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

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Message-ID: <20260105163305.00002306@huawei.com>
Date: Mon, 5 Jan 2026 16:34:37 +0000
From: Jonathan Cameron <jonathan.cameron@...wei.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 v2 03/45] arm_mpam: Use non-atomic bitops when modifying
 feature bitmap

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

Jonathan


> 
> Signed-off-by: Ben Horgan <ben.horgan@....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 {