linux-kernel - Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

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Date:   Wed, 2 Aug 2017 09:46:54 +0100
From:   Will Deacon <will.deacon@....com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

On Tue, Aug 01, 2017 at 03:18:18PM -0700, Paul E. McKenney wrote:
> On Tue, Aug 01, 2017 at 06:42:00PM +0200, Peter Zijlstra wrote:
> > On Tue, Aug 01, 2017 at 09:14:12AM -0700, Paul E. McKenney wrote:
> > > So if ARM really needs the litmus test with smp_rmb() to be allowed,
> > > we need to adjust the Linux-kernel memory model appropriately.  Which
> > > means that one of us needs to reach out to the usual suspects.  Would
> > > you like to do that, or would you like me to?
> > 
> > I'm really sad ARM8.1 LSE breaks this stuff.. It is rather counter
> > intuitive (then again, we _are_ talking barriers).
> 
> No argument.
> 
> Then again, when we said that the Linux kernel memory model would
> have a non-trivial rate of change, we weren't joking.
> 
> Will, is this the official description?
> 
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0557a.b/index.html
> 
> If so, is B6.1 what we should be looking at?

Sorry it's so tricky to find. The architecture document is here:

https://static.docs.arm.com/ddi0487/b/DDI0487B_a_armv8_arm.pdf

and in section C3.2.13 ("Atomic memory operations") it states:

| The ST<OP> instructions are not regarded as doing a read for the purpose
| of a DMB LD barrier.

Will