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

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Message-ID: <aMlxCB1Ob22kagbQ@e129823.arm.com>
Date: Tue, 16 Sep 2025 15:15:36 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <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,
	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

On Tue, Sep 16, 2025 at 03:07:09PM +0100, Mark Rutland wrote:
> On Tue, Sep 16, 2025 at 02:58:16PM +0100, Yeoreum Yun wrote:
> > Hi Mark,
> >
> > [...]
> > > > I think it's enough to use usafe_get_user() instead of get_user() in here
> > > > since when FEAT_LSUI enabled, it doeesn't need to call
> > > > uaccess_ttbr0_enable()/disable().
> > >
> > > Regardless of uaccess_ttbr0_enable() and uaccess_ttbr0_disable()
> > > specifically, API-wise unsafe_get_user() is only supposed to be called
> > > between user_access_begin() and user_access_end(), and there's some
> > > stuff we probably want to add there (e.g. might_fault(), which
> > > unsafe_get_user() lacks today).
> > >
> > > Do we call those?
> >
> > Yes when you're available.
> > As you mention, the difference seems might_fault(),
> > But I'm not sure whether that would be a reason to validate to use
> > get_user() instead of unsafe_get_user() taking a increase of instruction
> > of "nop" -- uaccess_ttbr0_enable()/disable() in LSUI
> > except the reason for DEUBG purpose.
>
> I think the practical impact of those NOPs is going to be neglible, and
> not worth optimizing for unless/until we have data demonstrating
> otherwise.
>
> If we want to strictly avoid those NOPs, I think that we should do a
> more general cleanup, and e.g. have variants of user_access_begin() and
> user_access_end() that do not mess with TTBR0. I don't think we need to
> do that for this series.
>
> For now, I think that you should either:
>
> * Use get_user().
>
> * Use user_access_begin() .. user_access_end() wrapping both
>   unsafe_get_user() and the user cmpxchg.

Understood. I'll use get_user() in this series after
getting some more comments from other about C version which I sent
before your suggestion (Sorry to miss your email before I sent).

Thanks :D

--
Sincerely,
Yeoreum Yun