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

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Date:   Thu, 31 Oct 2019 11:55:06 -0000
From:   "tip-bot2 for Alan Stern" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        linux-kernel@...r.kernel.org
Subject: [tip: core/rcu] tools/memory-model: Fix data race detection for
 unordered store and load

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     daebf24a8e8c6064cba3a330db9fe9376a137d2c
Gitweb:        https://git.kernel.org/tip/daebf24a8e8c6064cba3a330db9fe9376a137d2c
Author:        Alan Stern <stern@...land.harvard.edu>
AuthorDate:    Fri, 06 Sep 2019 16:57:22 -04:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Sat, 05 Oct 2019 11:58:14 -07:00

tools/memory-model: Fix data race detection for unordered store and load

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@...land.harvard.edu>
Acked-by: Andrea Parri <parri.andrea@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.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 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