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

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Message-ID: <aWpeZrgvSn2ab4VG@arm.com>
Date: Fri, 16 Jan 2026 15:51:02 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Ben Horgan <ben.horgan@....com>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
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Subject: Re: [PATCH v3 02/47] arm_mpam: Use non-atomic bitops when modifying
 feature bitmap

On Fri, Jan 16, 2026 at 12:12:53PM +0000, Ben Horgan wrote:
> On 1/16/26 12:02, Ben Horgan wrote:
> > On 1/16/26 11:57, Catalin Marinas wrote:
> >> 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).
> > 
> > Yes please.
> > 
> >>
> >> Is there a Fixes tag here for future reference?
> >>
> > 
> > Yes, the mpam_set/clear macros were introduced in
> 
> The mpam_set_clear() actually comes after in:
> c10ca83a7783 arm_mpam: Merge supported features during mpam_enable() into mpam_class
> but I think the fixes below is still the correct one as it is where we could 
> first start seeing the problem.
> > 
> > Fixes: 8c90dc68a5de ("arm_mpam: Probe the hardware features resctrl supports")

Yes, I left the original as that's the one first introducing the atomic
bitops on this structure.

-- 
Catalin