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

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Date:   Thu, 12 Jul 2018 21:52:42 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Alan Stern <stern@...land.harvard.edu>,
        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>
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?

powerpc have RCtso (and RCpc) but not RCsc unlock-lock, according to the
following indeed original terminology:

 - RCsc unlock-lock MUST ORDER:

  a) the WRITE and the READ below:

      WRITE x=1
      UNLOCK s
      LOCK s
      READ y

      as in a store-buffering test;

  b) the two WRITEs below:

      WRITE x=1
      UNLOCK s
      LOCK s
      WRITE y=1

      as in a message-passing test;

  c) the two READs below:

      READ x
      UNLOCK s
      LOCK s
      READ y

      as in a message-passing test;

  d) the READ and the WRITE below:

      READ x
      UNLOCK s
      LOCK s
      WRITE y

      as in a load-buffering test;

 - RCtso unlock-lock MUST ORDER b), c), d) above.

 - RCpc unlock-lock MUST ORDER none of the above.

AFAICT, all arch _in_ the current implementation have RCtso unlock-lock.


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

A simple answer is right above (place a sync somewhere in the sequence);
for benchmark results, I must defer...

  Andrea


> 
>            Linus