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

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Date:   Wed, 25 Oct 2017 03:18:58 -0700
From:   tip-bot for Mark Rutland <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     peterz@...radead.org, linux-kernel@...r.kernel.org, hpa@...or.com,
        mark.rutland@....com, johannes@...solutions.net,
        torvalds@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
        mingo@...nel.org, johannes.berg@...el.com, tglx@...utronix.de,
        davem@...emloft.net
Subject: [tip:locking/core] locking/atomics, net/average: Convert
 ACCESS_ONCE() to READ_ONCE()/WRITE_ONCE()

Commit-ID:  ef4d9af62f47e3070b00c3307a4d8eb5092bb9a2
Gitweb:     https://git.kernel.org/tip/ef4d9af62f47e3070b00c3307a4d8eb5092bb9a2
Author:     Mark Rutland <mark.rutland@....com>
AuthorDate: Mon, 23 Oct 2017 14:07:19 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 25 Oct 2017 11:01:00 +0200

locking/atomics, net/average: 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 pick up some uses, including those
in <linux/average.h>. As a preparatory step, this patch converts the
file to use {READ,WRITE}_ONCE() consistently.

At the same time, this patch addds missing includes necessary for
{READ,WRITE}_ONCE(), *BUG_ON*(), and ilog2().

----
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: Mark Rutland <mark.rutland@....com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Reviewed-by: Johannes Berg <johannes@...solutions.net>
Cc: David S. Miller <davem@...emloft.net>
Cc: Johannes Berg <johannes.berg@...el.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
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/1508792849-3115-9-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 include/linux/average.h | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/linux/average.h b/include/linux/average.h
index 7ddaf34..3f46229 100644
--- a/include/linux/average.h
+++ b/include/linux/average.h
@@ -1,6 +1,10 @@
 #ifndef _LINUX_AVERAGE_H
 #define _LINUX_AVERAGE_H
 
+#include <linux/bug.h>
+#include <linux/compiler.h>
+#include <linux/log2.h>
+
 /*
  * Exponentially weighted moving average (EWMA)
  *
@@ -48,7 +52,7 @@
 	static inline void ewma_##name##_add(struct ewma_##name *e,	\
 					     unsigned long val)		\
 	{								\
-		unsigned long internal = ACCESS_ONCE(e->internal);	\
+		unsigned long internal = READ_ONCE(e->internal);	\
 		unsigned long weight_rcp = ilog2(_weight_rcp);		\
 		unsigned long precision = _precision;			\
 									\
@@ -57,10 +61,10 @@
 		BUILD_BUG_ON((_precision) > 30);			\
 		BUILD_BUG_ON_NOT_POWER_OF_2(_weight_rcp);		\
 									\
-		ACCESS_ONCE(e->internal) = internal ?			\
+		WRITE_ONCE(e->internal, internal ?			\
 			(((internal << weight_rcp) - internal) +	\
 				(val << precision)) >> weight_rcp :	\
-			(val << precision);				\
+			(val << precision));				\
 	}
 
 #endif /* _LINUX_AVERAGE_H */