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:"... - The report preceded the patch; we might as well reflect this in the order of the tags. Thanks, Andrea
