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
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4596,6 +4596,36 @@
}
/**
+ * 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 @@
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