linux-kernel - Re: [PATCH v4 5/7] arm64/futex: add futex atomic operation with FEAT

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Message-ID: <aH4egvxnbQPu1XzE@J2N7QTR9R3>
Date: Mon, 21 Jul 2025 12:03:30 +0100
From: Mark Rutland <mark.rutland@....com>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: catalin.marinas@....com, will@...nel.org, broonie@...nel.org,
	oliver.upton@...ux.dev, ardb@...nel.org, frederic@...nel.org,
	james.morse@....com, joey.gouly@....com,
	scott@...amperecomputing.com, maz@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/7] arm64/futex: add futex atomic operation with
 FEAT_LSUI

On Mon, Jul 21, 2025 at 09:36:16AM +0100, Yeoreum Yun wrote:
> Current futex atomic operations are implemented with ll/sc instructions and
> clearing PSTATE.PAN.
> 
> Since Armv9.6, FEAT_LSUI supplies not only load/store instructions but
> also atomic operation for user memory access in kernel it doesn't need
> to clear PSTATE.PAN bit anymore.
> 
> With theses instructions some of futex atomic operations don't need to
> be implmented with ldxr/stlxr pair instead can be implmented with
> one atomic operation supplied by FEAT_LSUI.
> 
> However, some of futex atomic operations still need to use ll/sc way
> via ldtxr/stltxr supplied by FEAT_LSUI since there is no correspondant
> atomic instruction or doesn't support word size operation
> (i.e) eor, cas{mb}t But It's good to work without clearing PSTATE.PAN bit.

That's unfortunate; have we fed back to Arm's architecture folks that we
care about those cases?

> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> ---
>  arch/arm64/include/asm/futex_lsui.h | 132 ++++++++++++++++++++++++++++
>  1 file changed, 132 insertions(+)
>  create mode 100644 arch/arm64/include/asm/futex_lsui.h

This logic is introduced unused, and TBH I don't think this needs to be
in a separate header.

I reckon it's be better to keep all of this in <asm/futex.h> and rework
the series to:

(1) Factor out the existing LL/SC logic into separate LL/SC helpers in
    <asm/futex.h>, with an __llsc_ prefix, called by the existing
    functions.

(2) Add the new __lsui_ futex operations to <asm/futex.h>, along with
    code to select between the __llsc_ and __lsui_ versions.

We split the regular atomics different becuase there are *many* generic
atomic operations, but I don't think it's worthwhile to split the futex
logic over several headers.

Maybe it's worth having <asm/lsui.h>, but for now I reckon it's best to
also fold that into <asm/futex.h>, and we can split it out later if we
need it for something else.

Mark.