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Date: Mon, 1 Feb 2016 13:56:22 +0000
From: Will Deacon <will.deacon@arm.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        "Maciej W. Rozycki" <macro@imgtec.com>,
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        Ralf Baechle <ralf@linux-mips.org>, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org, boqun.feng@gmail.com
Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock()
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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)

> o	smp_store_release() -> lockless_dereference() (???)
> o	rcu_assign_pointer() -> rcu_dereference()
> o	smp_store_release() -> READ_ONCE(); if

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)
> 
> Seem reasonable, or am I missing some?

Looks alright to me.

Will