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

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Date:	Mon, 1 Feb 2016 19:54:58 -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 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:
> > > On Thu, Jan 28, 2016 at 02:31:31PM -0800, Paul E. McKenney wrote:
> > 
> > [ . . . ]
> > 
> > > > For Linux in general, this is a question: How strict do we want to be
> > > > about matching the type of write with the corresponding read?  My
> > > > default approach is to initially be quite strict and loosen as needed.
> > > > Here "quite strict" might mean requiring an rcu_assign_pointer() for
> > > > the write and rcu_dereference() for the read, as opposed to (say)
> > > > ACCESS_ONCE() for the read.  (I am guessing that this would be too
> > > > tight, but it makes a good example.)
> > > > 
> > > > Thoughts?
> > > 
> > > That sounds broadly sensible to me and allows rcu_assign_pointer and
> > > rcu_dereference to be used as drop-in replacements for release/acquire
> > > where local transitivity isn't required. However, I don't think we can
> > > rule out READ_ONCE/WRITE_ONCE interactions as they seem to be used
> > > already in things like the osq_lock (albeit without the address
> > > dependency).
> > 
> > Agreed.  So in the most strict case that I can imagine anyone putting
> > up with, we have the following pairings:
> 
> I think we can group these up:
> 
> Locally transitive:
> 
> > o	smp_store_release() -> smp_load_acquire() (locally transitive)
> 
> 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

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.

> Globally transitive:
> 
> > o	smp_mb(); WRITE_ONCE() -> READ_ONCE(); (globally transitive)
> > o	synchronize_rcu(); WRITE_ONCE() -> READ_ONCE(); (globally transitive)
> 
> RCU:
> 
> > o	synchronize_rcu(); WRITE_ONCE() -> rcu_read_lock(); READ_ONCE()
> > 		(strange and wonderful properties)

Agreed.

> > Seem reasonable, or am I missing some?
> 
> Looks alright to me.

So I have some litmus tests to generate.  ;-)

							Thnax, Paul