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. > > > > 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. > > > > Signed-off-by: Alan Stern <[email protected]> > > Reported-by: Herbert Xu <[email protected]> > > For the entire series: > > Acked-by: Andrea Parri <[email protected]> > > Two nits, but up to Paul AFAIAC: > > - This is a first time for "tools: memory-model:" in Subject; we were > kind of converging to "tools/memory-model:"...
Yeah, sure. That's the sort of detail I have a hard time remembering. > - The report preceded the patch; we might as well reflect this in the > order of the tags. Either way is okay with me. Alan
