linux-kernel - Re: [PATCH v8 5/5] arm64: futex: support futex with FEAT

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Message-ID: <aMq5DbqsXj6vP7Xe@e129823.arm.com>
Date: Wed, 17 Sep 2025 14:35:09 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: catalin.marinas@....com, 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 v8 5/5] arm64: futex: support futex with FEAT_LSUI

Hi Mark,

> On Wed, Sep 17, 2025 at 12:08:38PM +0100, Yeoreum Yun wrote:
> > +static __always_inline int
> > +__lsui_cmpxchg64(u64 __user *uaddr, u64 *oldval, u64 newval)
> > +{
> > +	int ret = 0;
> > +
> > +	asm volatile("// __lsui_cmpxchg64\n"
> > +	__LSUI_PREAMBLE
> > +"1:	casalt	%x2, %x3, %1\n"
> > +"2:\n"
> > +	_ASM_EXTABLE_UACCESS_ERR(1b, 2b, %w0)
> > +	: "+r" (ret), "+Q" (*uaddr), "+r" (*oldval)
> > +	: "r" (newval)
> > +	: "memory");
> > +
> > +	return ret;
> > +}
> > +
> > +static __always_inline int
> > +__lsui_cmpxchg32(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
> > +{
> > +	u64 __user *uaddr_al;
>
> Please use 'uaddr64' to match the other 64-bit variables.
>
> I assume that the '_al' suffix is meant to be short for 'aligned', but I
> think using '64' is more consistent and clearer.
>
> > +	u64 oval64, nval64, tmp;
>
> Likewise, 'orig64' would be clearer than 'tmp' here.

Thanks for your suggestion.
>
> > +	static const u64 hi_mask = IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN) ?
> > +		GENMASK_U64(63, 32): GENMASK_U64(31, 0);
> > +	static const u8 hi_shift = IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN) ? 32 : 0;
> > +	static const u8 lo_shift = IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN) ? 0 : 32;
> > +
> > +	uaddr_al = (u64 __user *) PTR_ALIGN_DOWN(uaddr, sizeof(u64));
> > +	if (get_user(oval64, uaddr_al))
> > +		return -EFAULT;
> > +
> > +	if ((u32 __user *)uaddr_al != uaddr) {
> > +		nval64 = ((oval64 & ~hi_mask) | ((u64)newval << hi_shift));
> > +		oval64 = ((oval64 & ~hi_mask) | ((u64)oldval << hi_shift));
> > +	} else {
> > +		nval64 = ((oval64 & hi_mask) | ((u64)newval << lo_shift));
> > +		oval64 = ((oval64 & hi_mask) | ((u64)oldval << lo_shift));
> > +	}
> > +
> > +	tmp = oval64;
> > +
> > +	if (__lsui_cmpxchg64(uaddr_al, &oval64, nval64))
> > +		return -EFAULT;
> > +
> > +	if (tmp != oval64)
> > +		return -EAGAIN;
>
> This means that we'll immediately return -EAGAIN upon a spurious failure
> (where the adjacent 4 bytes have changed), whereas the LL/SC ops would
> retry FUTEX_MAX_LOOPS before returning -EGAIN.
>
> I suspect we want to retry here (or in the immediate caller).

Right. I've thought about it but at the time of writing,
I return -EAGAIN immediately. Let's wait for other people's comments.

>
> > +
> > +	*oval = oldval;
> > +
> > +	return 0;
> > +}
>
> Aside from the retry issue, I *think* you can simplify this to something
> like:
>
> static __always_inline int
> __lsui_cmpxchg32(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
> {
> 	uaddr64 = (u64 __user *)PTR_ALIGN_DOWN(uaddr, sizeof(u64));
> 	u64 oval64, nval64, orig64;
>
> 	if (get_user(oval64, uaddr64)
> 		return -EFAULT;
>
> 	if (IS_ALIGNED(addr, sizeof(u64)) == IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN))  {
> 		FIELD_MODIFY(GENMASK_U64(31, 0), &oval64, oldval);
> 		FIELD_MODIFY(GENMASK_U64(31, 0), &nval64, newval);
> 	} else {
> 		FIELD_MODIFY(GENMASK_U64(63, 32), &oval64, oldval);
> 		FIELD_MODIFY(GENMASK_U64(63, 32), &nval64, newval);
> 	}
> 	orig64 = oval64;
>
> 	if (__lsui_cmpxchg64(uaddr_al, &oval64, nval64))
> 		return -EFAULT;
>
> 	if (oval64 != orig64)
> 		return -EAGAIN;
>
> 	*oval = oldval;
> 	return 0;
> }

Hmm I think this wouldn'b cover the case below when big-endianess used.

struct {
  u32 others 0x55667788;
  u32 futex = 0x11223344;
};

In this case, memory layout would be:

  55 66 77 88 11 22 33 44

So, the value of fetched oval64 is 0x5566778811223344;

So, it should modify the GENMASK_U64(31, 0) fields.
But, it tries to modify GENMASK_U64(63, 32) fields.

Thanks!

[...]
--
Sincerely,
Yeoreum Yun