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Date: Thu, 1 Oct 2015 14:27:16 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Ingo Molnar <mingo@kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will.deacon@arm.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Waiman Long <waiman.long@hp.com>
Subject: Re: [RFC v2 5/7] powerpc: atomic: Implement cmpxchg{,64}_* and
 atomic{,64}_cmpxchg_* variants
Message-ID: <20151001122715.GQ2881@worktop.programming.kicks-ass.net>
References: <1442418575-12297-1-git-send-email-boqun.feng@gmail.com>
 <1442418575-12297-6-git-send-email-boqun.feng@gmail.com>
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On Wed, Sep 16, 2015 at 11:49:33PM +0800, Boqun Feng wrote:
> Unlike other atomic operation variants, cmpxchg{,64}_acquire and
> atomic{,64}_cmpxchg_acquire don't have acquire semantics if the cmp part
> fails, so we need to implement these using assembly.

I think that is actually expected and documented. That is, a cmpxchg
only implies barriers on success. See:

  ed2de9f74ecb ("locking/Documentation: Clarify failed cmpxchg() memory ordering semantics")