cgroup: add cgroup_subsys->css_e_css_changed()

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@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

2 files changed, 19 insertions, 0 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index e717a39f22ea..3a04aeb8b5a1 100644
--- 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);
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index c8558693102b..69f033582a1a 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2836,6 +2836,24 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
                 }
         }
 
+        /*
+         * 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: