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Date: Thu, 1 Oct 2015 08:09:09 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>, 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>, Waiman Long <waiman.long@hp.com>
Subject: Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and
 atomic{,64}_xchg_* variants
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On Thu, Oct 01, 2015 at 02:24:40PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 16, 2015 at 11:49:32PM +0800, Boqun Feng wrote:
> > Implement xchg_relaxed and define atomic{,64}_xchg_* as xchg_relaxed,
> > based on these _relaxed variants, release/acquire variants can be built.
> > 
> > Note that xchg_relaxed and atomic_{,64}_xchg_relaxed are not compiler
> > barriers.
> 
> Hmm, and I note your previous patch creating the regular _relaxed
> thingies also removes the memory clobber.
> 
> And looking at the ARM _relaxed patch from Will, I see their _relaxed
> ops are also not a compiler barrier.
> 
> I must say I'm somewhat surprised by this level of relaxation, I had
> expected to only loose SMP barriers, not the program order ones.
> 
> Is there a good argument for this?

Yes, when we say "relaxed", we really mean relaxed.  ;-)

Both the CPU and the compiler are allowed to reorder around relaxed
operations.

							Thanx, Paul