On Fri, Jan 08, 2021 at 11:26:53AM +0100, Arnd Bergmann wrote:

On Fri, Jan 8, 2021 at 10:33 AM Will Deacon [off-list ref] wrote:

quoted

On Fri, Jan 08, 2021 at 10:19:56AM +0100, Arnd Bergmann wrote:

quoted

From: Arnd Bergmann <arnd@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.

Ah, I was hoping the compiler would've sorted that out, but then again, how
would it know? But doesn't this mean that whenever we get one caller passing
something like an __initdata pointer to a function, then that function needs
to be __always_inline for everybody? It feels like a slippery slope
considering the incentive to go back and replace it with 'inline' if the
caller goes away is very small.

Didn't we used to #define inline as __always_inline to avoid this situation?

Will

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