Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire and by locks

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Andrea Parri <andrea.parri@amarulasolutions.com>,
	Will Deacon <will.deacon@arm.com>,
	LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	Nicholas Piggin <npiggin@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Kernel development list <linux-kernel@vger.kernel.org>,
	dlustig@nvidia.com
Subject: Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire and by locks
Date: Fri, 6 Jul 2018 14:10:55 -0700	[thread overview]
Message-ID: <20180706211055.GN3593@linux.vnet.ibm.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1807061629280.1396-100000@iolanthe.rowland.org>

On Fri, Jul 06, 2018 at 04:37:21PM -0400, Alan Stern wrote:
> On Thu, 5 Jul 2018, Andrea Parri wrote:
> 
> > > At any rate, it looks like instead of strengthening the relation, I
> > > should write a patch that removes it entirely.  I also will add new,
> > > stronger relations for use with locking, essentially making spin_lock
> > > and spin_unlock be RCsc.
> > 
> > Thank you.
> > 
> > Ah let me put this forward: please keep an eye on the (generic)
> > 
> >   queued_spin_lock()
> >   queued_spin_unlock()
> > 
> > (just to point out an example). Their implementation (in part.,
> > the fast-path) suggests that if we will stick to RCsc lock then
> > we should also stick to RCsc acq. load from RMW and rel. store.
> 
> A very good point.  The implementation of those routines uses
> atomic_cmpxchg_acquire() to acquire the lock.  Unless this is
> implemented with an operation or fence that provides write-write
> ordering (in conjunction with a suitable release), qspinlocks won't
> have the ordering properties that we want.
> 
> I'm going to assume that the release operations used for unlocking
> don't need to have any extra properties; only the lock-acquire
> operations need to be special (i.e., stronger than a normal
> smp_load_acquire). This suggests that atomic RMW functions with acquire
> semantics should also use this stronger form of acquire.
> 
> Does anybody have a different suggestion?

The approach you suggest makes sense to me.  Will, Peter, Daniel, any
reasons why this approach would be a problem for you guys?

							Thanx, Paul