[PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[Miguel Ojeda]

On arm and loongarch64, or under `CONFIG_BUG=n`, the upcoming SRCU
abstractions from commit

  59cf3a5bda61 ("rust: sync: add SRCU abstraction")

use `warn_on!`, which in turn warns:

    error: constant `WARN_ON_FLAGS` is never used
       --> rust/kernel/bug.rs:126:19
        |
    126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
        |                     ^^^^^^^^^^^^^
        |
       ::: rust/kernel/sync/srcu.rs:106:12
        |
    106 |           if crate::warn_on!(
        |  ____________-
    107 | |             // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`
    108 | |             // and `srcu_readers_active()` only checks the active reader count.
    109 | |             unsafe { bindings::srcu_readers_active(ptr) }
    110 | |         ) {
        | |_________- in this macro invocation
        |
        = note: `-D dead-code` implied by `-D warnings`
        = help: to override `-D warnings` add `#[allow(dead_code)]`
        = note: this error originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)

The reason is that `warn_flags!` discards both arguments in those cases,
and so the `const` becomes unused. And since it is a macro, it went
unnoticed so far.

Thus fix it by simplifying by removing the `const`.

Cc: stable@vger.kernel.org
Fixes: dff64b072708 ("rust: Add warn_on macro")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
Either this or adding an underscore.

 rust/kernel/bug.rs | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
index ed943960f851..874adb93cdfe 100644
--- a/rust/kernel/bug.rs
+++ b/rust/kernel/bug.rs
@@ -123,9 +123,10 @@ macro_rules! warn_on {
         const _COND_STR: &str = file!();

         if cond {
-            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
-
-            $crate::warn_flags!(_COND_STR, WARN_ON_FLAGS);
+            $crate::warn_flags!(
+                _COND_STR,
+                $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN)
+            );
         }
         cond
     }};

base-commit: dc01dfb37b34beeefcfe1c3055364d41a4070c7e
--
2.55.0

Re: [PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[Miguel Ojeda]

On Sat, Aug 8, 2026 at 12:14 AM Miguel Ojeda <ojeda@kernel.org> wrote:
>
> On arm and loongarch64, or under `CONFIG_BUG=n`, the upcoming SRCU
> abstractions from commit
>
>   59cf3a5bda61 ("rust: sync: add SRCU abstraction")
>
> use `warn_on!`, which in turn warns:
>
>     error: constant `WARN_ON_FLAGS` is never used
>        --> rust/kernel/bug.rs:126:19
>         |
>     126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>         |                     ^^^^^^^^^^^^^
>         |
>        ::: rust/kernel/sync/srcu.rs:106:12
>         |
>     106 |           if crate::warn_on!(
>         |  ____________-
>     107 | |             // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`
>     108 | |             // and `srcu_readers_active()` only checks the active reader count.
>     109 | |             unsafe { bindings::srcu_readers_active(ptr) }
>     110 | |         ) {
>         | |_________- in this macro invocation
>         |
>         = note: `-D dead-code` implied by `-D warnings`
>         = help: to override `-D warnings` add `#[allow(dead_code)]`
>         = note: this error originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)
>
> The reason is that `warn_flags!` discards both arguments in those cases,
> and so the `const` becomes unused. And since it is a macro, it went
> unnoticed so far.
>
> Thus fix it by simplifying by removing the `const`.
>
> Cc: stable@vger.kernel.org
> Fixes: dff64b072708 ("rust: Add warn_on macro")
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Sorry, I forgot the Cc to Tomonori:

Cc: FUJITA Tomonori <fujita.tomonori@gmail.com>

Cheers,
Miguel

Re: [PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[FUJITA Tomonori]

On Sat,  8 Aug 2026 00:14:29 +0200
Miguel Ojeda <ojeda@kernel.org> wrote:

> On arm and loongarch64, or under `CONFIG_BUG=n`, the upcoming SRCU
> abstractions from commit
> 
>   59cf3a5bda61 ("rust: sync: add SRCU abstraction")
> 
> use `warn_on!`, which in turn warns:
> 
>     error: constant `WARN_ON_FLAGS` is never used
>        --> rust/kernel/bug.rs:126:19
>         |
>     126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
>         |                     ^^^^^^^^^^^^^
>         |
>        ::: rust/kernel/sync/srcu.rs:106:12
>         |
>     106 |           if crate::warn_on!(
>         |  ____________-
>     107 | |             // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`
>     108 | |             // and `srcu_readers_active()` only checks the active reader count.
>     109 | |             unsafe { bindings::srcu_readers_active(ptr) }
>     110 | |         ) {
>         | |_________- in this macro invocation
>         |
>         = note: `-D dead-code` implied by `-D warnings`
>         = help: to override `-D warnings` add `#[allow(dead_code)]`
>         = note: this error originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)
> 
> The reason is that `warn_flags!` discards both arguments in those cases,
> and so the `const` becomes unused. And since it is a macro, it went
> unnoticed so far.
> 
> Thus fix it by simplifying by removing the `const`.
> 
> Cc: stable@vger.kernel.org
> Fixes: dff64b072708 ("rust: Add warn_on macro")
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
> ---
> Either this or adding an underscore.
> 
>  rust/kernel/bug.rs | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
> index ed943960f851..874adb93cdfe 100644
> --- a/rust/kernel/bug.rs
> +++ b/rust/kernel/bug.rs
> @@ -123,9 +123,10 @@ macro_rules! warn_on {
>          const _COND_STR: &str = file!();
> 
>          if cond {
> -            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
> -
> -            $crate::warn_flags!(_COND_STR, WARN_ON_FLAGS);
> +            $crate::warn_flags!(
> +                _COND_STR,
> +                $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN)
> +            );
>          }
>          cond
>      }};
> 
> base-commit: dc01dfb37b34beeefcfe1c3055364d41a4070c7e
> --
> 2.55.0

I sent a fix for this bug a few days ago:

https://lore.kernel.org/all/20260801024841.786664-1-tomo@flapping.org/


If you prefer the simpler version, v1 might work.

https://lore.kernel.org/all/20260720035510.4048145-1-tomo@flapping.org/

Re: [PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[Miguel Ojeda]

On Sat, Aug 8, 2026 at 12:43 AM FUJITA Tomonori <tomo@flapping.org> wrote:
>
> I sent a fix for this bug a few days ago:
>
> https://lore.kernel.org/all/20260801024841.786664-1-tomo@flapping.org/

Ah, right, sorry! I also saw another one with `rusttest` for arm64 and
riscv64. Did you see that one? We probably need a `cfg(testlib)`
split.

Cheers,
Miguel

Re: [PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[FUJITA Tomonori]

On Sat, 8 Aug 2026 01:21:23 +0200
Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> wrote:

> On Sat, Aug 8, 2026 at 12:43 AM FUJITA Tomonori <tomo@flapping.org> wrote:
>>
>> I sent a fix for this bug a few days ago:
>>
>> https://lore.kernel.org/all/20260801024841.786664-1-tomo@flapping.org/
> 
> Ah, right, sorry! I also saw another one with `rusttest` for arm64 and
> riscv64. Did you see that one? We probably need a `cfg(testlib)`
> split.

Is this the one you saw?

error: invalid instruction mnemonic 'brk'
  --> rust/kernel/bug.rs:63:17
   |
63 | /                 concat!(
64 | |                     "/* {size} */",
65 | |                     include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_warn_asm.rs")),
66 | |                     include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_reachable_asm.rs")));
   | |_______________________________________________________________________________________________________^
   |
note: instantiated into assembly here
  --> <inline asm>:1:115
   |
 1 |     /* 8 */.pushsection __bug_table,"aw"; .align 2; 14470: .long 14471f - .;.short 2305;.align 2; .popsection; 14471:brk 0x800
   |                                                                                                                      ^^^

It shows up when `make rusttest` runs with `ARCH=` set to an
architecture other than the host's.

I posted a `cfg(testlib)` split for it:

https://lore.kernel.org/all/20260808022608.1125174-1-tomo@flapping.org/


While at it, a question: what is `rusttest` meant to cover when
`ARCH=` is not the host's?

Re: [PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[Miguel Ojeda]

On Sat, Aug 8, 2026 at 7:30 AM FUJITA Tomonori <tomo@flapping.org> wrote:
>
> Is this the one you saw?

Yeah, exactly.

> I posted a `cfg(testlib)` split for it:
>
> https://lore.kernel.org/all/20260808022608.1125174-1-tomo@flapping.org/

Thanks! It seems good to me.

> While at it, a question: what is `rusttest` meant to cover when
> `ARCH=` is not the host's?

`rusttest` is meant to run tests on the host, regardless of the target.

There aren't many tests there nowadays, since most of them moved to
the actual kernel when we got support for that, but we can still do
there things like testing proc macros.

We could also use it for more things in the future, perhaps...

Cheers,
Miguel

Re: [PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[FUJITA Tomonori]

On Sat, 8 Aug 2026 21:40:49 +0200
Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> wrote:

>> I posted a `cfg(testlib)` split for it:
>>
>> https://lore.kernel.org/all/20260808022608.1125174-1-tomo@flapping.org/
> 
> Thanks! It seems good to me.

Thanks!

>> While at it, a question: what is `rusttest` meant to cover when
>> `ARCH=` is not the host's?
> 
> `rusttest` is meant to run tests on the host, regardless of the target.

Then `jump_label.rs` needs the same treatment?

`arch_static_branch!` embeds the generated arch asm too. It is not a
problem today only because, like `warn_on!` until now, it has no user
inside the `kernel` crate itself.

Re: [PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[Miguel Ojeda]

On Sun, Aug 9, 2026 at 2:50 AM FUJITA Tomonori <tomo@flapping.org> wrote:
>
> Then `jump_label.rs` needs the same treatment?
>
> `arch_static_branch!` embeds the generated arch asm too. It is not a
> problem today only because, like `warn_on!` until now, it has no user
> inside the `kernel` crate itself.

Yeah, I think we should do it to avoid surprises later.

Good eyes!

Cheers,
Miguel

Re: [PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[Gary Guo]

On Sat Aug 8, 2026 at 12:21 AM BST, Miguel Ojeda wrote:
> On Sat, Aug 8, 2026 at 12:43 AM FUJITA Tomonori <tomo@flapping.org> wrote:
>>
>> I sent a fix for this bug a few days ago:
>>
>> https://lore.kernel.org/all/20260801024841.786664-1-tomo@flapping.org/
>
> Ah, right, sorry! I also saw another one with `rusttest` for arm64 and
> riscv64. Did you see that one? We probably need a `cfg(testlib)`
> split.

I am not sure how much value does `rusttest` still provide, given that KUnit can
test much more stuff in an actual envirionment. The fact that we have to do
special casing does suggest to me that we might want to remove it.

Best,
Gary