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Fri, 7 Aug 2026 18:43:44 -0400 (EDT) Date: Sat, 08 Aug 2026 07:43:41 +0900 (JST) Message-Id: <20260808.074341.1056015191905819805.tomo@flapping.org> To: ojeda@kernel.org Cc: arnd@arndb.de, krzk@kernel.org, alexandre.belloni@bootlin.com, linusw@kernel.org, chenhuacai@kernel.org, boqun@kernel.org, gary@garyguo.net, bjorn3_gh@protonmail.com, lossin@kernel.org, a.hindborg@kernel.org, aliceryhl@google.com, tmgross@umich.edu, dakr@kernel.org, daniel.almeida@collabora.com, tamird@kernel.org, acourbot@nvidia.com, work@onurozkan.dev, rust-for-linux@vger.kernel.org, fustini@kernel.org, linux-arm-kernel@lists.infradead.org, soc@lists.linux.dev, kernel@xen0n.name, loongarch@lists.linux.dev, stable@vger.kernel.org Subject: Re: [PATCH] rust: bug: fix `dead_code` warning in `warn_on!` From: FUJITA Tomonori In-Reply-To: <20260807221429.155946-1-ojeda@kernel.org> References: <20260807221429.155946-1-ojeda@kernel.org> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit On Sat, 8 Aug 2026 00:14:29 +0200 Miguel Ojeda 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 > --- > 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/