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

From: Will Deacon <will.deacon@arm.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
	Andrea Parri <andrea.parri@amarulasolutions.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>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire and by locks
Date: Wed, 4 Jul 2018 12:53:31 +0100	[thread overview]
Message-ID: <20180704115331.GA26941@arm.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1806221458480.1454-100000@iolanthe.rowland.org>

Hi Alan,

On Fri, Jun 22, 2018 at 03:11:37PM -0400, Alan Stern wrote:
> On Fri, 22 Jun 2018, Will Deacon wrote:
> > On Fri, Jun 22, 2018 at 02:09:04PM -0400, Alan Stern wrote:
> > > On Fri, 22 Jun 2018, Will Deacon wrote:
> > > > Could we drop the acquire/release stuff from the patch and limit this change
> > > > to locking instead?
> > > 
> > > The LKMM uses the same CAT code for acquire/release and lock/unlock.  
> > > (In essence, it considers a lock to be an acquire and an unlock to be a
> > > release; everything else follows from that.)  Treating one differently
> > > from the other in these tests would require some significant changes.
> > > It wouldn't be easy.
> > 
> > It would be boring if it was easy ;) I think this is a case of the tail
> > wagging the dog.
> > 
> > Paul -- please can you drop this patch until we've resolved this discussion?
> 
> Agreed.  It sounds like we'll need two versions of the Rel and Acq sets
> in the memory model; one for RCpc and one for RCsc.  smp_load_acquire
> and smp_store_release will use the former, and locking will use the
> latter.
> 
> Would it suffice to have this duplication just for release, using a
> single version of acquire?  What would happen on ARMv8 or RISC-V if an
> RCsc release was read by an RCpc acquire?  Or vice versa?

On Arm, RCsc release can forward to an RCpc acquire. We don't have an RCpc
release instruction.

Will