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

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Message-ID: <20160127233836.GQ4503@linux.vnet.ibm.com>
Date:	Wed, 27 Jan 2016 15:38:36 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"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, torvalds@...ux-foundation.org,
	boqun.feng@...il.com
Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock()

On Wed, Jan 27, 2016 at 03:21:58PM +0000, Will Deacon wrote:
> 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

Each paragraph being a separate thread, correct?  If so, agreed.

> and I suppose a variant of that:
> 
> 
> Wx=1
> WyRel=1
> 
> RyAcq=1
> Wz=1
> 
> Rz=1
> <address dependency>
> Rx=0

Agreed, this would be needed as well, along with the read-read and
read-write variants.  I picked the write-read version (Will's first
test above) because write-read reordering is the most likely on
hardware that I am aware of.

							Thanx, Paul