++
>  rust/kernel/jump_label.rs       | 30 ++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs              |  1 +
>  5 files changed, 48 insertions(+)
> 
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index ae82e9c941af..e0846e7e93e6 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -14,6 +14,7 @@
>  #include <linux/ethtool.h>
>  #include <linux/firmware.h>
>  #include <linux/jiffies.h>
> +#include <linux/jump_label.h>
>  #include <linux/mdio.h>
>  #include <linux/phy.h>
>  #include <linux/refcount.h>
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 30f40149f3a9..17e1b60d178f 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -12,6 +12,7 @@
>  #include "build_assert.c"
>  #include "build_bug.c"
>  #include "err.c"
> +#include "jump_label.c"
>  #include "kunit.c"
>  #include "mutex.c"
>  #include "page.c"
> diff --git a/rust/helpers/jump_label.c b/rust/helpers/jump_label.c
> new file mode 100644
> index 000000000000..6948cae5738f
> --- /dev/null
> +++ b/rust/helpers/jump_label.c
> @@ -0,0 +1,15 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * Copyright (C) 2024 Google LLC.
> + */
> +
> +#include <linux/jump_label.h>
> +
> +#ifndef CONFIG_JUMP_LABEL
> +int rust_helper_static_key_count(struct static_key *key)
> +{
> +	return static_key_count(key);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_static_key_count);

^ Explicit export should be removed. This only works because we didn't
remove export.h from all helpers.c yet, but there's a patch to do
that and this will stop working.

You can add a Reviewed-by from me with this fixed.

> +#endif
> diff --git a/rust/kernel/jump_label.rs b/rust/kernel/jump_label.rs
> new file mode 100644
> index 000000000000..4b7655b2a022
> --- /dev/null
> +++ b/rust/kernel/jump_label.rs
> @@ -0,0 +1,30 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (C) 2024 Google LLC.
> +
> +//! Logic for static keys.
> +//!
> +//! C header: [`include/linux/jump_label.h`](srctree/include/linux/jump_label.h).
> +
> +/// Branch based on a static key.
> +///
> +/// Takes three arguments:
> +///
> +/// * `key` - the path to the static variable containing the `static_key`.
> +/// * `keytyp` - the type of `key`.
> +/// * `field` - the name of the field of `key` that contains the `static_key`.
> +///
> +/// # Safety
> +///
> +/// The macro must be used with a real static key defined by C.
> +#[macro_export]
> +macro_rules! static_branch_unlikely {
> +    ($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 $crate::bindings::static_key = _key.cast();
> +
> +        $crate::bindings::static_key_count(_key.cast_mut()) > 0
> +    }};
> +}
> +pub use static_branch_unlikely;
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index b5f4b3ce6b48..708ff817ccc3 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -36,6 +36,7 @@
>  pub mod firmware;
>  pub mod init;
>  pub mod ioctl;
> +pub mod jump_label;
>  #[cfg(CONFIG_KUNIT)]
>  pub mod kunit;
>  pub mod list;
>