linux-kernel - Re: [RFC][PATCH 04/31] locking,arm: Implement atomic{,64}_fetch_{add,sub,and,andnot,or,xor}{,_relaxed,_acquire,

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Message-ID: <20160422113527.GD10289@arm.com>
Date:	Fri, 22 Apr 2016 12:35:27 +0100
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	torvalds@...ux-foundation.org, mingo@...nel.org,
	tglx@...utronix.de, paulmck@...ux.vnet.ibm.com,
	boqun.feng@...il.com, waiman.long@....com, fweisbec@...il.com,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	rth@...ddle.net, vgupta@...opsys.com, linux@....linux.org.uk,
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	ysato@...rs.sourceforge.jp, rkuo@...eaurora.org,
	tony.luck@...el.com, geert@...ux-m68k.org, james.hogan@...tec.com,
	ralf@...ux-mips.org, dhowells@...hat.com, jejb@...isc-linux.org,
	mpe@...erman.id.au, schwidefsky@...ibm.com, dalias@...c.org,
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	arnd@...db.de, dbueso@...e.de, fengguang.wu@...el.com
Subject: Re: [RFC][PATCH 04/31] locking,arm: Implement
 atomic{,64}_fetch_{add,sub,and,andnot,or,xor}{,_relaxed,_acquire,_release}()

On Fri, Apr 22, 2016 at 11:04:17AM +0200, Peter Zijlstra wrote:
> Implement FETCH-OP atomic primitives, these are very similar to the
> existing OP-RETURN primitives we already have, except they return the
> value of the atomic variable _before_ modification.
> 
> This is especially useful for irreversible operations -- such as
> bitops (because it becomes impossible to reconstruct the state prior
> to modification).
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/arm/include/asm/atomic.h |  108 ++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 98 insertions(+), 10 deletions(-)
> 
> --- a/arch/arm/include/asm/atomic.h
> +++ b/arch/arm/include/asm/atomic.h
> @@ -77,8 +77,36 @@ static inline int atomic_##op##_return_r
>  	return result;							\
>  }

[...]

> +static inline long long							\
> +atomic64_fetch_##op##_relaxed(long long i, atomic64_t *v)		\
> +{									\
> +	long long result, val;						\
> +	unsigned long tmp;						\
> +									\
> +	prefetchw(&v->counter);						\
> +									\
> +	__asm__ __volatile__("@ atomic64_fetch_" #op "\n"		\
> +"1:	ldrexd	%0, %H0, [%4]\n"					\
> +"	" #op1 " %Q1, %Q0, %Q5\n"					\
> +"	" #op2 " %R1, %R0, %R5\n"					\
> +"	strexd	%2, %1, %H0, [%4]\n"					\

You want %H1 here.

With that:

Acked-by: Will Deacon <will.deacon@....com>

Will