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Subject[PATCH tip/core/rcu 01/32] tools/memory-model: Fix data race detection for unordered store and load
Date
From: Alan Stern <stern@rowland.harvard.edu>

Currently the Linux Kernel Memory Model gives an incorrect response
for the following litmus test:

C plain-WWC

{}

P0(int *x)
{
WRITE_ONCE(*x, 2);
}

P1(int *x, int *y)
{
int r1;
int r2;
int r3;

r1 = READ_ONCE(*x);
if (r1 == 2) {
smp_rmb();
r2 = *x;
}
smp_rmb();
r3 = READ_ONCE(*x);
WRITE_ONCE(*y, r3 - 1);
}

P2(int *x, int *y)
{
int r4;

r4 = READ_ONCE(*y);
if (r4 > 0)
WRITE_ONCE(*x, 1);
}

exists (x=2 /\ 1:r2=2 /\ 2:r4=1)

The memory model says that the plain read of *x in P1 races with the
WRITE_ONCE(*x) in P2.

The problem is that we have a write W and a read R related by neither
fre or rfe, but rather W ->coe W' ->rfe R, where W' is an intermediate
write (the WRITE_ONCE() in P0). In this situation there is no
particular ordering between W and R, so either a wr-vis link from W to
R or an rw-xbstar link from R to W would prove that the accesses
aren't concurrent.

But the LKMM only looks for a wr-vis link, which is equivalent to
assuming that W must execute before R. This is not necessarily true
on non-multicopy-atomic systems, as the WWC pattern demonstrates.

This patch changes the LKMM to accept either a wr-vis or a reverse
rw-xbstar link as a proof of non-concurrency.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
tools/memory-model/linux-kernel.cat | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index ea2ff4b..2a9b4fe 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -197,7 +197,7 @@ empty (wr-incoh | rw-incoh | ww-incoh) as plain-coherence
(* Actual races *)
let ww-nonrace = ww-vis & ((Marked * W) | rw-xbstar) & ((W * Marked) | wr-vis)
let ww-race = (pre-race & co) \ ww-nonrace
-let wr-race = (pre-race & (co? ; rf)) \ wr-vis
+let wr-race = (pre-race & (co? ; rf)) \ wr-vis \ rw-xbstar^-1
let rw-race = (pre-race & fr) \ rw-xbstar

flag ~empty (ww-race | wr-race | rw-race) as data-race
--
2.9.5
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