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

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Date:   Wed, 25 Oct 2017 03:17:43 -0700
From:   tip-bot for Mark Rutland <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     tglx@...utronix.de, sakari.ailus@...ux.intel.com,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, mark.rutland@....com,
        paulmck@...ux.vnet.ibm.com, hpa@...or.com, peterz@...radead.org,
        mchehab@...nel.org
Subject: [tip:locking/core] locking/atomics, media/dvb_ringbuffer: Convert
 ACCESS_ONCE() to READ_ONCE()/WRITE_ONCE()

Commit-ID:  24fbd6e04e4e4793ec9dd9c2e007b0b38d2ec3ab
Gitweb:     https://git.kernel.org/tip/24fbd6e04e4e4793ec9dd9c2e007b0b38d2ec3ab
Author:     Mark Rutland <mark.rutland@....com>
AuthorDate: Mon, 23 Oct 2017 14:07:16 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 25 Oct 2017 11:00:58 +0200

locking/atomics, media/dvb_ringbuffer: 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 DVB ringbuffer 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: Mark Rutland <mark.rutland@....com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Sakari Ailus <sakari.ailus@...ux.intel.com>
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/1508792849-3115-6-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 drivers/media/dvb-core/dvb_ringbuffer.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/media/dvb-core/dvb_ringbuffer.c b/drivers/media/dvb-core/dvb_ringbuffer.c
index 2322af1..5301162 100644
--- a/drivers/media/dvb-core/dvb_ringbuffer.c
+++ b/drivers/media/dvb-core/dvb_ringbuffer.c
@@ -66,12 +66,12 @@ ssize_t dvb_ringbuffer_free(struct dvb_ringbuffer *rbuf)
 {
 	ssize_t free;
 
-	/* ACCESS_ONCE() to load read pointer on writer side
+	/* READ_ONCE() to load read pointer on writer side
 	 * this pairs with smp_store_release() in dvb_ringbuffer_read(),
 	 * dvb_ringbuffer_read_user(), dvb_ringbuffer_flush(),
 	 * or dvb_ringbuffer_reset()
 	 */
-	free = ACCESS_ONCE(rbuf->pread) - rbuf->pwrite;
+	free = READ_ONCE(rbuf->pread) - rbuf->pwrite;
 	if (free <= 0)
 		free += rbuf->size;
 	return free-1;
@@ -143,7 +143,7 @@ ssize_t dvb_ringbuffer_read_user(struct dvb_ringbuffer *rbuf, u8 __user *buf, si
 		todo -= split;
 		/* smp_store_release() for read pointer update to ensure
 		 * that buf is not overwritten until read is complete,
-		 * this pairs with ACCESS_ONCE() in dvb_ringbuffer_free()
+		 * this pairs with READ_ONCE() in dvb_ringbuffer_free()
 		 */
 		smp_store_release(&rbuf->pread, 0);
 	}
@@ -168,7 +168,7 @@ void dvb_ringbuffer_read(struct dvb_ringbuffer *rbuf, u8 *buf, size_t len)
 		todo -= split;
 		/* smp_store_release() for read pointer update to ensure
 		 * that buf is not overwritten until read is complete,
-		 * this pairs with ACCESS_ONCE() in dvb_ringbuffer_free()
+		 * this pairs with READ_ONCE() in dvb_ringbuffer_free()
 		 */
 		smp_store_release(&rbuf->pread, 0);
 	}