Re: [PATCH v2 03/45] arm_mpam: Use non-atomic bitops when modifying feature bitmap

Jonathan Cameron
2026-01-05T16:34:42Z

On Fri, 19 Dec 2025 18:11:05 +0000
Ben Horgan <ben.horgan@arm.com> wrote:

> In the test__props_mismatch() kunit test we rely on the struct mpam_props
> being packed to ensure memcmp doesn't consider packing. Making it packed
> reduces the alignment of the features bitmap and so breaks a requirement
> for the use of atomics. As we don't rely on the set/clear of these bits
> being atomic, just make them non-atomic.

oh. That's a bit horrible.

Whilst I guess no one is likely to 'upgrade' these it feels odd enough that
I'd be tempted to leave a breadcrumb of a comment next to them on why
they must be the non atomic versions.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

Jonathan

>
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> ---
> drivers/resctrl/mpam_internal.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index 17cdc3080d58..aaaf31ca9210 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -200,8 +200,8 @@ struct mpam_props {
> } PACKED_FOR_KUNIT;
>
> #define mpam_has_feature(_feat, x) test_bit(_feat, (x)->features)
> -#define mpam_set_feature(_feat, x) set_bit(_feat, (x)->features)
> -#define mpam_clear_feature(_feat, x) clear_bit(_feat, (x)->features)
> +#define mpam_set_feature(_feat, x) __set_bit(_feat, (x)->features)
> +#define mpam_clear_feature(_feat, x) __clear_bit(_feat, (x)->features)
>
> /* The values for MSMON_CFG_MBWU_FLT.RWBW */
> enum mon_filter_options {