Re: [PATCH v3 0/7] rust: build_assert: document and fix use with function arguments

[Gary Guo <gary@garyguo.net>]

On Mon, 08 Dec 2025 11:46:58 +0900
Alexandre Courbot <acourbot@nvidia.com> wrote:

> `build_assert` relies on the compiler to optimize out its error path,
> lest build fails with the dreaded error:
> 
>     ERROR: modpost: "rust_build_error" [path/to/module.ko] undefined!
> 
> It has been observed that very trivial code performing I/O accesses
> (sometimes even using an immediate value) would seemingly randomly fail
> with this error whenever `CLIPPY=1` was set. The same behavior was also
> observed until different, very similar conditions [1][2].
> 
> The cause, as pointed out by Gary Guo [3], appears to be that the
> failing function is eventually using `build_assert` with its argument,
> but is only annotated with `#[inline]`. This gives the compiler freedom
> to not inline the function, which it notably did when Clippy was active,
> triggering the error.

That's an interesting observation, so `#[inline]` is fine without
clippy but `#[inline(always)]` is needed when Clippy is used?

I know Clippy would affect codegen but this might be a first concrete
example that it actually creates (non-perf) issues that I've countered
in practice.

> 
> The fix is to annotate functions passing their argument to
> `build_assert` with `#[inline(always)]`, telling the compiler to be as
> aggressive as possible with their inlining. This is also the correct
> behavior as inlining is mandatory for correct behavior in these cases.

Yeah, I suppose when you draw parallelism with C `BUILD_BUG` macro,
there are a few users of that in other macros, which are kinda
force-inlined.

> 
> This series fixes all possible points of failure in the kernel crate,
> and adds documentation to `build_assert` explaining how to properly
> inline functions for which this behavior may arise.
> 
> [1] https://lore.kernel.org/all/DEEUYUOAEZU3.1J1HM2YQ10EX1@nvidia.com/
> [2] https://lore.kernel.org/all/A1A280D4-836E-4D75-863E-30B1C276C80C@collabora.com/
> [3] https://lore.kernel.org/all/20251121143008.2f5acc33.gary@garyguo.net/
> 
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
> Changes in v3:
> - Add "Fixes:" tags.
> - CC stable on fixup patches.
> - Link to v2: https://patch.msgid.link/20251128-io-build-assert-v2-0-a9ea9ce7d45d@nvidia.com
> 
> Changes in v2:
> - Turn into a series and address other similar cases in the kernel crate.
> - Link to v1: https://patch.msgid.link/20251127-io-build-assert-v1-1-04237f2e5850@nvidia.com
> 
> ---
> Alexandre Courbot (7):
>       rust: build_assert: add instructions for use with function arguments
>       rust: io: always inline functions using build_assert with arguments
>       rust: cpufreq: always inline functions using build_assert with arguments
>       rust: bits: always inline functions using build_assert with arguments
>       rust: sync: refcount: always inline functions using build_assert with arguments
>       rust: irq: always inline functions using build_assert with arguments
>       rust: num: bounded: add missing comment for always inlined function
> 
>  rust/kernel/bits.rs          | 6 ++++--
>  rust/kernel/build_assert.rs  | 7 ++++++-
>  rust/kernel/cpufreq.rs       | 2 ++
>  rust/kernel/io.rs            | 9 ++++++---
>  rust/kernel/io/resource.rs   | 2 ++
>  rust/kernel/irq/flags.rs     | 2 ++
>  rust/kernel/num/bounded.rs   | 1 +
>  rust/kernel/sync/refcount.rs | 3 ++-
>  8 files changed, 25 insertions(+), 7 deletions(-)
> ---
> base-commit: ba65a4e7120a616d9c592750d9147f6dcafedffa
> change-id: 20251127-io-build-assert-3579a5bfb81c
> 
> Best regards,