linux-kernel - Re: [RFC 3/5] powerpc: atomic: implement atomic{,64}_{add,sub}_return

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Message-ID: <20150901190027.GP1612@arm.com>
Date:	Tue, 1 Sep 2015 20:00:27 +0100
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Boqun Feng <boqun.feng@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>
Subject: Re: [RFC 3/5] powerpc: atomic: implement
 atomic{,64}_{add,sub}_return_* variants

On Fri, Aug 28, 2015 at 04:39:21PM +0100, Peter Zijlstra wrote:
> On Fri, Aug 28, 2015 at 10:16:02PM +0800, Boqun Feng wrote:
> > Ah.. just read through the thread you mentioned, I might misunderstand
> > you, probably because I didn't understand RCpc well..
> > 
> > You are saying that in a RELEASE we -might- switch from smp_lwsync() to
> > smp_mb() semantically, right? I guess this means we -might- switch from
> > RCpc to RCsc, right?
> > 
> > If so, I think I'd better to wait until we have a conclusion for this.
> 
> Yes, the difference between RCpc and RCsc is in the meaning of RELEASE +
> ACQUIRE. With RCsc that implies a full memory barrier, with RCpc it does
> not.

We've discussed this before, but for the sake of completeness, I don't
think we're fully RCsc either because we don't order the actual RELEASE
operation again a subsequent ACQUIRE operation:


P0
smp_store_release(&x, 1);
foo = smp_load_acquire(&y);

P1
smp_store_release(&y, 1);
bar = smp_load_acquire(&x);

We allow foo == bar == 0, which is prohibited by SC.


However, we *do* enforce ordering on any prior or subsequent accesses
for the code snippet above (the release and acquire combine to give a
full barrier), which makes these primitives well suited to things like
message passing.

Will
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