From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: [PATCH] Revert "cgroup: remove redundant variable in cgroup_mount()" Date: Fri, 26 Sep 2014 05:53:52 +0100 Message-ID: <20140926045352.GN7996@ZenIV.linux.org.uk> References: <1411704205-28995-1-git-send-email-lizefan@huawei.com> <20140926042617.GA14426@htj.dyndns.org> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20140926042617.GA14426-Gd/HAXX7CRxy/B6EtB590w@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Tejun Heo Cc: Zefan Li , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org On Fri, Sep 26, 2014 at 12:26:17AM -0400, Tejun Heo wrote: > On Fri, Sep 26, 2014 at 12:03:25PM +0800, Zefan Li wrote: > > 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 > > Reported-by: Andrey Wagin > > Signed-off-by: Zefan Li > > Applied to cgroup/for-3.18. Er? It's a clear regression since 3.16; shouldn't it go into mainline before 3.17-final? Another thing: AFAICS, if kernfs_fill_super() fails, you get unbalanced kernfs_put() on the cleanup path - kernfs_kill_sb() will be called in those cases as well. IOW, we'd better move that kernfs_get(info->root->kn); in kernfs_fill_super() all way up to the point before kernfs_get_inode(). Something like this: Signed-off-by: Al Viro --- diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c index f973ae9..e5e92e3 100644 --- a/fs/kernfs/mount.c +++ b/fs/kernfs/mount.c @@ -74,6 +74,7 @@ static int kernfs_fill_super(struct super_block *sb, unsigned long magic) sb->s_magic = magic; sb->s_op = &kernfs_sops; sb->s_time_gran = 1; + kernfs_get(info->root->kn); /* get root inode, initialize and unlock it */ mutex_lock(&kernfs_mutex); @@ -90,7 +91,6 @@ static int kernfs_fill_super(struct super_block *sb, unsigned long magic) pr_debug("%s: could not get root dentry!\n", __func__); return -ENOMEM; } - kernfs_get(info->root->kn); root->d_fsdata = info->root->kn; sb->s_root = root; sb->s_d_op = &kernfs_dops;