On 2014/6/25 23:00, Tejun Heo wrote:
> Hey,
>
> On Wed, Jun 25, 2014 at 09:56:31AM +0800, Li Zefan wrote:
>>> Hmmm? Why does that matter? The only region in cgroup_mount() which
>>> needs to be put inside such mutex would be root lookup, no?
>>
>> unfortunately that won't help. I think what you suggest is:
>>
>> cgroup_mount()
>> {
>> mutex_lock();
>> lookup_cgroup_root();
>> mutex_unlock();
>> kernfs_mount();
>> }
>>
>> cgroup_kill_sb()
>> {
>> mutex_lock();
>> percpu_ref_kill();
>> mutex_Unlock();
>> kernfs_kill_sb();
>> }
>>
>> See, we may still be destroying the superblock after we've succeeded
>> in getting the refcnt of cgroup root.
>
> Sure, but now the decision to kill is synchronized so the other side
> can interlock with it. e.g.
>
> cgroup_mount()
> {
> mutex_lock();
> lookup_cgroup_root();
> if (root isn't killed yet)
> root->this_better_stay_alive++;
> mutex_unlock();
> kernfs_mount();
> }
>
> cgroup_kill_sb()
> {
> mutex_lock();
> if (check whether root can be killed)
> percpu_ref_kill();
> mutex_unlock();
> if (the above condition was true)
> kernfs_kill_sb();
> }
>
This looks nasty, and I don't think it's correct. If we skip the call
to kernfs_kill_sb(), kernfs_super_info won't be freed but super_block
will, so we will end up either leaking memory or accessing invalid
memory. There are other problems like returning with sb->s_umount still
held.
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