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

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Message-ID: <aMrAic2NbXTDsctJ@e129823.arm.com>
Date: Wed, 17 Sep 2025 15:07:05 +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,

[...]

> > > 		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;
>
> Ok, so the entire struct is aligned to 8 bytes, and the 'futex' field is
> 4 bytes after that (and not itself aligned to 8 bytes). In that case:
[...]

Sorry and thanks for explation. I've misread the condition as

  if (IS_ALIGNED(uaddr, sizeof(64) && IS_ENABLED(CONFIG_CPU_LITTLE_ENDIAN)

Again. sorry for my bad eyes :\


[...]

--
Sincerely,
Yeoreum Yun