linux-kernel - Re: [PATCH powerpc/next 1/2] powerpc: Make value-returning atomics fully ordered

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Message-ID: <20151102022602.GL4122@linux.vnet.ibm.com>
Date:	Sun, 1 Nov 2015 18:26:02 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <will.deacon@....com>, stable@...r.kernel.org
Subject: Re: [PATCH powerpc/next 1/2] powerpc: Make value-returning atomics
 fully ordered

On Mon, Nov 02, 2015 at 09:30:31AM +0800, Boqun Feng wrote:
> According to memory-barriers.txt:
> 
> > Any atomic operation that modifies some state in memory and returns
> > information about the state (old or new) implies an SMP-conditional
> > general memory barrier (smp_mb()) on each side of the actual
> > operation ...
> 
> Which mean these operations should be fully ordered. However on PPC,
> PPC_ATOMIC_ENTRY_BARRIER is the barrier before the actual operation,
> which is currently "lwsync" if SMP=y. The leading "lwsync" can not
> guarantee fully ordered atomics, according to Paul Mckenney:
> 
> https://lkml.org/lkml/2015/10/14/970
> 
> To fix this, we define PPC_ATOMIC_ENTRY_BARRIER as "sync" to guarantee
> the fully-ordered semantics.
> 
> This also makes futex atomics fully ordered, which can avoid possible
> memory ordering problems if userspace code relies on futex system call
> for fully ordered semantics.
> 
> Cc: <stable@...r.kernel.org> # 3.4+
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
> These two are separated and splited from the patchset of powerpc atomic
> variants implementation, whose link is:
> 
> https://lkml.org/lkml/2015/10/26/141
> 
> Based on next branch of powerpc tree, tested by 0day.
> 
>  arch/powerpc/include/asm/synch.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/synch.h b/arch/powerpc/include/asm/synch.h
> index e682a71..c508686 100644
> --- a/arch/powerpc/include/asm/synch.h
> +++ b/arch/powerpc/include/asm/synch.h
> @@ -44,7 +44,7 @@ static inline void isync(void)
>  	MAKE_LWSYNC_SECTION_ENTRY(97, __lwsync_fixup);
>  #define PPC_ACQUIRE_BARRIER	 "\n" stringify_in_c(__PPC_ACQUIRE_BARRIER)
>  #define PPC_RELEASE_BARRIER	 stringify_in_c(LWSYNC) "\n"
> -#define PPC_ATOMIC_ENTRY_BARRIER "\n" stringify_in_c(LWSYNC) "\n"
> +#define PPC_ATOMIC_ENTRY_BARRIER "\n" stringify_in_c(sync) "\n"
>  #define PPC_ATOMIC_EXIT_BARRIER	 "\n" stringify_in_c(sync) "\n"
>  #else
>  #define PPC_ACQUIRE_BARRIER
> -- 
> 2.6.2
> 

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