linux-kernel - Re: [PATCH v7 RESEND 5/6] arm64: futex: small optimisation for __llsc_futex_atomic

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Message-ID: <aML2fwGfeubY6HgA@e129823.arm.com>
Date: Thu, 11 Sep 2025 17:19:11 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Will Deacon <will@...nel.org>
Cc: catalin.marinas@....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 v7 RESEND 5/6] arm64: futex: small optimisation for
 __llsc_futex_atomic_set()

Hi Will,

[...]

> >  arch/arm64/include/asm/futex.h | 43 ++++++++++++++++++++++++++++------
> >  1 file changed, 36 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
> > index ab7003cb4724..22a6301a9f3d 100644
> > --- a/arch/arm64/include/asm/futex.h
> > +++ b/arch/arm64/include/asm/futex.h
> > @@ -13,7 +13,7 @@
> >
> >  #define LLSC_MAX_LOOPS	128 /* What's the largest number you can think of? */
> >
> > -#define LLSC_FUTEX_ATOMIC_OP(op, insn)					\
> > +#define LLSC_FUTEX_ATOMIC_OP(op, asm_op)				\
> >  static __always_inline int						\
> >  __llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval)	\
> >  {									\
> > @@ -24,7 +24,7 @@ __llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval)	\
> >  	asm volatile("// __llsc_futex_atomic_" #op "\n"			\
> >  "	prfm	pstl1strm, %2\n"					\
> >  "1:	ldxr	%w1, %2\n"						\
> > -	insn "\n"							\
> > +"	" #asm_op "	%w3, %w1, %w5\n"				\
> >  "2:	stlxr	%w0, %w3, %2\n"						\
> >  "	cbz	%w0, 3f\n"						\
> >  "	sub	%w4, %w4, %w0\n"					\
> > @@ -46,11 +46,40 @@ __llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval)	\
> >  	return ret;							\
> >  }
> >
> > -LLSC_FUTEX_ATOMIC_OP(add, "add	%w3, %w1, %w5")
> > -LLSC_FUTEX_ATOMIC_OP(or, "orr	%w3, %w1, %w5")
> > -LLSC_FUTEX_ATOMIC_OP(and, "and	%w3, %w1, %w5")
> > -LLSC_FUTEX_ATOMIC_OP(eor, "eor	%w3, %w1, %w5")
> > -LLSC_FUTEX_ATOMIC_OP(set, "mov	%w3, %w5")
> > +LLSC_FUTEX_ATOMIC_OP(add, add)
> > +LLSC_FUTEX_ATOMIC_OP(or, orr)
> > +LLSC_FUTEX_ATOMIC_OP(and, and)
> > +LLSC_FUTEX_ATOMIC_OP(eor, eor)
> > +
> > +static __always_inline int
> > +__llsc_futex_atomic_set(int oparg, u32 __user *uaddr, int *oval)
> > +{
> > +	unsigned int loops = LLSC_MAX_LOOPS;
> > +	int ret, oldval;
> > +
> > +	uaccess_enable_privileged();
> > +	asm volatile("//__llsc_futex_xchg\n"
> > +"	prfm	pstl1strm, %2\n"
> > +"1:	ldxr	%w1, %2\n"
> > +"2:	stlxr	%w0, %w4, %2\n"
> > +"	cbz	%w3, 3f\n"
> > +"	sub	%w3, %w3, %w0\n"
> > +"	cbnz	%w3, 1b\n"
> > +"	mov	%w0, %w5\n"
> > +"3:\n"
> > +"	dmb	ish\n"
> > +	_ASM_EXTABLE_UACCESS_ERR(1b, 3b, %w0)
> > +	_ASM_EXTABLE_UACCESS_ERR(2b, 3b, %w0)
> > +	: "=&r" (ret), "=&r" (oldval), "+Q" (*uaddr), "+r" (loops)
> > +	: "r" (oparg), "Ir" (-EAGAIN)
> > +	: "memory");
> > +	uaccess_disable_privileged();
> > +
> > +	if (!ret)
> > +		*oval = oldval;
>
> Hmm, I'm really not sure this is worthwhile. I doubt the "optimisation"
> actually does anything and adding a whole new block of asm just for the
> SET case isn't much of an improvement on the maintainability side, either.

TBH, I had the same question, but I thought this code seems to modify
freqenetly, I decide even a small optimisation -- reduce one instruction
only.

But I don't have strong opinion for this patch.
If it's not good for maintainability perspective,
This patch can be dropped.

Thanks!

--
Sincerely,
Yeoreum Yun