[PATCH v2 14/14] cgroup: implement css_tryget()

Tejun Heo
2014-05-16T16:07:59Z

>From 141fdfadf18c10fed33624bbd414fcc0cb74d7ca Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Fri, 16 May 2014 11:40:38 -0400

Implement css_tryget() which tries to grab a cgroup_subsys_state's
reference as long as it already hasn't reached zero. Combined with
the recent css iterator changes to include offline && !released csses
during traversal, this can be used to access csses regardless of its
online state.

v2: Take the new flag CSS_NO_REF into account.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
---
include/linux/cgroup.h | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b769999..4afe544 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -113,6 +113,24 @@ static inline void css_get(struct cgroup_subsys_state *css)
}

/**
+ * css_tryget - try to obtain a reference on the specified css
+ * @css: target css
+ *
+ * Obtain a reference on @css unless it already has reached zero and is
+ * being released. This function doesn't care whether @css is on or
+ * offline. The caller naturally needs to ensure that @css is accessible
+ * but doesn't have to be holding a reference on it - IOW, RCU protected
+ * access is good enough for this function. Returns %true if a reference
+ * count was successfully obtained; %false otherwise.
+ */
+static inline bool css_tryget(struct cgroup_subsys_state *css)
+{
+ if (!(css->flags & CSS_NO_REF))
+ return percpu_ref_tryget(&css->refcnt);
+ return true;
+}
+
+/**
* css_tryget_online - try to obtain a reference on the specified css if online
* @css: target css
*
--
1.9.0