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/[email protected]/ > > > > > > 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 <[email protected]> > > > > 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 <[email protected]> > 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/[email protected]/ > > [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
