linux-kernel - Re: [PATCH] s390/locking: Reenable optimistic spinning

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Date:	Wed, 6 Aug 2014 10:34:53 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>, mingo@...nel.org,
	Peter Zijlstra <peterz@...radead.org>, stable@...r.kernel.org
Subject: Re: [PATCH] s390/locking: Reenable optimistic spinning

On Tue, Aug 05, 2014 at 09:57:51AM +0200, Christian Borntraeger wrote:
> commit 4badad352a6bb202ec68afa7a574c0bb961e5ebc (locking/mutex: Disable
> optimistic spinning on some architectures) fenced spinning for
> architectures without proper cmpxchg.
> There is no need to disable mutex spinning on s390, though:
> The instructions CS,CSG and friends provide the proper guarantees.
> (We dont implement cmpxchg with locks).
> 
> Please note: This is a quick fix for s390, suitable for stable
> as 3.16 unintenionally disabled optimistic spinning.
> 
> The proper fix is probably to use ARCH_HAVE_NMI_SAFE_CMPXCHG
> instead of introducing ARCH_SUPPORTS_ATOMIC_RMW.
> 
> Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
> CC: <mingo@...nel.org>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: stable@...r.kernel.org
> ---
>  arch/s390/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index 9a0ea87..eab3c32 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -63,6 +63,7 @@ config S390
>  	select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
>  	select ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
>  	select ARCH_HAVE_NMI_SAFE_CMPXCHG
> +	select ARCH_SUPPORTS_ATOMIC_RMW
>  	select ARCH_INLINE_READ_LOCK
>  	select ARCH_INLINE_READ_LOCK_BH
>  	select ARCH_INLINE_READ_LOCK_IRQ

This can go in via the s390 tree. I applied your patch and changed
it a bit to keep Kconfig sorted.
Thanks!

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