Re: [PATCH] cgroup: superblock can't be released with active dentries

[Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

2012/5/24 23:41, Tejun Heo wrote:

>>From 88787c483106c5830a46d18deaffdc1e70929af7 Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Date: Thu, 24 May 2012 08:24:39 -0700
> 
> 48ddbe1946 "cgroup: make css->refcnt clearing on cgroup removal
> optional" allowed a css to linger after the associated cgroup is
> removed.  As a css holds a reference on the cgroup's dentry, it means
> that cgroup dentries may linger for a while.
> 
> cgroup_create() does grab an active reference on the superblock to
> prevent it from going away while there are !root cgroups; however, the
> reference is put from cgroup_diput() which is invoked on cgroup
> removal, so cgroup dentries which are removed but persisting due to
> lingering csses already have released their superblock active refs
> allowing superblock to be killed while those dentries are around.
> 
> Given the right condition, this makes cgroup_kill_sb() call
> kill_litter_super() with dentries with non-zero d_count leading to
> BUG() in shrink_dcache_for_umount_subtree().
> 
> Fix it by adding cgroup_dops->d_release() operation and moving
> deactivate_super() to it.  cgroup_diput() now marks dentry->d_fsdata
> with itself if superblock should be deactivated and cgroup_d_release()
> deactivates the superblock on dentry release.
> 
> Signed-off-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Reported-by: Sasha Levin <levinsasha928-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Tested-by: Sasha Levin <levinsasha928-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> LKML-Reference: <CA+1xoqe5hMuxzCRhMy7J0XchDk2ZnuxOHJKikROk1-ReAzcT6g-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>


Acked-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>