linux-kernel - [tip:locking/core] locking/atomics, rcutorture/formal: Prepare for ACCESS

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Date:   Wed, 25 Oct 2017 03:20:40 -0700
From:   tip-bot for Mark Rutland <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     tglx@...utronix.de, paulmck@...ux.vnet.ibm.com,
        torvalds@...ux-foundation.org, mark.rutland@....com, hpa@...or.com,
        mingo@...nel.org, peterz@...radead.org,
        linux-kernel@...r.kernel.org
Subject: [tip:locking/core] locking/atomics, rcutorture/formal: Prepare for
 ACCESS_ONCE() removal

Commit-ID:  5cd38016d0c88e3f67528c1a7380a6d5c90859e9
Gitweb:     https://git.kernel.org/tip/5cd38016d0c88e3f67528c1a7380a6d5c90859e9
Author:     Mark Rutland <mark.rutland@....com>
AuthorDate: Mon, 23 Oct 2017 14:07:23 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 25 Oct 2017 11:01:04 +0200

locking/atomics, 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 definitions of {READ,WRITE}_ONCE() in the rcutorture formal
tests, and removes the unused ACCESS_ONCE() helper. There should be no
functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@....com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: davem@...emloft.net
Cc: linux-arch@...r.kernel.org
Cc: mpe@...erman.id.au
Cc: shuah@...nel.org
Cc: snitzer@...hat.com
Cc: thor.thayer@...ux.intel.com
Cc: tj@...nel.org
Cc: viro@...iv.linux.org.uk
Cc: will.deacon@....com
Link: http://lkml.kernel.org/r/1508792849-3115-13-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h | 5 ++---
 1 file changed, 2 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..cc27b9e 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 +34,7 @@
 #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 READ_ONCE(x) (*(volatile typeof(x) *) &(x))
+#define WRITE_ONCE(x) ((*(volatile typeof(x) *) &(x)) = (val))

 #endif