Revert "cgroup: remove redundant variable in cgroup_mount()"

This reverts commit 0c7bf3e8cab7900e17ce7f97104c39927d835469.

If there are child cgroups in the cgroupfs and then we umount it,
the superblock will be destroyed but the cgroup_root will be kept
around. When we mount it again, cgroup_mount() will find this
cgroup_root and allocate a new sb for it.

So with this commit we will be trapped in a dead loop in the case
described above, because kernfs_pin_sb() keeps returning NULL.

Currently I don't see how we can avoid using both pinned_sb and
new_sb, so just revert it.

Cc: Al Viro <viro@ZenIV.linux.org.uk>
Reported-by: Andrey Wagin <avagin@gmail.com>
Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 5eb20cd1709c56956ace0820699bef5735475a77..f873c468131648fa422bdaab9adb2faf8230297c 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1694,6 +1694,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
        struct dentry *dentry;
        int ret;
        int i;
+       bool new_sb;
 
        /*
         * The first time anyone tries to mount a cgroup, enable the list
@@ -1784,7 +1785,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                 * path is super cold.  Let's just sleep a bit and retry.
                 */
                pinned_sb = kernfs_pin_sb(root->kf_root, NULL);
-               if (IS_ERR_OR_NULL(pinned_sb) ||
+               if (IS_ERR(pinned_sb) ||
                    !percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {
                        mutex_unlock(&cgroup_mutex);
                        if (!IS_ERR_OR_NULL(pinned_sb))
@@ -1830,16 +1831,18 @@ out_free:
                return ERR_PTR(ret);
 
        dentry = kernfs_mount(fs_type, flags, root->kf_root,
-                               CGROUP_SUPER_MAGIC, NULL);
-       if (IS_ERR(dentry) || pinned_sb)
+                               CGROUP_SUPER_MAGIC, &new_sb);
+       if (IS_ERR(dentry) || !new_sb)
                cgroup_put(&root->cgrp);
 
        /*
         * If @pinned_sb, we're reusing an existing root and holding an
         * extra ref on its sb.  Mount is complete.  Put the extra ref.
         */
-       if (pinned_sb)
+       if (pinned_sb) {
+               WARN_ON(new_sb);
                deactivate_super(pinned_sb);
+       }
 
        return dentry;
 }