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Date: Thu, 1 Oct 2015 12:41:00 -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
Message-ID: <20151001194100.GL4043@linux.vnet.ibm.com>
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On Thu, Oct 01, 2015 at 08:23:09PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 01, 2015 at 11:03:01AM -0700, Paul E. McKenney wrote:
> > On Thu, Oct 01, 2015 at 07:13:04PM +0200, Peter Zijlstra wrote:
> > > On Thu, Oct 01, 2015 at 08:09:09AM -0700, Paul E. McKenney wrote:
> > > > On Thu, Oct 01, 2015 at 02:24:40PM +0200, Peter Zijlstra wrote:
> > > 
> > > > > 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.
> > > 
> > > Is this documented somewhere, because I completely missed this part.
> > 
> > Well, yes, these need to be added to the documentation.  I am assuming
> > that Will is looking to have the same effect as C11 memory_order_relaxed,
> > which is relaxed in this sense.  If he has something else in mind,
> > he needs to tell us what it is and why.  ;-)
> 
> I suspect he is; but I'm not _that_ up to date on the whole C11 stuff.

Lucky you!  ;-)

							Thanx, Paul