Re: [PATCH v5 2/4] atomic: Specify alignment for atomic_t and atomic64_t

[Arnd Bergmann]

On Tue, Dec 16, 2025, at 07:31, Finn Thain wrote:
> Some recent commits incorrectly assumed 4-byte alignment of locks.
> That assumption fails on Linux/m68k (and, interestingly, would have
> failed on Linux/cris also). The jump label implementation makes a
> similar alignment assumption.
>
> The expectation that atomic_t and atomic64_t variables will be naturally
> aligned seems reasonable, as indeed they are on 64-bit architectures.
> But atomic64_t isn't naturally aligned on csky, m68k, microblaze, nios2,
> openrisc and sh. Neither atomic_t nor atomic64_t are naturally aligned
> on m68k.
>
> This patch brings a little uniformity by specifying natural alignment
> for atomic types. One benefit is that atomic64_t variables do not get
> split across a page boundary. The cost is that some structs grow which
> leads to cache misses and wasted memory.

Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>