Date:   Mon, 24 Jun 2019 09:18:11 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <andrea.parri@...rulasolutions.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>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] tools: memory-model: Improve data-race detection

On Mon, Jun 24, 2019 at 11:39:23AM -0400, Alan Stern wrote:
> On Mon, 24 Jun 2019, Paul E. McKenney wrote:
> 
> > On Sun, Jun 23, 2019 at 09:34:55PM -0700, Paul E. McKenney wrote:
> > > On Sun, Jun 23, 2019 at 11:15:06AM -0400, Alan Stern wrote:
> > > > On Sun, 23 Jun 2019, Akira Yokosawa wrote:
> > > > 
> > > > > Hi Paul and Alan,
> > > > > 
> > > > > On 2019/06/22 8:54, Paul E. McKenney wrote:
> > > > > > On Fri, Jun 21, 2019 at 10:25:23AM -0400, Alan Stern wrote:
> > > > > >> On Fri, 21 Jun 2019, Andrea Parri wrote:
> > > > > >>
> > > > > >>> On Thu, Jun 20, 2019 at 11:55:58AM -0400, Alan Stern wrote:
> > > > > >>>> Herbert Xu recently reported a problem concerning RCU and compiler
> > > > > >>>> barriers.  In the course of discussing the problem, he put forth a
> > > > > >>>> litmus test which illustrated a serious defect in the Linux Kernel
> > > > > >>>> Memory Model's data-race-detection code.
> > > > > 
> > > > > I was not involved in the mail thread and wondering what the litmus test
> > > > > looked like. Some searching of the archive has suggested that Alan presented
> > > > > a properly formatted test based on Herbert's idea in [1].
> > > > > 
> > > > > [1]: https://lore.kernel.org/lkml/Pine.LNX.4.44L0.1906041026570.1731-100000@iolanthe.rowland.org/
> > > > 
> > > > Yes, that's it.  The test is also available at:
> > > > 
> > > > https://github.com/paulmckrcu/litmus/blob/master/manual/plain/C-S-rcunoderef-2.litmus
> > > > 
> > > > Alan
> > > > 
> > > > > If this is the case, adding the link (or message id) in the change
> > > > > log would help people see the circumstances, I suppose.
> > > > > Paul, can you amend the change log?
> > > > > 
> > > > > I ran herd7 on said litmus test at both "lkmm" and "dev" of -rcu and
> > > > > confirmed that this patch fixes the result.
> > > > > 
> > > > > So,
> > > > > 
> > > > > Tested-by: Akira Yokosawa <akiyks@...il.com>
> > > 
> > > Thank you both!  I will apply these changes tomorrow morning, Pacific Time.
> > 
> > And done.  Please see below for the updated commit.
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit 46a020e9464aff884df56e5fd483134c8801e39f
> > Author: Alan Stern <stern@...land.harvard.edu>
> > Date:   Thu Jun 20 11:55:58 2019 -0400
> > 
> >     tools/memory-model: Improve data-race detection
> >     
> >     Herbert Xu recently reported a problem concerning RCU and compiler
> >     barriers.  In the course of discussing the problem, he put forth a
> >     litmus test which illustrated a serious defect in the Linux Kernel
> >     Memory Model's data-race-detection code [1].
> >     
> >     The defect was that the LKMM assumed visibility and executes-before
> >     ordering of plain accesses had to be mediated by marked accesses.  In
> >     Herbert's litmus test this wasn't so, and the LKMM claimed the litmus
> >     test was allowed and contained a data race although neither is true.
> >     
> >     In fact, plain accesses can be ordered by fences even in the absence
> >     of marked accesses.  In most cases this doesn't matter, because most
> >     fences only order accesses within a single thread.  But the rcu-fence
> >     relation is different; it can order (and induce visibility between)
> >     accesses in different threads -- events which otherwise might be
> >     concurrent.  This makes it relevant to data-race detection.
> >     
> >     This patch makes two changes to the memory model to incorporate the
> >     new insight:
> >     
> >             If a store is separated by a fence from another access,
> >             the store is necessarily visible to the other access (as
> >             reflected in the ww-vis and wr-vis relations).  Similarly,
> >             if a load is separated by a fence from another access then
> >             the load necessarily executes before the other access (as
> >             reflected in the rw-xbstar relation).
> >     
> >             If a store is separated by a strong fence from a marked access
> >             then it is necessarily visible to any access that executes
> >             after the marked access (as reflected in the ww-vis and wr-vis
> >             relations).
> >     
> >     With these changes, the LKMM gives the desired result for Herbert's
> >     litmus test and other related ones [2].
> >     
> >     [1]     https://lore.kernel.org/lkml/Pine.LNX.4.44L0.1906041026570.1731-100000@iolanthe.rowland.org/
> >     
> >     [2]     https://github.com/paulmckrcu/litmus/blob/master/manual/plain/C-S-rcunoderef-1.litmus
> >             https://github.com/paulmckrcu/litmus/blob/master/manual/plain/C-S-rcunoderef-2.litmus
> >             https://github.com/paulmckrcu/litmus/blob/master/manual/plain/C-S-rcunoderef-3.litmus
> >             https://github.com/paulmckrcu/litmus/blob/master/manual/plain/C-S-rcunoderef-4.litmus
> 
> Please add:
> 
> https://github.com/paulmckrcu/litmus/blob/master/manual/plain/strong-vis.litmus

Done, and calling this version final.  Thank you all again!

							Thanx, Paul

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