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

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Date:	Tue, 2 Feb 2016 20:12:30 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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
Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock()

On Tue, Feb 02, 2016 at 11:45:59AM +0000, Will Deacon wrote:
> On Tue, Feb 02, 2016 at 01:19:04PM +0800, Boqun Feng wrote:
> > On Mon, Feb 01, 2016 at 07:54:58PM -0800, Paul E. McKenney wrote:
> > > On Mon, Feb 01, 2016 at 01:56:22PM +0000, Will Deacon wrote:
> > > > On Fri, Jan 29, 2016 at 02:22:53AM -0800, Paul E. McKenney wrote:
> > > > > On Fri, Jan 29, 2016 at 09:59:59AM +0000, Will Deacon wrote:
> > > > Locally transitive chain termination:
> > > > 
> > > > (i.e. these can't be used to extend a chain)
> > > 
> > > Agreed.
> > > 
> > > > > o	smp_store_release() -> lockless_dereference() (???)
> > > > > o	rcu_assign_pointer() -> rcu_dereference()
> > > > > o	smp_store_release() -> READ_ONCE(); if
> > 
> > Just want to make sure, this one is actually:
> > 
> > o	smp_store_release() -> READ_ONCE(); if ;<WRITE_ONCE()>
> > 
> > right? Because control dependency only orders READ->WRITE.
> > 
> > If so, do we also need to take the following pairing into consideration?
> > 
> > o	smp_store_release() -> READ_ONCE(); if ;smp_rmb(); <ACCESS_ONCE()>
> > 
> > > 
> > > I am OK with the first and last, but I believe that the middle one
> > > has real use cases.  So the rcu_assign_pointer() -> rcu_dereference()
> > > case needs to be locally transitive.
> > > 
> > 
> > Hmm... I don't think we should differ rcu_dereference() and
> > lockless_dereference(). One reason: list_for_each_entry_rcu() are using
> > lockless_dereference() right now, which means we used to think
> > rcu_dereference() and lockless_dereference() are interchangeable, right?
> > 
> > Besides, Will, what's the reason of having a locally transitive chain
> > termination? Because on some architectures RELEASE->DEPENDENCY pairs may
> > not be locally transitive?
> 
> Well, the following ISA2 test is permitted on ARM:
> 
> 
> P0:
> Wx=1
> WyRel=1	// rcu_assign_pointer
> 
> P1:
> Ry=1	// rcu_dereference

What if a <addr> dependency is added here? Same result?

> WzRel=1 // rcu_assign_pointer
> 
> P2:
> Rz=1	// rcu_dereference
> <addr>
> Rx=0
> 
> 
> Make one of the rcu_dereferences an ACQUIRE and the behaviour is
> forbidden.
> 
> Paul: what use-cases did you have in mind?
> 
> Will

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