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Date: Tue, 2 Feb 2016 20:12:30 +0800
From: Boqun Feng <boqun.feng@gmail.com>
To: Will Deacon <will.deacon@arm.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Maciej W. Rozycki" <macro@imgtec.com>,
        David Daney <ddaney@caviumnetworks.com>,
        =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= <mans@mansr.com>,
        Ralf Baechle <ralf@linux-mips.org>, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org
Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock()
Message-ID: <20160202121230.GE1239@fixme-laptop.cn.ibm.com>
References: <20160127152158.GJ2390@arm.com>
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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:
> > > >=20
> > > > (i.e. these can't be used to extend a chain)
> > >=20
> > > Agreed.
> > >=20
> > > > > o	smp_store_release() -> lockless_dereference() (???)
> > > > > o	rcu_assign_pointer() -> rcu_dereference()
> > > > > o	smp_store_release() -> READ_ONCE(); if
> >=20
> > Just want to make sure, this one is actually:
> >=20
> > o	smp_store_release() -> READ_ONCE(); if ;<WRITE_ONCE()>
> >=20
> > right? Because control dependency only orders READ->WRITE.
> >=20
> > If so, do we also need to take the following pairing into consideration?
> >=20
> > o	smp_store_release() -> READ_ONCE(); if ;smp_rmb(); <ACCESS_ONCE()>
> >=20
> > >=20
> > > 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.
> > >=20
> >=20
> > 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?
> >=20
> > 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?
>=20
> Well, the following ISA2 test is permitted on ARM:
>=20
>=20
> P0:
> Wx=3D1
> WyRel=3D1	// rcu_assign_pointer
>=20
> P1:
> Ry=3D1	// rcu_dereference

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

> WzRel=3D1 // rcu_assign_pointer
>=20
> P2:
> Rz=3D1	// rcu_dereference
> <addr>
> Rx=3D0
>=20
>=20
> Make one of the rcu_dereferences an ACQUIRE and the behaviour is
> forbidden.
>=20
> Paul: what use-cases did you have in mind?
>=20
> Will

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