Re: [PATCH v12 6/7] arm64: futex: support futex with FEAT_LSUI

Catalin Marinas
2026-02-10T16:45:19Z

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