cgroup: make css_sets pin the associated cgroups

Currently, css_sets don't pin the associated cgroups.  This is okay as
a cgroup with css_sets associated are not allowed to be removed;
however, to help resource tracking for zombie tasks, this is scheduled
to change such that a cgroup can be removed even when it has css_sets
associated as long as none of them are populated.

To ensure that a cgroup doesn't go away while css_sets are still
associated with it, make each associated css_set hold a reference on
the cgroup if non-root.

v2: Root cgroups are special and shouldn't be ref'd by css_sets.

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

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index ab5c9a5a059627e2be100becf7c351e200e082da..ea87340af87158d7f31de61aab8d50e8fca556c9 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -705,6 +705,8 @@ static void put_css_set_locked(struct css_set *cset)
        list_for_each_entry_safe(link, tmp_link, &cset->cgrp_links, cgrp_link) {
                list_del(&link->cset_link);
                list_del(&link->cgrp_link);
+               if (cgroup_parent(link->cgrp))
+                       cgroup_put(link->cgrp);
                kfree(link);
        }
 
@@ -919,6 +921,9 @@ static void link_css_set(struct list_head *tmp_links, struct css_set *cset,
         * is sorted by order of hierarchy creation
         */
        list_add_tail(&link->cgrp_link, &cset->cgrp_links);
+
+       if (cgroup_parent(cgrp))
+               cgroup_get(cgrp);
 }
 
 /**
@@ -4998,10 +5003,7 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 
        lockdep_assert_held(&cgroup_mutex);
 
-       /*
-        * css_set_rwsem synchronizes access to ->cset_links and prevents
-        * @cgrp from being removed while put_css_set() is in progress.
-        */
+       /* css_set_rwsem synchronizes access to ->cset_links */
        down_read(&css_set_rwsem);
        empty = list_empty(&cgrp->cset_links);
        up_read(&css_set_rwsem);