linux-kernel - Re: [PATCH v5 tip/core/locking 6/7] locking: Add an smp_mb__after_unlock

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Date:	Tue, 10 Dec 2013 13:37:26 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu,
	Linux-Arch <linux-arch@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 tip/core/locking 6/7] locking: Add an
 smp_mb__after_unlock_lock() for UNLOCK+LOCK barrier

On Mon, Dec 09, 2013 at 05:28:02PM -0800, Paul E. McKenney wrote:
> diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> index f89da808ce31..abf645799991 100644
> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -84,4 +84,6 @@ do {									\
>  	___p1;								\
>  })
>  
> +#define smp_mb__after_unlock_lock()	do { } while (0)
> +
>  #endif /* _ASM_POWERPC_BARRIER_H */

Didn't ben said ppc actually violates the current unlock+lock assumtion
and therefore this barrier woulnd't actually be a nop on ppc
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