linux-kernel - Re: [PATCH RESEND v7 4/6] arm64: futex: refactor futex atomic operation

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Message-ID: <aMfrR0vserl/hfZ3@e129823.arm.com>
Date: Mon, 15 Sep 2025 11:32:39 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: will@...nel.org, broonie@...nel.org, maz@...nel.org,
	oliver.upton@...ux.dev, joey.gouly@....com, james.morse@....com,
	ardb@...nel.org, scott@...amperecomputing.com,
	suzuki.poulose@....com, yuzenghui@...wei.com, mark.rutland@....com,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v7 4/6] arm64: futex: refactor futex atomic
 operation

Hi Catalin,

> On Sat, Aug 16, 2025 at 04:19:27PM +0100, Yeoreum Yun wrote:
> > diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
> > index bc06691d2062..ab7003cb4724 100644
> > --- a/arch/arm64/include/asm/futex.h
> > +++ b/arch/arm64/include/asm/futex.h
> > @@ -7,17 +7,21 @@
> >
> >  #include <linux/futex.h>
> >  #include <linux/uaccess.h>
> > +#include <linux/stringify.h>
> >
> >  #include <asm/errno.h>
> >
> > -#define FUTEX_MAX_LOOPS	128 /* What's the largest number you can think of? */
> > +#define LLSC_MAX_LOOPS	128 /* What's the largest number you can think of? */
>
> I just noticed - you might as well leave the name as is here, especially
> if in patch 6 you align down address and use CAS on a 64-bit value as
> per Will's comment (and it's no longer LLSC). I think renaming this is
> unnecessary.

Okay. I'll restore to use origin name.
But I think LSUI wouldn't be used with CAS according to patch 6's
comments from you and additionally i think
chaning the CAS would make a failure because of
change of unrelated field. i.e)

struct user_structure{
  uint32 futex;
  uint32 some_value;
};

In this case, the change of some_value from user side could make a
failure of futex atomic operation.

So I think it would be better to keep the current LLSC implementation
in LSUI.

Thanks.

--
Sincerely,
Yeoreum Yun