linux-kernel - Re: [RFC PATCH] arm64: cmpxchg.h: Bring ldxr and stxr closer

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Message-ID: <20150227201526.GH24818@arm.com>
Date:	Fri, 27 Feb 2015 20:15:30 +0000
From:	Will Deacon <will.deacon@....com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	Steve Capper <steve.capper@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] arm64: cmpxchg.h: Bring ldxr and stxr closer

On Fri, Feb 27, 2015 at 08:09:17PM +0000, Pranith Kumar wrote:
> ARM64 documentation recommends keeping exclusive loads and stores as close as
> possible. Any instructions which do not depend on the value loaded should be
> moved outside. 
> 
> In the current implementation of cmpxchg(), there is a mov instruction which can
> be pulled before the load exclusive instruction without any change in
> functionality. This patch does that change.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> ---
>  arch/arm64/include/asm/cmpxchg.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)

[...]

> @@ -166,11 +166,11 @@ static inline int __cmpxchg_double(volatile void *ptr1, volatile void *ptr2,
>  		VM_BUG_ON((unsigned long *)ptr2 - (unsigned long *)ptr1 != 1);
>  		do {
>  			asm volatile("// __cmpxchg_double8\n"
> +			"	mov	%w0, #0\n"
>  			"	ldxp	%0, %1, %2\n"

Seriously, you might want to test this before you mindlessly make changes to
low-level synchronisation code. Not only is the change completely unnecessary
but it is actively harmful.

Have a good weekend,

Will
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