cgroups: make cgroup_path() RCU-safe

Fix races between /proc/sched_debug by freeing cgroup objects via an RCU
callback.  Thus any cgroup reference obtained from an RCU-safe source will
remain valid during the RCU section.  Since dentries are also RCU-safe,
this allows us to traverse up the tree safely.

Additionally, make cgroup_path() check for a NULL cgrp->dentry to avoid
trying to report a path for a partially-created cgroup.

[lizf@cn.fujitsu.com: call deactive_super() in cgroup_diput()]
Signed-off-by: Paul Menage <menage@google.com>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Tested-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index f68dfd8dd53a..73d1c730c3c4 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -116,7 +116,7 @@ struct cgroup {
         struct list_head children;      /* my children */
 
         struct cgroup *parent;  /* my parent */
-        struct dentry *dentry;          /* cgroup fs entry */
+        struct dentry *dentry;          /* cgroup fs entry, RCU protected */
 
         /* Private pointers for each registered subsystem */
         struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
@@ -145,6 +145,9 @@ struct cgroup {
         int pids_use_count;
         /* Length of the current tasks_pids array */
         int pids_length;
+
+        /* For RCU-protected deletion */
+        struct rcu_head rcu_head;
 };
 
 /* A css_set is a structure holding pointers to a set of