Messages in this thread | | | Date | Mon, 24 Jun 2019 09:18:11 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 3/3] tools: memory-model: Improve data-race detection |
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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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