[PATCH tip/core/rcu 01/32] tools/memory-model: Fix data race detection for unordered store and load

From: paulmck@kernel.org
To: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	mingo@kernel.org
Cc: stern@rowland.harvard.edu, parri.andrea@gmail.com,
	will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com,
	npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk,
	luc.maranget@inria.fr, akiyks@gmail.com,
	"Paul E . McKenney" <paulmck@linux.ibm.com>
Subject: [PATCH tip/core/rcu 01/32] tools/memory-model: Fix data race detection for unordered store and load
Date: Wed,  2 Oct 2019 17:26:19 -0700	[thread overview]
Message-ID: <20191003002650.11249-1-paulmck@kernel.org> (raw)
In-Reply-To: <20191003001039.GA8027@paulmck-ThinkPad-P72>

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

