path: root/tools
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authorAlan Stern <stern@rowland.harvard.edu>2019-06-20 11:55:58 -0400
committerPaul E. McKenney <paulmck@linux.ibm.com>2019-06-24 09:08:54 -0700
commit4289ee7d5a8343eaddd0986f8fb492868e2f546f (patch)
treef05b3d5b42f7038696f971a718de56f661cde3a5 /tools
parent15aa25cbf0ccc4bd63ed6f2a8065decb7f5e6f89 (diff)
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 https://github.com/paulmckrcu/litmus/blob/master/manual/plain/strong-vis.litmus Reported-by: Herbert Xu <herbert@gondor.apana.org.au> Signed-off-by: Alan Stern <stern@rowland.harvard.edu> Acked-by: Andrea Parri <andrea.parri@amarulasolutions.com> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com> Tested-by: Akira Yokosawa <akiyks@gmail.com>
Diffstat (limited to 'tools')
1 files changed, 5 insertions, 3 deletions
diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index ca2f4297b4e6..ea2ff4b94074 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -179,9 +179,11 @@ let r-post-bounded = (nonrw-fence | ([~Noreturn] ; fencerel(Rmb) ; [R4rmb]))? ;
(* Visibility and executes-before for plain accesses *)
-let ww-vis = w-post-bounded ; vis ; w-pre-bounded
-let wr-vis = w-post-bounded ; vis ; r-pre-bounded
-let rw-xbstar = r-post-bounded ; xbstar ; w-pre-bounded
+let ww-vis = fence | (strong-fence ; xbstar ; w-pre-bounded) |
+ (w-post-bounded ; vis ; w-pre-bounded)
+let wr-vis = fence | (strong-fence ; xbstar ; r-pre-bounded) |
+ (w-post-bounded ; vis ; r-pre-bounded)
+let rw-xbstar = fence | (r-post-bounded ; xbstar ; w-pre-bounded)
(* Potential races *)
let pre-race = ext & ((Plain * M) | ((M \ IW) * Plain))