cgroup: implement css_tryget()

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>

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b769999..4afe544 100644 (file)
--- 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
  *