linux-kernel - Re: [RFC][PATCH] mips: Fix arch_spin

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Date:	Wed, 27 Jan 2016 15:21:58 +0000
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Maciej W. Rozycki" <macro@...tec.com>,
	David Daney <ddaney@...iumnetworks.com>,
	Måns Rullgård <mans@...sr.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	linux-kernel@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	torvalds@...ux-foundation.org, boqun.feng@...il.com
Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock()

On Wed, Jan 27, 2016 at 03:54:21PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 27, 2016 at 11:43:48AM +0000, Will Deacon wrote:
> > Do you know whether a SYNC 18 (RELEASE) followed in program order by a
> > SYNC 17 (ACQUIRE) creates a full barrier (i.e. something like SYNC 16)?
> > 
> > If not, you may need to implement smp_mb__after_unlock_lock for RCU
> > to ensure globally transitive unlock->lock ordering should you decide
> > to relax your locking barriers.
> 
> You know that is a tricky question. Maybe its easier if you give the 3
> cpu litmus test that goes with it.

Sure, I was building up to that. I just wanted to make sure the basics
were there (program-order, so same CPU) before we go any further. It
sounds like they are, so that's promising.

> Maciej, the tricky point is what, if any, effect the
> SYNC_RELEASE+SYNC_ACQUIRE pair has on an unrelated CPU. Please review
> the TRANSITIVITY section in Documentation/memory-barriers.txt and
> replace <general barrier> with the RELEASE+ACQUIRE pair.
> 
> We've all (Will, Paul and me) had much 'fun' trying to decipher the
> MIPS64r6 manual but failed to reach a conclusion on this.

For the inter-thread case, Paul had a previous example along the lines
of:


Wx=1
WyRel=1

RyAcq=1
Rz=0

Wz=1
smp_mb()
Rx=0


and I suppose a variant of that:


Wx=1
WyRel=1

RyAcq=1
Wz=1

Rz=1
<address dependency>
Rx=0


Will