[PATCH] arm64: make atomic helpers __always_inline

[Nick Desaulniers]

On Fri, Jan 8, 2021 at 2:27 AM Arnd Bergmann <arnd at kernel.org> wrote:
>
> On Fri, Jan 8, 2021 at 10:33 AM Will Deacon <will at kernel.org> wrote:
> > On Fri, Jan 08, 2021 at 10:19:56AM +0100, Arnd Bergmann wrote:
> > > From: Arnd Bergmann <arnd at arndb.de>
> > >
> > > With UBSAN enabled and building with clang, there are occasionally
> > > warnings like
> > >
> > > WARNING: modpost: vmlinux.o(.text+0xc533ec): Section mismatch in reference from the function arch_atomic64_or() to the variable .init.data:numa_nodes_parsed
> > > The function arch_atomic64_or() references
> > > the variable __initdata numa_nodes_parsed.
> > > This is often because arch_atomic64_or lacks a __initdata
> > > annotation or the annotation of numa_nodes_parsed is wrong.
> > >
> > > for functions that end up not being inlined as intended but operating
> > > on __initdata variables. Mark these as __always_inline, along with
> > > the corresponding asm-generic wrappers.
> >
> > Hmm, I don't fully grok this. Why does it matter if a non '__init' function
> > is called with a pointer to some '__initdata'? Or is the reference coming
> > from somewhere else? (where?).
>
> There are (at least) three ways for gcc to deal with a 'static inline'
> function:
>
> a) fully inline it as the __always_inline attribute does
> b) not inline it at all, treating it as a regular static function
> c) create a specialized version with different calling conventions
>
> In this case, clang goes with option c when it notices that all
> callers pass the same constant pointer. This means we have a
> synthetic
>
> static noinline long arch_atomic64_or(long i)
> {
>         return __lse_ll_sc_body(atomic64_fetch_or, i, &numa_nodes_parsed);
> }
>
> which is a few bytes shorter than option b as it saves a load in the
> caller. This function definition however violates the kernel's rules
> for section references, as the synthetic version is not marked __init.

Interesting, I didn't know LLVM could do that.  Do you have a simpler
test case? Maybe I could just fix that in LLVM. (I would guess that
when synthesizing a function from an existing function, the new
function needs to copy the original functions attributes as well).

-- 
Thanks,
~Nick Desaulniers