From: Alan Stern stern@rowland.harvard.edu
[ Upstream commit daebf24a8e8c6064cba3a330db9fe9376a137d2c ]
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@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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 ea2ff4b94074..2a9b4fe4a84e 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