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

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Message-ID: <aMkLb6jPiSbzeRWL@arm.com>
Date: Tue, 16 Sep 2025 08:02:07 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Will Deacon <will@...nel.org>
Cc: Yeoreum Yun <yeoreum.yun@....com>, 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, mark.rutland@....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 Mon, Sep 15, 2025 at 09:35:55PM +0100, Will Deacon wrote:
> On Mon, Sep 15, 2025 at 08:40:33PM +0100, Catalin Marinas wrote:
> > On Mon, Sep 15, 2025 at 11:32:39AM +0100, Yeoreum Yun wrote:
> > > So I think it would be better to keep the current LLSC implementation
> > > in LSUI.
> > 
> > I think the code would look simpler with LL/SC but you can give it a try
> > and post the code sample here (not in a new series).
> 
> If you stick the cas*t instruction in its own helper say, cmpxchg_user(),
> then you can do all the shifting/masking in C and I don't reckon it's
> that bad. It means we (a) get rid of exclusives, which is the whole
> point of this and (b) don't have to mess around with PAN.

We get rid of PAN toggling already since FEAT_LSUI introduces
LDTXR/STTXR. But, I'm all for CAS if it doesn't look too bad. Easier
I think if we do a get_user() of a u64 and combine it with the futex u32
while taking care of CPU endianness. All in a loop. Hopefully the
compiler is smart enough to reduce masking/or'ing to fewer instructions.

-- 
Catalin