cgroup: fix top cgroup refcnt leak

As mount() and kill_sb() is not a one-to-one match, If we mount the same
cgroupfs in serveral mount points, and then umount all of them, kill_sb()
will be called only once.

Try:
        # mount -t cgroup -o cpuacct xxx /cgroup
        # mount -t cgroup -o cpuacct xxx /cgroup2
        # cat /proc/cgroups | grep cpuacct
        cpuacct 2       1       1
        # umount /cgroup
        # umount /cgroup2
        # cat /proc/cgroups | grep cpuacct
        cpuacct 2       1       1

You'll see cgroupfs will never be freed.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index fede3d3f28ff..0dfc7324c789 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1487,6 +1487,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
         struct cgroup_sb_opts opts;
         struct dentry *dentry;
         int ret;
+        bool new_sb;
 
         /*
          * The first time anyone tries to mount a cgroup, enable the list
@@ -1603,8 +1604,8 @@ out_unlock:
         if (ret)
                 return ERR_PTR(ret);
 
-        dentry = kernfs_mount(fs_type, flags, root->kf_root, NULL);
-        if (IS_ERR(dentry))
+        dentry = kernfs_mount(fs_type, flags, root->kf_root, &new_sb);
+        if (IS_ERR(dentry) || !new_sb)
                 cgroup_put(&root->cgrp);
         return dentry;
 }