Re: [PATCH] [BUGFIX]cgroup: fix potential deadlock in pre_destroy.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 12 Nov 2008 10:23:55 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> KAMEZAWA Hiroyuki wrote:
> > Balbir, Paul, Li, How about this ?
> > =
> > As Balbir pointed out, memcg's pre_destroy handler has potential deadlock.
> > 
> > It has following lock sequence.
> > 
> > 	cgroup_mutex (cgroup_rmdir)
> > 	    -> pre_destroy
> > 		-> mem_cgroup_pre_destroy
> > 			-> force_empty
> > 			   -> lru_add_drain_all->
> > 			      -> schedule_work_on_all_cpus
> >                                  -> get_online_cpus -> cpuhotplug.lock.
> > 
> > But, cpuset has following.
> > 	cpu_hotplug.lock (call notifier)
> > 		-> cgroup_mutex. (within notifier)
> > 
> > Then, this lock sequence should be fixed.
> > 
> > Considering how pre_destroy works, it's not necessary to holding
> > cgroup_mutex() while calling it. 
> > 
> > As side effect, we don't have to wait at this mutex while memcg's force_empty
> > works.(it can be long when there are tons of pages.)
> > 
> > Note: memcg is an only user of pre_destroy, now.
> > 
> 
> I thought about this and it seems promising. My concern is that with
> cgroup_mutex given, the state of cgroup within pre-destroy will be
> unpredictable. I suspect, if pre-destory really needs cgroup_mutex, we can hold
> it within pre-destroy.
> 
I agree.

> BTW, your last check, does not seem right
> 
> +	if (atomic_read(&cgrp->count)
> +	    || list_empty(&cgrp->children)
> 
> Why should list_empty() result in EBUSY, shouldn't it be !list_empty()?
> 
> +	    || cgroup_has_css_refs(cgrp)) {
>
Oh, my bad...

will fix soon.

Thanks,
-Kame