Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Peter Zijlstra <peterz@infradead.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	Will Deacon <will.deacon@arm.com>,
	Akira Yokosawa <akiyks@gmail.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Daniel Lustig <dlustig@nvidia.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	Nick Piggin <npiggin@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
Date: Tue, 17 Jul 2018 12:52:42 -0700	[thread overview]
Message-ID: <20180717195242.GV12945@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180717194001.GA3781@andrea>

On Tue, Jul 17, 2018 at 09:40:01PM +0200, Andrea Parri wrote:
> > > That said, I don't understand the powerpc memory ordering. I thought
> > > the rules were "isync on lock, lwsync on unlock".
> > > 
> > > That's what the AIX docs imply, at least.
> > > 
> > > In particular, I find:
> > > 
> > >   "isync is not a memory barrier instruction, but the
> > > load-compare-conditional branch-isync sequence can provide this
> > > ordering property"
> > > 
> > > so why are you doing "sync/lwsync", when it sounds like "isync/lwsync"
> > > (for lock/unlock) is the right thing and would already give memory
> > > barrier semantics?
> > 
> > The PowerPC guys will correct me if I miss something here...
> 
> [Same here.]
> 
> > The isync provides ordering roughly similar to lwsync, but nowhere near
> > as strong as sync, and it is sync that would be needed to cause lock
> > acquisition to provide full ordering.
> 
> IIRC, ctrl+isync is even *weaker* than lwsync in certain respects, e.g.,
> the former doesn't provide A-cumulativity according to the architectural
> intent.
> 
> 
> >The reason for using lwsync instead
> > of isync is that the former proved to be faster on recent hardware.
> 
> Interesting; can you add some references about this?

Sadly, no.  I just asked why all the isyncs were being rewritten by
lwsyncs some years back, and that is the answer I got.  I trust the
people answering, so didn't dig further.

							Thanx, Paul

>   Andrea
> 
> 
> > The reason that the kernel still has the ability to instead generate
> > isync instructions is that some older PowerPC hardware does not provide
> > the lwsync instruction.  If the hardware does support lwsync, the isync
> > instructions are overwritten with lwsync at boot time.
> > 
> > 							Thanx, Paul
> > 
> 