linux-kernel - Re: [PATCH v12 6/7] arm64: futex: support futex with FEAT

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Message-ID: <aYtgmZZhAKAvtfaK@arm.com>
Date: Tue, 10 Feb 2026 16:45:13 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
	linux-kselftest@...r.kernel.org, will@...nel.org, maz@...nel.org,
	broonie@...nel.org, oliver.upton@...ux.dev, miko.lenczewski@....com,
	kevin.brodsky@....com, ardb@...nel.org, suzuki.poulose@....com,
	lpieralisi@...nel.org, scott@...amperecomputing.com,
	joey.gouly@....com, yuzenghui@...wei.com, pbonzini@...hat.com,
	shuah@...nel.org, mark.rutland@....com, arnd@...db.de
Subject: Re: [PATCH v12 6/7] arm64: futex: support futex with FEAT_LSUI

I wonder whether we can shorten this function a bit. Not sure it would
be more readable but it would be shorter.

On Wed, Jan 21, 2026 at 07:06:21PM +0000, Yeoreum Yun wrote:
> +static __always_inline int
> +__lsui_cmpxchg32(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
> +{
> +	u64 __user *uaddr64;
> +	bool futex_on_lo;
> +	int ret, i;
> +	u32 other, orig_other;
> +	union {
> +		struct futex_on_lo {
> +			u32 val;
> +			u32 other;
> +		} lo_futex;
> +
> +		struct futex_on_hi {
> +			u32 other;
> +			u32 val;
> +		} hi_futex;
> +
> +		u64 raw;
> +	} oval64, orig64, nval64;

	union {
		u32 futex[2];
		u64 raw;
	}

> +
> +	uaddr64 = (u64 __user *) PTR_ALIGN_DOWN(uaddr, sizeof(u64));
> +	futex_on_lo = IS_ALIGNED((unsigned long)uaddr, sizeof(u64));

	futex_pos = (unsigned long)uaddr & 4 ? 1 : 0;

> +
> +	if (futex_on_lo) {
> +		oval64.lo_futex.val = oldval;
> +		ret = get_user(oval64.lo_futex.other, uaddr + 1);
> +	} else {
> +		oval64.hi_futex.val = oldval;
> +		ret = get_user(oval64.hi_futex.other, uaddr - 1);
> +	}

and here use

	get_user(oval64.raw, uaddr64);
	futex[futex_pos] = oldval;

> +
> +	if (ret)
> +		return -EFAULT;
> +
> +	ret = -EAGAIN;
> +	for (i = 0; i < FUTEX_MAX_LOOPS; i++) {
> +		orig64.raw = nval64.raw = oval64.raw;
> +
> +		if (futex_on_lo)
> +			nval64.lo_futex.val = newval;
> +		else
> +			nval64.hi_futex.val = newval;
> +
> +		if (__lsui_cmpxchg64(uaddr64, &oval64.raw, nval64.raw))
> +			return -EFAULT;
> +
> +		if (futex_on_lo) {
> +			oldval = oval64.lo_futex.val;
> +			other = oval64.lo_futex.other;
> +			orig_other = orig64.lo_futex.other;
> +		} else {
> +			oldval = oval64.hi_futex.val;
> +			other = oval64.hi_futex.other;
> +			orig_other = orig64.hi_futex.other;
> +		}

Something similar here to use futex[futex_pos].

We probably also need to check that the user pointer is 32-bit aligned
and return -EFAULT if not.

-- 
Catalin