cgroup: make cgroup_write_event_control() use css_from_dir() instead of __d_cgrp()

cgroup_event will be moved to its only user - memcg.  Replace
__d_cgrp() usage with css_from_dir(), which is already exported.  This
also simplifies the code a bit.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 12237a2..e76698d 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4041,7 +4041,7 @@ static int cgroup_write_event_control(struct cgroup_subsys_state *dummy_css,
 {
        struct cgroup *cgrp = dummy_css->cgroup;
        struct cgroup_event *event;
-       struct cgroup *cgrp_cfile;
+       struct cgroup_subsys_state *cfile_css;
        unsigned int efd, cfd;
        struct file *efile;
        struct file *cfile;
@@ -4103,7 +4103,8 @@ static int cgroup_write_event_control(struct cgroup_subsys_state *dummy_css,
        }
 
        /*
-        * Determine the css of @cfile and associate @event with it.
+        * Determine the css of @cfile, verify it belongs to the same
+        * cgroup as cgroup.event_control, and associate @event with it.
         * Remaining events are automatically removed on cgroup destruction
         * but the removal is asynchronous, so take an extra ref.
         */
@@ -4111,23 +4112,14 @@ static int cgroup_write_event_control(struct cgroup_subsys_state *dummy_css,
 
        ret = -EINVAL;
        event->css = cgroup_css(cgrp, event->cft->ss);
-       if (event->css && css_tryget(event->css))
+       cfile_css = css_from_dir(cfile->f_dentry->d_parent, event->cft->ss);
+       if (event->css && event->css == cfile_css && css_tryget(event->css))
                ret = 0;
 
        rcu_read_unlock();
        if (ret)
                goto out_put_cfile;
 
-       /*
-        * The file to be monitored must be in the same cgroup as
-        * cgroup.event_control is.
-        */
-       cgrp_cfile = __d_cgrp(cfile->f_dentry->d_parent);
-       if (cgrp_cfile != cgrp) {
-               ret = -EINVAL;
-               goto out_put_css;
-       }
-
        if (!event->cft->register_event || !event->cft->unregister_event) {
                ret = -EINVAL;
                goto out_put_css;