cgroup: factor out kill_css()

Factor out css ref killing from cgroup_destroy_locked() into
kill_css().  We're gonna add more to the path and the factored out
function will eventually be called from other places too.

While at it, replace open coded percpu_ref_get() with css_get() for
consistency.  This shouldn't cause any functional difference as the
function is not used for root cgroups.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3c4c4b0..7b7575f 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4596,6 +4596,36 @@ static void css_killed_ref_fn(struct percpu_ref *ref)
 }
 
 /**
+ * kill_css - destroy a css
+ * @css: css to destroy
+ *
+ * This function initiates destruction of @css by putting its base
+ * reference.  ->css_offline() will be invoked asynchronously once
+ * css_tryget() is guaranteed to fail and when the reference count reaches
+ * zero, @css will be released.
+ */
+static void kill_css(struct cgroup_subsys_state *css)
+{
+       /*
+        * Killing would put the base ref, but we need to keep it alive
+        * until after ->css_offline().
+        */
+       css_get(css);
+
+       /*
+        * cgroup core guarantees that, by the time ->css_offline() is
+        * invoked, no new css reference will be given out via
+        * css_tryget().  We can't simply call percpu_ref_kill() and
+        * proceed to offlining css's because percpu_ref_kill() doesn't
+        * guarantee that the ref is seen as killed on all CPUs on return.
+        *
+        * Use percpu_ref_kill_and_confirm() to get notifications as each
+        * css is confirmed to be seen as killed on all CPUs.
+        */
+       percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn);
+}
+
+/**
  * cgroup_destroy_locked - the first stage of cgroup destruction
  * @cgrp: cgroup to be destroyed
  *
@@ -4641,30 +4671,12 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
                return -EBUSY;
 
        /*
-        * Block new css_tryget() by killing css refcnts.  cgroup core
-        * guarantees that, by the time ->css_offline() is invoked, no new
-        * css reference will be given out via css_tryget().  We can't
-        * simply call percpu_ref_kill() and proceed to offlining css's
-        * because percpu_ref_kill() doesn't guarantee that the ref is seen
-        * as killed on all CPUs on return.
-        *
-        * Use percpu_ref_kill_and_confirm() to get notifications as each
-        * css is confirmed to be seen as killed on all CPUs.
-        * cgroup_destroy_css_killed() will be invoked to perform the rest
-        * of destruction once the percpu refs of all css's are confirmed
-        * to be killed.
+        * Initiate massacre of all css's.  cgroup_destroy_css_killed()
+        * will be invoked to perform the rest of destruction once the
+        * percpu refs of all css's are confirmed to be killed.
         */
-       for_each_root_subsys(cgrp->root, ss) {
-               struct cgroup_subsys_state *css = cgroup_css(cgrp, ss->subsys_id);
-
-               /*
-                * Killing would put the base ref, but we need to keep it
-                * alive until after ->css_offline.
-                */
-               percpu_ref_get(&css->refcnt);
-
-               percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn);
-       }
+       for_each_root_subsys(cgrp->root, ss)
+               kill_css(cgroup_css(cgrp, ss->subsys_id));
 
        /*
         * Mark @cgrp dead.  This prevents further task migration and child