[PATCH 2/3 cgroup/for-3.19] cgroup: add cgroup_subsys->css_e_css_changed()

[Tejun Heo]

Add a new cgroup_subsys operatoin ->css_e_css_changed().  This is
invoked if any of the effective csses seen from the css's cgroup may
have changed.  This will be used to implement cgroup writeback
support.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Li Zefan <lizefan@xxxxxxxxxx>
---
 include/linux/cgroup.h |    1 +
 kernel/cgroup.c        |   18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -615,6 +615,7 @@ struct cgroup_subsys {
 	void (*css_released)(struct cgroup_subsys_state *css);
 	void (*css_free)(struct cgroup_subsys_state *css);
 	void (*css_reset)(struct cgroup_subsys_state *css);
+	void (*css_e_css_changed)(struct cgroup_subsys_state *css);
 
 	int (*can_attach)(struct cgroup_subsys_state *css,
 			  struct cgroup_taskset *tset);
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2836,6 +2836,24 @@ static ssize_t cgroup_subtree_control_wr
 		}
 	}
 
+	/*
+	 * The effective csses of all the descendants (excluding @cgrp) may
+	 * have changed.  Subsystems can optionally subscribe to this event
+	 * by implementing ->css_e_css_changed() which is invoked if any of
+	 * the effective csses seen from the css's cgroup may have changed.
+	 */
+	for_each_subsys(ss, ssid) {
+		struct cgroup_subsys_state *this_css = cgroup_css(cgrp, ss);
+		struct cgroup_subsys_state *css;
+
+		if (!ss->css_e_css_changed || !this_css)
+			continue;
+
+		css_for_each_descendant_pre(css, this_css)
+			if (css != this_css)
+				ss->css_e_css_changed(css);
+	}
+
 	kernfs_activate(cgrp->kn);
 	ret = 0;
 out_unlock:
--
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