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

[Balbir Singh <balbir@linux.vnet.ibm.com>]

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.

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)) {


-- 
	Balbir