linux-kernel - [PATCH percpu/for-3.11 1/2] percpu-refcount: add __must_check to percpu_ref_init() and don't use ACCESS_ONCE() in percpu_ref_kill

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Message-ID: <20130613035201.GD9451@mtj.dyndns.org>
Date:	Wed, 12 Jun 2013 20:52:01 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>,
	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Oleg Nesterov <oleg@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: [PATCH percpu/for-3.11 1/2] percpu-refcount: add __must_check to
 percpu_ref_init() and don't use ACCESS_ONCE() in percpu_ref_kill_rcu()

Two small changes.

* Unlike most init functions, percpu_ref_init() allocates memory and
  may fail.  Let's mark it with __must_check in case the caller
  forgets.

* percpu_ref_kill_rcu() is unnecessarily using ACCESS_ONCE() to
  dereference @ref->pcpu_count, which can be misleading.  The pointer
  is guaranteed to be valid and visible and can't change underneath
  the function.  Drop ACCESS_ONCE().

Signed-off-by: Tejun Heo <tj@...nel.org>
---
 include/linux/percpu-refcount.h |    3 ++-
 lib/percpu-refcount.c           |    4 +---
 2 files changed, 3 insertions(+), 4 deletions(-)

--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -67,7 +67,8 @@ struct percpu_ref {
 	struct rcu_head		rcu;
 };
 
-int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release);
+int __must_check percpu_ref_init(struct percpu_ref *ref,
+				 percpu_ref_func_t *release);
 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 				 percpu_ref_func_t *confirm_kill);
 
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -57,12 +57,10 @@ int percpu_ref_init(struct percpu_ref *r
 static void percpu_ref_kill_rcu(struct rcu_head *rcu)
 {
 	struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
-	unsigned __percpu *pcpu_count;
+	unsigned __percpu *pcpu_count = ref->pcpu_count;
 	unsigned count = 0;
 	int cpu;
 
-	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
-
 	/* Mask out PCPU_REF_DEAD */
 	pcpu_count = (unsigned __percpu *)
 		(((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
--
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