[Devel] [PATCH RHEL7 COMMIT] ms/percpu-refcount: add __must_check to percpu_ref_init() and don't use ACCESS_ONCE() in percpu_ref_kill_rcu()

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-229.7.2-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-229.7.2.vz7.6.3
------>
commit 82f6802b3f09878172024c57ed12cf2da92cccd3
Author: Vladimir Davydov <vdavydov at parallels.com>
Date:   Fri Aug 28 14:49:23 2015 +0400

    ms/percpu-refcount: add __must_check to percpu_ref_init() and don't use ACCESS_ONCE() in percpu_ref_kill_rcu()
    
    Patchset description:
    
    Pulling upstream patches converting css refcnt to percpu_ref.
    
    https://jira.sw.ru/browse/PSBM-34174
    
    Kent Overstreet (2):
      percpu: implement generic percpu refcounting
      percpu-refcount: Don't use silly cmpxchg()
    
    Tejun Heo (9):
      percpu-refcount: consistently use plain (non-sched) RCU
      percpu-refcount: cosmetic updates
      percpu-refcount: add __must_check to percpu_ref_init() and don't use
        ACCESS_ONCE() in percpu_ref_kill_rcu()
      percpu-refcount: implement percpu_ref_cancel_init()
      percpu-refcount: implement percpu_tryget() along with
        percpu_ref_kill_and_confirm()
      percpu-refcount: use RCU-sched insted of normal RCU
      cgroup: reorder the operations in cgroup_destroy_locked()
      cgroup: split cgroup destruction into two steps
      cgroup: use percpu refcnt for cgroup_subsys_states
    
    ===
    This patch description:
    
    From: Tejun Heo <tj at kernel.org>
    
    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 at kernel.org>
    (cherry picked from commit acac7883ee7bcc32476963bce7baf73d44574dd1)
    Signed-off-by: Vladimir Davydov <vdavydov at parallels.com>
---
 include/linux/percpu-refcount.h | 3 ++-
 lib/percpu-refcount.c           | 4 +---
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index b61bd6f..8146aa9 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -66,7 +66,8 @@ 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(struct percpu_ref *ref);
 
 #define PCPU_STATUS_BITS	2
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 9a78e55..b35eaac 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -57,12 +57,10 @@ int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release)
 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);