Re: [PATCH v2 1/6] ARM: cmpxchg: always inline __arch_xchg() and __cmpxchg()

[Karl Mehltretter <kmehltretter@gmail.com>]

On Thu, Oct 08, 2026 at 11:01:32AM +0100, Gary Guo wrote:
> > v6.19 links. Commit ab717dd98bee ("rust: helpers: Add i8/i16 atomic
> > xchg_acquire helpers") in v7.0 triggers __bad_xchg. Commit
> > ac8f06ade38a ("rust: sync: atomic: Add Atomic<*{mut,const} T> support")
> > in v7.1 also triggers __bad_cmpxchg.
> 
> I think the issue is that when you get multiple function calls in a function,
> GCC starts to think that not inlining can result in code deduplication where
> that is not actually true.
> 
> But I am surprised that it hasn't been seen in the past, perhaps there're only
> calls with single size from any translation unit previously?

The sizes were already mixed. It is the number of callers.

Commit 5dbc0a692459 ("rust: helpers: Add i8/i16 atomic xchg helpers")
has 1-, 2- and 4-byte xchg() in helpers.c and no __bad_xchg. The next
commit, ab717dd98bee, adds two more callers of the same sizes, the
xchg_acquire helpers, and has it.

> The fix LGTM, but Sashiko's report is worth looking into.

It's a valid concern. __generic_cmpxchg_local() has the same weakness
and patch 1 does not touch it. No file in the tree fails today. A test
file triggers it on ARMv5 with gcc 8.1.0 and CC_OPTIMIZE_FOR_SIZE:

- 12 valid cmpxchg_local() callers link
- 24 leave an undefined reference to wrong_size_cmpxchg
- __always_inline on __generic_cmpxchg_local() removes it

helpers.c stays below that. The ARMv4T and ARMv5TE kernels of this
series link with gcc 8.1.0. I will send this asm-generic change as a
separate patch.

Thanks,
Karl