Re: [PATCH v1] rust: bug: prevent dead_code warning from warn_on!'s flags constant

On Sun, Jul 19, 2026 at 10:54:24PM -0700, Boqun Feng wrote:
> On Mon, Jul 20, 2026 at 12:55:10PM +0900, FUJITA Tomonori wrote:
> > From: FUJITA Tomonori <[email protected]>
> > 
> > Fix the following dead_code warning on some configurations in an
> > atomic development branch:
> > 
> > warning: constant `WARN_ON_FLAGS` is never used
> >    --> /linux/rust/kernel/bug.rs:126:19
> >     |
> > 126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
> >     |                     ^^^^^^^^^^^^^
> >     |
> >    ::: /linux/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: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
> >     = note: this warning originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)
> > 
> > The warn_on! macro always defines a WARN_ON_FLAGS constant and hands it
> > to warn_flags!. On configurations where warn_flags! does not reference
> > its flags argument (the LOONGARCH/ARM variant, which only calls
> > WARN_ON(), and the !CONFIG_BUG no-op variant), the constant is left
> > unused and triggers a dead_code warning.
> > 
> > The warning only surfaces for callers inside the kernel crate itself:
> > uses from other crates expand an external macro, so the lint is
> > suppressed there.
> > 
> > Rename the constant to _WARN_ON_FLAGS so the underscore marks it as
> > possibly-unused, matching the existing _COND_STR constant in the same
> > macro. The name is still referenceable, so configurations that do use
> > the flags are unaffected.
> > 
> > Signed-off-by: FUJITA Tomonori <[email protected]>
> 
> [Cc Paul]
> 
> Do we need a "Fixes" tag here?

Future bug-chasers would thank us for a "Fixes" tag.  ;-)

> FWIW:
> 
> Reviewed-by: Boqun Feng <[email protected]>

And thank you both for the fix.  I will keep the Rust-SRCU series.
Until further notice, anyway.  ;-)

							Thanx, Paul
> Regards,
> Boqun
> 
> > ---
> >  rust/kernel/bug.rs | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/rust/kernel/bug.rs b/rust/kernel/bug.rs
> > index ed943960f851..2fd7ee25abcb 100644
> > --- a/rust/kernel/bug.rs
> > +++ b/rust/kernel/bug.rs
> > @@ -123,9 +123,9 @@ macro_rules! warn_on {
> >          const _COND_STR: &str = file!();
> >  
> >          if cond {
> > -            const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
> > +            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, _WARN_ON_FLAGS);
> >          }
> >          cond
> >      }};
> > 
> > base-commit: 880c43b185ca52239e75bc546cc4f4d9154d0fed
> > -- 
> > 2.43.0
> >