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

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Date:	Tue, 2 Feb 2016 11:45:59 +0000
From:	Will Deacon <will.deacon@....com>
To:	Boqun Feng <boqun.feng@...il.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 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
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