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SubjectRe: [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@gmail.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@rowland.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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