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>

1 files changed, 5 insertions, 13 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 12237a291d88..e76698dd6c08 100644
--- 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;