[PATCH] Documentation: circular-buffers: use READ_ONCE()

From: Mark Rutland
Date: Wed Nov 16 2016 - 06:13:56 EST

While the {READ,WRITE}_ONCE() macros should be used in preference to
ACCESS_ONCE(), the circular buffer documentation uses the latter

To point people in the right direction, and as a step towards the
eventual removal of ACCESS_ONCE(), update the documentation to use
READ_ONCE(), as ACCESS_ONCE() is only used in a reader context in the
circular buffer documentation.

Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: linux-doc@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Documentation/circular-buffers.txt | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/circular-buffers.txt b/Documentation/circular-buffers.txt
index 88951b1..4a824d2 100644
--- a/Documentation/circular-buffers.txt
+++ b/Documentation/circular-buffers.txt
@@ -161,7 +161,7 @@ The producer will look something like this:

unsigned long head = buffer->head;
/* The spin_unlock() and next spin_lock() provide needed ordering. */
- unsigned long tail = ACCESS_ONCE(buffer->tail);
+ unsigned long tail = READ_ONCE(buffer->tail);

if (CIRC_SPACE(head, tail, buffer->size) >= 1) {
/* insert one item into the buffer */
@@ -222,7 +222,7 @@ This will instruct the CPU to make sure the index is up to date before reading
the new item, and then it shall make sure the CPU has finished reading the item
before it writes the new tail pointer, which will erase the item.

-Note the use of ACCESS_ONCE() and smp_load_acquire() to read the
+Note the use of READ_ONCE() and smp_load_acquire() to read the
opposition index. This prevents the compiler from discarding and
reloading its cached value - which some compilers will do across
smp_read_barrier_depends(). This isn't strictly needed if you can