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

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Date:	Mon, 2 Nov 2015 10:14:24 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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>

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
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