linux-kernel - [PATCH 13/13] rcutorture: formal: prepare for ACCESS

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Message-Id: <1507573730-8083-14-git-send-email-mark.rutland@arm.com>
Date:   Mon,  9 Oct 2017 19:28:50 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     linux-kernel@...r.kernel.org
Cc:     Mark Rutland <mark.rutland@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH 13/13] rcutorture: formal: prepare for ACCESS_ONCE() removal

For several reasons, it is desirable to use {READ,WRITE}_ONCE() in
preference to ACCESS_ONCE(), and new code is expected to use one of the
former. So far, there's been no reason to change most existing uses of
ACCESS_ONCE(), as these aren't currently harmful.

However, for some features it is necessary to instrument reads and
writes separately, which is not possible with ACCESS_ONCE(). This
distinction is critical to correct operation.

The bulk of the kernel code can be transformed via Coccinelle to use
{READ,WRITE}_ONCE(), though this only modifies users of ACCESS_ONCE(),
and not the implementation itself. As such, it has the potential to
break homebrew ACCESS_ONCE() macros seen in some user code in the kernel
tree (e.g. the virtio code, as fixed in commit ea9156fb3b71d9f7).

To avoid fragility if/when that transformation occurs, this patch
reworks the rcutorture formal tests to use an intermediate
__ACCESS_ONCE() helper, which will avoid {READ,WRITE}_ONCE() being
turned into tautological definitions. There should be no functional
change as a result of this patch.

Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Mark Rutland <mark.rutland@....com>
---
 tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h b/tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h
index 6687acc..ee4e4f8 100644
--- a/tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h
+++ b/tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h
@@ -34,8 +34,9 @@
 #define rs_smp_mb() do {} while (0)
 #endif

-#define ACCESS_ONCE(x) (*(volatile typeof(x) *) &(x))
-#define READ_ONCE(x) ACCESS_ONCE(x)
-#define WRITE_ONCE(x, val) (ACCESS_ONCE(x) = (val))
+#define __ACCESS_ONCE(x) (*(volatile typeof(x) *) &(x))
+#define ACCESS_ONCE(x) __ACCESS_ONCE(x)
+#define READ_ONCE(x) __ACCESS_ONCE(x)
+#define WRITE_ONCE(x, val) (__ACCESS_ONCE(x) = (val))

 #endif
-- 
1.9.1