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

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Message-ID: <20180713110851.GY2494@hirez.programming.kicks-ass.net>
Date:   Fri, 13 Jul 2018 13:08:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Alan Stern <stern@...land.harvard.edu>,
        andrea.parri@...rulasolutions.com,
        Will Deacon <will.deacon@....com>,
        Akira Yokosawa <akiyks@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nick Piggin <npiggin@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and
 remove it for ordinary release/acquire

On Thu, Jul 12, 2018 at 11:10:58AM -0700, Linus Torvalds wrote:
> On Thu, Jul 12, 2018 at 11:05 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > The locking pattern is fairly simple and shows where RCpc comes apart
> > from expectation real nice.
> 
> So who does RCpc right now for the unlock-lock sequence? Somebody
> mentioned powerpc. Anybody else?

RISC-V followed, because the LKMM currently states it is allowed, in
fact LKMM is currently weaker than even PowerPC, which is what this
current discussion is about.

A number of people, including myself, are arguing for stronger lock
ordering (RCsc) but getting the LKMM to be at least as strong as Power
(RCtsc as coined by Daniel) which disallows full RCpc.

> How nasty would be be to make powerpc conform? I will always advocate
> tighter locking and ordering rules over looser ones..

mpe did a micro-bench a little while ago:

  http://lkml.iu.edu/hypermail/linux/kernel/1804.0/01990.html

which says 30% more expensive for uncontended lock+unlock. Which I admit
is fairly yuck. No macro bench results though.