linux-kernel - Re: [PATCH v3 02/47] arm_mpam: Use non-atomic bitops when modifying feature bitmap

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Message-ID: <aWonpc52-BMzcx8Q@arm.com>
Date: Fri, 16 Jan 2026 11:57:25 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Ben Horgan <ben.horgan@....com>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
	baolin.wang@...ux.alibaba.com, carl@...amperecomputing.com,
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Subject: Re: [PATCH v3 02/47] arm_mpam: Use non-atomic bitops when modifying
 feature bitmap

On Mon, Jan 12, 2026 at 04:58:29PM +0000, Ben Horgan 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.
> 
> Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>
> Signed-off-by: Ben Horgan <ben.horgan@....com>
> ---
> Changes since v2:
> Add comment (Jonathan)
> ---
>  drivers/resctrl/mpam_internal.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index 17cdc3080d58..e8971842b124 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -200,8 +200,12 @@ 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)
> +/*
> + * The non-atomic get/set operations are used because if struct mpam_props is
> + * packed, the alignment requirements for atomics aren't met.
> + */
> +#define mpam_set_feature(_feat, x)	__set_bit(_feat, (x)->features)
> +#define mpam_clear_feature(_feat, x)	__clear_bit(_feat, (x)->features)

After discussing privately, I can see how test__props_mismatch() can
end up with unaligned atomics on the mmap_props::features array. Happy to
pick it up for 6.19 (probably the first patch as well, though that's
harmless).

Is there a Fixes tag here for future reference?

-- 
Catalin