[Devel] Re: [PATCH v5 3/3] cgroups: make procs file writable

[David Rientjes]

On Thu, 30 Dec 2010, Li Zefan wrote:

> > This needs to be done with cgroup_lock() instead of callback_mutex since 
> > the post_clone() callback will store to cs->mems_allowed on 
> > cgroup_clone().
> > 
> 
> Then cpuset_post_clone() breaks the lock rule:
> 
>  * A task must hold both mutexes to modify cpusets...
>  ...
>  * If a task is only holding callback_mutex, then it has read-only
>  * access to cpusets.
> 
> But that's Ok, because cgroup_clone() is called during the creation of
> the new cgroup, so no one can access the cpuset at that time.
> 

I'm saying that if cpusets implements a cgroup_clone() handler that the 
locking will break with only callback_mutex here because the only 
synchronization after the new cgroup dentry is added is cgroup_lock() that 
is always held when a post_clone callback is invoked and reading a mems 
file may race since it is accessible before the task is attached in the 
cgroup_clone() case.  It's not a problem right now but may subtly break if 
cpusets were to use cgroup_clone().
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