cgroups: fix lock inconsistency in cgroup_clone()

I fixed a bug in cgroup_clone() in Linus' tree in commit 7b574b7
("cgroups: fix a race between cgroup_clone and umount") without noticing
there was a cleanup patch in -mm tree that should be rebased (now commit
104cbd5, "cgroups: use task_lock() for access tsk->cgroups safe in
cgroup_clone()"), thus resulted in lock inconsistency.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 6 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2ae7cb47dbfa..0066092de19a 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2993,20 +2993,21 @@ int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *subsys,
                 mutex_unlock(&cgroup_mutex);
                 return 0;
         }
-        task_lock(tsk);
-        cg = tsk->cgroups;
-        parent = task_cgroup(tsk, subsys->subsys_id);
 
         /* Pin the hierarchy */
-        if (!atomic_inc_not_zero(&parent->root->sb->s_active)) {
+        if (!atomic_inc_not_zero(&root->sb->s_active)) {
                 /* We race with the final deactivate_super() */
                 mutex_unlock(&cgroup_mutex);
                 return 0;
         }
 
         /* Keep the cgroup alive */
+        task_lock(tsk);
+        parent = task_cgroup(tsk, subsys->subsys_id);
+        cg = tsk->cgroups;
         get_css_set(cg);
         task_unlock(tsk);
+
         mutex_unlock(&cgroup_mutex);
 
         /* Now do the VFS work to create a cgroup */
@@ -3045,7 +3046,7 @@ int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *subsys,
                 mutex_unlock(&inode->i_mutex);
                 put_css_set(cg);
 
-                deactivate_super(parent->root->sb);
+                deactivate_super(root->sb);
                 /* The cgroup is still accessible in the VFS, but
                  * we're not going to try to rmdir() it at this
                  * point. */
@@ -3071,7 +3072,7 @@ int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *subsys,
         mutex_lock(&cgroup_mutex);
         put_css_set(cg);
         mutex_unlock(&cgroup_mutex);
-        deactivate_super(parent->root->sb);
+        deactivate_super(root->sb);
         return ret;
 }