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

From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will.deacon@arm.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	LKMM Maintainers -- 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>,
	Nicholas Piggin <npiggin@gmail.com>,
	Kernel development list <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
Date: Thu, 12 Jul 2018 19:52:28 +0200	[thread overview]
Message-ID: <20180712175228.GB3533@andrea> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1807121236470.1306-100000@iolanthe.rowland.org>

> It seems reasonable to ask people to learn that locks have stronger
> ordering guarantees than RMW atomics do.  Maybe not the greatest
> situation in the world, but one I think we could live with.

Yeah, this was one of my main objections.

> > Hence my proposal to strenghten rmw-acquire, because that is the basic
> > primitive used to implement lock.
> 
> That was essentially what the v2 patch did.  (And my reasoning was
> basically the same as what you have just outlined.  There was one
> additional element: smp_store_release() is already strong enough for
> TSO; the acquire is what needs to be stronger in the memory model.)

Mmh? see my comments to v2 (and your reply, in part., the part "At
least, it's not a valid general-purpose implementation".).

> > Another, and I like this proposal least, is to introduce a new barrier
> > to make this all work.
> 
> This apparently boils down to two questions:
> 
> 	Should spin_lock/spin_unlock be RCsc?
> 
> 	Should rmw-acquire be strong enough so that smp_store_release + 
> 	rmw-acquire is RCtso?
> 
> If both answers are No, we end up with the v3 patch.  If the first
> answer is No and the second is Yes, we end up with the v2 patch.  The
> problem is that different people seem to want differing answers.

Again, maybe you're confonding v2 with v1?

  Andrea

> 
> (The implicit third question, "Should spin_lock/spin_unlock be RCtso?",
> seems to be pretty well settled at this point -- by Peter's and Will's
> vociferousness if nothing else -- despite Andrea's reservations.  
> However I admit it would be nice to have one or two examples showing
> that the kernel really needs this.)
> 
> Alan
> 