Re: [PATCH 5/5] cgroup: fix a race between cgroup_mount() and cgroup_kill_sb()

[Li Zefan <lizefan@huawei.com>]

On 2014/6/21 3:35, Tejun Heo wrote:
> Hello, Li.
> 
> Sorry about the long delay.
> 
> On Thu, Jun 12, 2014 at 02:33:05PM +0800, Li Zefan wrote:
>> We've converted cgroup to kernfs so cgroup won't be intertwined with
>> vfs objects and locking, but there are dark areas.
>>
>> Run two instances of this script concurrently:
>>
>>     for ((; ;))
>>     {
>>     	mount -t cgroup -o cpuacct xxx /cgroup
>>     	umount /cgroup
>>     }
>>
>> After a while, I saw two mount processes were stuck at retrying, because
>> they were waiting for a subsystem to become free, but the root associated
>> with this subsystem never got freed.
>>
>> This can happen, if thread A is in the process of killing superblock but
>> hasn't called percpu_ref_kill(), and at this time thread B is mounting
>> the same cgroup root and finds the root in the root list and performs
>> percpu_ref_try_get().
>>
>> To fix this, we increase the refcnt of the superblock instead of increasing
>> the percpu refcnt of cgroup root.
> 
> Ah, right.  Gees, I'm really hating the fact that we have ->mount but
> not ->umount.  However, can't we make it a bit simpler by just
> introducing a mutex protecting looking up and refing up an existing
> root and a sb going away?  The only problem is that the refcnt being
> killed isn't atomic w.r.t. new live ref coming up, right?  Why not
> just add a mutex around them so that they can't race?
> 

Well, kill_sb() is called with sb->s_umount held, while kernfs_mount()
returned with sb->s_umount held, so adding a mutex will lead to ABBA
deadlock.