linux-kernel - [tip:locking/core] locking/atomics, mm: Convert ACCESS_ONCE() to READ_ONCE()/WRITE

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Message-ID: <tip-b03a0fe0c5e4b46dcd400d27395b124499554a71@git.kernel.org>
Date:   Wed, 25 Oct 2017 03:21:30 -0700
From:   "tip-bot for Paul E. McKenney" <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     peterz@...radead.org, torvalds@...ux-foundation.org,
        tglx@...utronix.de, hpa@...or.com, mark.rutland@....com,
        will.deacon@....com, paulmck@...ux.vnet.ibm.com,
        linux-kernel@...r.kernel.org, mingo@...nel.org
Subject: [tip:locking/core] locking/atomics, mm: Convert ACCESS_ONCE() to
 READ_ONCE()/WRITE_ONCE()

Commit-ID:  b03a0fe0c5e4b46dcd400d27395b124499554a71
Gitweb:     https://git.kernel.org/tip/b03a0fe0c5e4b46dcd400d27395b124499554a71
Author:     Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
AuthorDate: Mon, 23 Oct 2017 14:07:25 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 25 Oct 2017 11:01:06 +0200

locking/atomics, mm: Convert ACCESS_ONCE() to READ_ONCE()/WRITE_ONCE()

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.

It's possible to transform the bulk of kernel code using the Coccinelle
script below. However, this doesn't handle comments, leaving references
to ACCESS_ONCE() instances which have been removed. As a preparatory
step, this patch converts the mm code and comments to use
{READ,WRITE}_ONCE() consistently.

----
virtual patch

@ depends on patch @
expression E1, E2;
@@

- ACCESS_ONCE(E1) = E2
+ WRITE_ONCE(E1, E2)

@ depends on patch @
expression E;
@@

- ACCESS_ONCE(E)
+ READ_ONCE(E)
----

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Acked-by: Will Deacon <will.deacon@....com>
Acked-by: Mark Rutland <mark.rutland@....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
Link: http://lkml.kernel.org/r/1508792849-3115-15-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 mm/memory.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index a728bed..cae514e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3891,9 +3891,9 @@ static int handle_pte_fault(struct vm_fault *vmf)
 		/*
 		 * some architectures can have larger ptes than wordsize,
 		 * e.g.ppc44x-defconfig has CONFIG_PTE_64BIT=y and
-		 * CONFIG_32BIT=y, so READ_ONCE or ACCESS_ONCE cannot guarantee
-		 * atomic accesses.  The code below just needs a consistent
-		 * view for the ifs and we later double check anyway with the
+		 * CONFIG_32BIT=y, so READ_ONCE cannot guarantee atomic
+		 * accesses.  The code below just needs a consistent view
+		 * for the ifs and we later double check anyway with the
 		 * ptl lock held. So here a barrier will do.
 		 */
 		barrier();