[Devel] [PATCH RHEL7 COMMIT] ms/percpu-refcount: Don't use silly cmpxchg()

[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 337bb797aa4aa5eca030d634d0a9874290511db5
Author: Vladimir Davydov <vdavydov at parallels.com>
Date:   Fri Aug 28 14:49:21 2015 +0400

    ms/percpu-refcount: Don't use silly cmpxchg()
    
    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: Kent Overstreet <koverstreet at google.com>
    
    The cmpxchg() was just to ensure the debug check didn't race, which was
    a bit excessive. The caller is supposed to do the appropriate
    synchronization, which means percpu_ref_kill() can just do a simple
    store.
    
    Signed-off-by: Kent Overstreet <koverstreet at google.com>
    Signed-off-by: Tejun Heo <tj at kernel.org>
    (cherry picked from commit c1ae6e9b4db00023b9caed72af49a93abad46452)
    Signed-off-by: Vladimir Davydov <vdavydov at parallels.com>
---
 lib/percpu-refcount.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 6f0ffd7..1a17399 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -107,22 +107,11 @@ static void percpu_ref_kill_rcu(struct rcu_head *rcu)
  */
 void percpu_ref_kill(struct percpu_ref *ref)
 {
-	unsigned __percpu *pcpu_count, *old, *new;
+	WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,
+		  "percpu_ref_kill() called more than once!\n");
 
-	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
-
-	do {
-		if (REF_STATUS(pcpu_count) == PCPU_REF_DEAD) {
-			WARN(1, "percpu_ref_kill() called more than once!\n");
-			return;
-		}
-
-		old = pcpu_count;
-		new = (unsigned __percpu *)
-			(((unsigned long) pcpu_count)|PCPU_REF_DEAD);
-
-		pcpu_count = cmpxchg(&ref->pcpu_count, old, new);
-	} while (pcpu_count != old);
+	ref->pcpu_count = (unsigned __percpu *)
+		(((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);
 
 	call_rcu(&ref->rcu, percpu_ref_kill_rcu);
 }