lkml.org 
[lkml]   [2019]   [Jun]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 3/3] tools: memory-model: Improve data-race detection
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

Alan

\
 
 \ /
  Last update: 2019-06-24 17:39    [W:0.072 / U:4.672 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site