Re: [RFC v2 2/3] atomic: Specify alignment for atomic_t and atomic64_t

[Finn Thain <fthain@linux-m68k.org>]

On Wed, 1 Oct 2025, Arnd Bergmann wrote:

> >> Since there is nothing telling the compiler that the 'old' argument 
> >> to atomic*_try_cmpcxchg() needs to be naturally aligned, maybe that 
> >> check should be changed to only test for the ABI-guaranteed 
> >> alignment? I think that would still be needed on x86-32.
> >>  
> >
> > I don't know why we would check the alignment of the 'old' quantity. 
> > It's going to be loaded into a register before being used, right?
> 
> I was wondering about that as well, but checking for alignof(*old) 
> probably can't hurt. The only architectures that actually have a custom 
> arch_try_cmpxchg*() are s390 and x86 and those don't care about 
> alignmnent of 'old', but it's possible that another architecture that 
> can't handle unaligned load/store would add an inline asm implementation 
> in the future and break if an alignment fixup happens in the middle of 
> an ll/sc loop.
> 

That hypothetical future requirement seems improbable to me. Moreover, 
would such an architecture have a need for CONFIG_DEBUG_ATOMIC?