[12/13] workqueue: kill off ACCESS_ONCE()

Message ID 1507573730-8083-13-git-send-email-mark.rutland@arm.com
State New
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Series
  • Preparatory work to kill off ACCESS_ONCE()
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Commit Message

Mark Rutland Oct. 9, 2017, 6:28 p.m.
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>

Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
---
 kernel/workqueue.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

-- 
1.9.1

Comments

Tejun Heo Oct. 10, 2017, 2:06 p.m. | #1
On Mon, Oct 09, 2017 at 07:28:49PM +0100, Mark Rutland wrote:
> 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.

...
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>

> Cc: Lai Jiangshan <jiangshanlai@gmail.com>

> Cc: Tejun Heo <tj@kernel.org>


Acked-by: Tejun Heo <tj@kernel.org>


If you want me to route it through the workqueue tree, please let me
know.

Thanks.

-- 
tejun
Mark Rutland Oct. 10, 2017, 4:21 p.m. | #2
On Tue, Oct 10, 2017 at 07:06:29AM -0700, Tejun Heo wrote:
> On Mon, Oct 09, 2017 at 07:28:49PM +0100, Mark Rutland wrote:

> > 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.

> ...

> > Signed-off-by: Mark Rutland <mark.rutland@arm.com>

> > Cc: Lai Jiangshan <jiangshanlai@gmail.com>

> > Cc: Tejun Heo <tj@kernel.org>

> 

> Acked-by: Tejun Heo <tj@kernel.org>


Thanks!

> If you want me to route it through the workqueue tree, please let me

> know.


I'm not sure what the plan is for merging just yet, but I will let you
know as soon as that's figured out.

Thanks,
Mark.

Patch

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);