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@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: davem@davemloft.net
Cc: linux-arch@vger.kernel.org
Cc: mpe@ellerman.id.au
Cc: shuah@kernel.org
Cc: snitzer@redhat.com
Cc: thor.thayer@linux.intel.com
Cc: viro@zeniv.linux.org.uk
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1508792849-3115-12-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 64d0edf428f850f2e5cfed94970cb74491eb6b61..39831b2f3c5f8aa9ff0e7317d714d0fe1bae7e46 100644 (file)
--- 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);