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-rw-r--r--include/linux/cgroup.h11
-rw-r--r--kernel/cgroup.c9
2 files changed, 15 insertions, 5 deletions
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 5db8138a0482..b0d5f53ae5e1 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -225,6 +225,9 @@ struct cgroup {
struct list_head pidlists;
struct mutex pidlist_mutex;
+ /* dummy css with NULL ->ss, points back to this cgroup */
+ struct cgroup_subsys_state dummy_css;
+
/* For css percpu_ref killing and RCU-protected deletion */
struct rcu_head rcu_head;
struct work_struct destroy_work;
@@ -668,7 +671,13 @@ struct cgroup_subsys_state *css_parent(struct cgroup_subsys_state *css)
{
struct cgroup *parent_cgrp = css->cgroup->parent;
- return parent_cgrp ? parent_cgrp->subsys[css->ss->subsys_id] : NULL;
+ if (!parent_cgrp)
+ return NULL;
+
+ if (css->ss)
+ return parent_cgrp->subsys[css->ss->subsys_id];
+ else
+ return &parent_cgrp->dummy_css;
}
/**
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 583f8f66a7e1..c049992f1ffa 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1365,6 +1365,7 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
INIT_LIST_HEAD(&cgrp->release_list);
INIT_LIST_HEAD(&cgrp->pidlists);
mutex_init(&cgrp->pidlist_mutex);
+ cgrp->dummy_css.cgroup = cgrp;
INIT_LIST_HEAD(&cgrp->event_list);
spin_lock_init(&cgrp->event_list_lock);
simple_xattrs_init(&cgrp->xattrs);
@@ -2285,7 +2286,7 @@ static struct cgroup_subsys_state *cgroup_file_css(struct cfent *cfe)
if (cft->ss)
return cgrp->subsys[cft->ss->subsys_id];
- return NULL;
+ return &cgrp->dummy_css;
}
/* A buffer size big enough for numbers or short strings */
@@ -2467,7 +2468,7 @@ static int cgroup_file_open(struct inode *inode, struct file *file)
* unpinned either on open failure or release. This ensures that
* @css stays alive for all file operations.
*/
- if (css && !css_tryget(css))
+ if (css->ss && !css_tryget(css))
return -ENODEV;
if (cft->read_map || cft->read_seq_string) {
@@ -2477,7 +2478,7 @@ static int cgroup_file_open(struct inode *inode, struct file *file)
err = cft->open(inode, file);
}
- if (css && err)
+ if (css->ss && err)
css_put(css);
return err;
}
@@ -2491,7 +2492,7 @@ static int cgroup_file_release(struct inode *inode, struct file *file)
if (cft->release)
ret = cft->release(inode, file);
- if (css)
+ if (css->ss)
css_put(css);
return ret;
}