cgroup: replace css_set walking populated test with testing cgrp->nr_populated_csets

Implement trivial cgroup_has_tasks() which tests whether
cgrp->nr_populated_csets is zero and replace the explicit local
populated test in cgroup_subtree_control().  This simplifies the code
and cgroup_has_tasks() will be used in more places later.

Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 5fe2644..d5b6231 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -325,6 +325,11 @@ static struct cgroup *cgroup_parent(struct cgroup *cgrp)
        return NULL;
 }
 
+static bool cgroup_has_tasks(struct cgroup *cgrp)
+{
+       return cgrp->nr_populated_csets;
+}
+
 /* subsystems visibly enabled on a cgroup */
 static u16 cgroup_control(struct cgroup *cgrp)
 {
@@ -2971,28 +2976,9 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
         * Except for the root, subtree_control must be zero for a cgroup
         * with tasks so that child cgroups don't compete against tasks.
         */
-       if (enable && cgroup_parent(cgrp)) {
-               struct cgrp_cset_link *link;
-
-               /*
-                * Because namespaces pin csets too, @cgrp->cset_links
-                * might not be empty even when @cgrp is empty.  Walk and
-                * verify each cset.
-                */
-               spin_lock_irq(&css_set_lock);
-
-               ret = 0;
-               list_for_each_entry(link, &cgrp->cset_links, cset_link) {
-                       if (css_set_populated(link->cset)) {
-                               ret = -EBUSY;
-                               break;
-                       }
-               }
-
-               spin_unlock_irq(&css_set_lock);
-
-               if (ret)
-                       goto out_unlock;
+       if (enable && cgroup_parent(cgrp) && cgroup_has_tasks(cgrp)) {
+               ret = -EBUSY;
+               goto out_unlock;
        }
 
        /* save and update control masks and prepare csses */