* [PATCH v2] rust: add mark_used macro
@ 2026-09-09 12:02 FUJITA Tomonori
2026-09-30 9:50 ` FUJITA Tomonori
0 siblings, 1 reply; 6+ messages in thread
From: FUJITA Tomonori @ 2026-09-09 12:02 UTC (permalink / raw)
To: aliceryhl, gary, ojeda, rostedt
Cc: a.hindborg, acourbot, bjorn3_gh, boqun, dakr, daniel.almeida,
lossin, mathieu.desnoyers, mhiramat, tamird, tmgross, work,
linux-trace-kernel, rust-for-linux, FUJITA Tomonori
From: FUJITA Tomonori <fujita.tomonori@gmail.com>
Macros whose expansion depends on the kernel configuration end up
ignoring some of their arguments, resulting in unused warnings.
Add a macro for the `if false { _ = ...; }` idiom used to avoid them,
and convert the existing open-coded users in `warn_flags!` and
`declare_trace!`.
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
v2:
- Drop the comment in `declare_trace!`. The macro name says the same thing.
- Add Gary's Reviewed-by.
- Rebase on rust-next.
v1: https://lore.kernel.org/rust-for-linux/20260810075144.1245761-1-tomo@flapping.org/
---
rust/kernel/bug.rs | 18 ++++--------------
rust/kernel/lib.rs | 12 ++++++++++++
rust/kernel/tracepoint.rs | 4 +---
3 files changed, 17 insertions(+), 17 deletions(-)
diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
index 3566f0234ca4..c1e4eb7d4a70 100644
--- a/rust/kernel/bug.rs
+++ b/rust/kernel/bug.rs
@@ -55,9 +55,7 @@ macro_rules! warn_flags {
($file:expr, $flags:expr) => {
const FLAGS: u32 = $crate::bindings::BUGFLAG_WARNING | $flags;
- if false {
- _ = $file;
- }
+ $crate::mark_used!($file);
// SAFETY:
// - `flags` and `size` are all compile-time constants, preventing
@@ -83,9 +81,7 @@ macro_rules! warn_flags {
#[cfg(all(CONFIG_BUG, CONFIG_UML))]
macro_rules! warn_flags {
($file:expr, $flags:expr) => {
- if false {
- _ = $file;
- }
+ $crate::mark_used!($file);
// SAFETY: It is always safe to call `warn_slowpath_fmt()`
// with a valid null-terminated string.
@@ -106,10 +102,7 @@ macro_rules! warn_flags {
#[cfg(all(CONFIG_BUG, any(CONFIG_LOONGARCH, CONFIG_ARM)))]
macro_rules! warn_flags {
($file:expr, $flags:expr) => {
- if false {
- _ = $file;
- _ = $flags;
- }
+ $crate::mark_used!($file, $flags);
// SAFETY: It is always safe to call `WARN_ON()`.
unsafe { $crate::bindings::WARN_ON(true) }
@@ -121,10 +114,7 @@ macro_rules! warn_flags {
#[cfg(any(testlib, not(CONFIG_BUG)))]
macro_rules! warn_flags {
($file:expr, $flags:expr) => {
- if false {
- _ = $file;
- _ = $flags;
- }
+ $crate::mark_used!($file, $flags);
};
}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 4d5c96ddc49c..3b38e6b8196b 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -227,6 +227,18 @@ macro_rules! container_of {
#[doc(hidden)]
pub fn assert_same_type<T>(_: T, _: T) {}
+/// Marks the given expressions as used.
+///
+/// This avoids "unused" warnings in kernel configurations that do not otherwise use them.
+#[macro_export]
+macro_rules! mark_used {
+ ($($e:expr),* $(,)?) => {
+ if false {
+ $( _ = $e; )*
+ }
+ };
+}
+
/// Helper for `.rs.S` files.
#[doc(hidden)]
#[macro_export]
diff --git a/rust/kernel/tracepoint.rs b/rust/kernel/tracepoint.rs
index c6e80aa99e8e..29d2d2d0aca8 100644
--- a/rust/kernel/tracepoint.rs
+++ b/rust/kernel/tracepoint.rs
@@ -38,9 +38,7 @@ macro_rules! declare_trace {
#[cfg(not(CONFIG_TRACEPOINTS))]
{
- // If tracepoints are disabled, insert a trivial use of each argument
- // to avoid unused argument warnings.
- $( let _unused = $argname; )*
+ $crate::mark_used!($($argname),*);
}
}
)*}
base-commit: 28924df2a08f440c73991b83028032c901de2ae4
--
2.43.0
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH v2] rust: add mark_used macro
2026-09-09 12:02 [PATCH v2] rust: add mark_used macro FUJITA Tomonori
@ 2026-09-30 9:50 ` FUJITA Tomonori
2026-10-01 11:05 ` Alice Ryhl
0 siblings, 1 reply; 6+ messages in thread
From: FUJITA Tomonori @ 2026-09-30 9:50 UTC (permalink / raw)
To: aliceryhl, rostedt
Cc: gary, ojeda, a.hindborg, acourbot, bjorn3_gh, boqun, dakr,
daniel.almeida, lossin, mathieu.desnoyers, mhiramat, tamird,
tmgross, work, linux-trace-kernel, rust-for-linux,
fujita.tomonori
Hi Alice, Steven,
On Wed, 9 Sep 2026 21:02:10 +0900
FUJITA Tomonori <tomo@flapping.org> wrote:
> Macros whose expansion depends on the kernel configuration end up
> ignoring some of their arguments, resulting in unused warnings.
>
> Add a macro for the `if false { _ = ...; }` idiom used to avoid them,
> and convert the existing open-coded users in `warn_flags!` and
> `declare_trace!`.
>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
> v2:
> - Drop the comment in `declare_trace!`. The macro name says the same thing.
> - Add Gary's Reviewed-by.
> - Rebase on rust-next.
> v1: https://lore.kernel.org/rust-for-linux/20260810075144.1245761-1-tomo@flapping.org/
> ---
> rust/kernel/bug.rs | 18 ++++--------------
> rust/kernel/lib.rs | 12 ++++++++++++
> rust/kernel/tracepoint.rs | 4 +---
> 3 files changed, 17 insertions(+), 17 deletions(-)
(snip)
> diff --git a/rust/kernel/tracepoint.rs b/rust/kernel/tracepoint.rs
> index c6e80aa99e8e..29d2d2d0aca8 100644
> --- a/rust/kernel/tracepoint.rs
> +++ b/rust/kernel/tracepoint.rs
> @@ -38,9 +38,7 @@ macro_rules! declare_trace {
>
> #[cfg(not(CONFIG_TRACEPOINTS))]
> {
> - // If tracepoints are disabled, insert a trivial use of each argument
> - // to avoid unused argument warnings.
> - $( let _unused = $argname; )*
> + $crate::mark_used!($($argname),*);
> }
> }
> )*}
>
Gentle ping. Could you take a look at the tracepoint.rs part? If it
looks fine, I think it is easiest to take the patch through the Rust
tree.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v2] rust: add mark_used macro
2026-09-30 9:50 ` FUJITA Tomonori
@ 2026-10-01 11:05 ` Alice Ryhl
2026-10-01 12:04 ` FUJITA Tomonori
0 siblings, 1 reply; 6+ messages in thread
From: Alice Ryhl @ 2026-10-01 11:05 UTC (permalink / raw)
To: FUJITA Tomonori
Cc: rostedt, gary, ojeda, a.hindborg, acourbot, bjorn3_gh, boqun,
dakr, daniel.almeida, lossin, mathieu.desnoyers, mhiramat, tamird,
tmgross, work, linux-trace-kernel, rust-for-linux,
fujita.tomonori
On Wed, Sep 30, 2026 at 06:50:28PM +0900, FUJITA Tomonori wrote:
> > diff --git a/rust/kernel/tracepoint.rs b/rust/kernel/tracepoint.rs
> > index c6e80aa99e8e..29d2d2d0aca8 100644
> > --- a/rust/kernel/tracepoint.rs
> > +++ b/rust/kernel/tracepoint.rs
> > @@ -38,9 +38,7 @@ macro_rules! declare_trace {
> >
> > #[cfg(not(CONFIG_TRACEPOINTS))]
> > {
> > - // If tracepoints are disabled, insert a trivial use of each argument
> > - // to avoid unused argument warnings.
> > - $( let _unused = $argname; )*
> > + $crate::mark_used!($($argname),*);
> > }
> > }
> > )*}
> >
>
> Gentle ping. Could you take a look at the tracepoint.rs part? If it
> looks fine, I think it is easiest to take the patch through the Rust
> tree.
Why are we adding this macro?
I think it's better to prefer avoiding macros if we can. The current
code is familiar to people who know Rust idioms, and now you are
replacing it with a kernel-specific macro that such people will not be
familiar with.
Alice
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v2] rust: add mark_used macro
2026-10-01 11:05 ` Alice Ryhl
@ 2026-10-01 12:04 ` FUJITA Tomonori
2026-10-01 13:02 ` Miguel Ojeda
0 siblings, 1 reply; 6+ messages in thread
From: FUJITA Tomonori @ 2026-10-01 12:04 UTC (permalink / raw)
To: aliceryhl, ojeda
Cc: tomo, rostedt, gary, a.hindborg, acourbot, bjorn3_gh, boqun, dakr,
daniel.almeida, lossin, mathieu.desnoyers, mhiramat, tamird,
tmgross, work, linux-trace-kernel, rust-for-linux,
fujita.tomonori
On Thu, 1 Oct 2026 11:05:52 +0000
Alice Ryhl <aliceryhl@google.com> wrote:
> On Wed, Sep 30, 2026 at 06:50:28PM +0900, FUJITA Tomonori wrote:
>> > diff --git a/rust/kernel/tracepoint.rs b/rust/kernel/tracepoint.rs
>> > index c6e80aa99e8e..29d2d2d0aca8 100644
>> > --- a/rust/kernel/tracepoint.rs
>> > +++ b/rust/kernel/tracepoint.rs
>> > @@ -38,9 +38,7 @@ macro_rules! declare_trace {
>> >
>> > #[cfg(not(CONFIG_TRACEPOINTS))]
>> > {
>> > - // If tracepoints are disabled, insert a trivial use of each argument
>> > - // to avoid unused argument warnings.
>> > - $( let _unused = $argname; )*
>> > + $crate::mark_used!($($argname),*);
>> > }
>> > }
>> > )*}
>> >
>>
>> Gentle ping. Could you take a look at the tracepoint.rs part? If it
>> looks fine, I think it is easiest to take the patch through the Rust
>> tree.
>
> Why are we adding this macro?
>
> I think it's better to prefer avoiding macros if we can. The current
> code is familiar to people who know Rust idioms, and now you are
> replacing it with a kernel-specific macro that such people will not be
> familiar with.
The macro was suggested by Miguel:
https://lore.kernel.org/rust-for-linux/CANiq72nWcqd3aSKdPqOSRRQBuwWNW-s2EOscYszQ9kSbmrKxcw@mail.gmail.com/
I think you also have a point.
Miguel, what do you think?
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v2] rust: add mark_used macro
2026-10-01 12:04 ` FUJITA Tomonori
@ 2026-10-01 13:02 ` Miguel Ojeda
2026-10-01 21:37 ` FUJITA Tomonori
0 siblings, 1 reply; 6+ messages in thread
From: Miguel Ojeda @ 2026-10-01 13:02 UTC (permalink / raw)
To: FUJITA Tomonori
Cc: aliceryhl, ojeda, rostedt, gary, a.hindborg, acourbot, bjorn3_gh,
boqun, dakr, daniel.almeida, lossin, mathieu.desnoyers, mhiramat,
tamird, tmgross, work, linux-trace-kernel, rust-for-linux,
fujita.tomonori
On Thu, Oct 1, 2026 at 2:04 PM FUJITA Tomonori <tomo@flapping.org> wrote:
>
> Miguel, what do you think?
I suggested it due to seeing the multi-line combination of `if false`
and `_ = ...`, and the fact that we will probably want to reuse the
combination in other macros. It makes the concept more explicit,
easier to grep, and follows other cases we have created for similar
things like this (e.g. `static_assert!`).
Having said that, I do prefer using plain language instead of macros
where possible/reasonable, i.e. I don't like introducing macros and
more concepts in general just for the sake of it. On the other hand,
sometimes language features aren't the best spellings to be honest
(like the `const _` "abuse") and some people prefer abbreviations
(like the discussion on `SZ_*` constants).
Either way is fine, obviously.
Cheers,
Miguel
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v2] rust: add mark_used macro
2026-10-01 13:02 ` Miguel Ojeda
@ 2026-10-01 21:37 ` FUJITA Tomonori
0 siblings, 0 replies; 6+ messages in thread
From: FUJITA Tomonori @ 2026-10-01 21:37 UTC (permalink / raw)
To: miguel.ojeda.sandonis
Cc: tomo, aliceryhl, ojeda, rostedt, gary, a.hindborg, acourbot,
bjorn3_gh, boqun, dakr, daniel.almeida, lossin, mathieu.desnoyers,
mhiramat, tamird, tmgross, work, linux-trace-kernel,
rust-for-linux, fujita.tomonori
On Thu, 1 Oct 2026 15:02:40 +0200
Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> wrote:
> On Thu, Oct 1, 2026 at 2:04 PM FUJITA Tomonori <tomo@flapping.org> wrote:
>>
>> Miguel, what do you think?
>
> I suggested it due to seeing the multi-line combination of `if false`
> and `_ = ...`, and the fact that we will probably want to reuse the
> combination in other macros. It makes the concept more explicit,
> easier to grep, and follows other cases we have created for similar
> things like this (e.g. `static_assert!`).
>
> Having said that, I do prefer using plain language instead of macros
> where possible/reasonable, i.e. I don't like introducing macros and
> more concepts in general just for the sake of it. On the other hand,
> sometimes language features aren't the best spellings to be honest
> (like the `const _` "abuse") and some people prefer abbreviations
> (like the discussion on `SZ_*` constants).
>
> Either way is fine, obviously.
Thanks, I'll drop it.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2026-09-09 12:02 [PATCH v2] rust: add mark_used macro FUJITA Tomonori
2026-09-30 9:50 ` FUJITA Tomonori
2026-10-01 11:05 ` Alice Ryhl
2026-10-01 12:04 ` FUJITA Tomonori
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