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

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Date:   Wed, 25 Oct 2017 03:20:16 -0700
From:   tip-bot for Mark Rutland <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     torvalds@...ux-foundation.org, peterz@...radead.org,
        mingo@...nel.org, tj@...nel.org, jiangshanlai@...il.com,
        hpa@...or.com, linux-kernel@...r.kernel.org,
        paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
        mark.rutland@....com
Subject: [tip:locking/core] locking/atomics, workqueue: Convert
 ACCESS_ONCE() to READ_ONCE()/WRITE_ONCE()

Commit-ID:  c95491ed6d6a958743d82bea1d053819988da418
Gitweb:     https://git.kernel.org/tip/c95491ed6d6a958743d82bea1d053819988da418
Author:     Mark Rutland <mark.rutland@....com>
AuthorDate: Mon, 23 Oct 2017 14:07:22 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 25 Oct 2017 11:01:03 +0200

locking/atomics, workqueue: 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 workqueue 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>
Acked-by: Tejun Heo <tj@...nel.org>
Cc: Lai Jiangshan <jiangshanlai@...il.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: viro@...iv.linux.org.uk
Cc: will.deacon@....com
Link: http://lkml.kernel.org/r/1508792849-3115-12-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/workqueue.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 64d0edf..39831b2 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -4647,7 +4647,7 @@ static void rebind_workers(struct worker_pool *pool)
 		 * concurrency management.  Note that when or whether
 		 * @worker clears REBOUND doesn't affect correctness.
 		 *
-		 * ACCESS_ONCE() is necessary because @worker->flags may be
+		 * WRITE_ONCE() is necessary because @worker->flags may be
 		 * tested without holding any lock in
 		 * wq_worker_waking_up().  Without it, NOT_RUNNING test may
 		 * fail incorrectly leading to premature concurrency
@@ -4656,7 +4656,7 @@ static void rebind_workers(struct worker_pool *pool)
 		WARN_ON_ONCE(!(worker_flags & WORKER_UNBOUND));
 		worker_flags |= WORKER_REBOUND;
 		worker_flags &= ~WORKER_UNBOUND;
-		ACCESS_ONCE(worker->flags) = worker_flags;
+		WRITE_ONCE(worker->flags, worker_flags);
 	}
 
 	spin_unlock_irq(&pool->lock);