Re: [PATCH] [BUGFIX]cgroup: fix potential deadlock in pre_destroy(v2)

[Balbir Singh]

KAMEZAWA Hiroyuki wrote:
> This is fixed one. Thank you for all help.
> 
> Regards,
> -Kame
> ==
> 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
> 		-> cpu_hotplug.lock. (lru_add_drain_all->
> 				      schedule_work->
>                                       get_online_cpus)
> 
> 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.)
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> 
> ---
>  kernel/cgroup.c |   14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> Index: mmotm-2.6.28-Nov10/kernel/cgroup.c
> ===================================================================
> --- mmotm-2.6.28-Nov10.orig/kernel/cgroup.c
> +++ mmotm-2.6.28-Nov10/kernel/cgroup.c
> @@ -2475,10 +2475,7 @@ static int cgroup_rmdir(struct inode *un
>  		mutex_unlock(&cgroup_mutex);
>  		return -EBUSY;
>  	}
> -
> -	parent = cgrp->parent;
> -	root = cgrp->root;
> -	sb = root->sb;
> +	mutex_unlock(&cgroup_mutex);
> 
>  	/*
>  	 * Call pre_destroy handlers of subsys. Notify subsystems
> @@ -2486,7 +2483,14 @@ static int cgroup_rmdir(struct inode *un
>  	 */
>  	cgroup_call_pre_destroy(cgrp);
> 
> -	if (cgroup_has_css_refs(cgrp)) {
> +	mutex_lock(&cgroup_mutex);
> +	parent = cgrp->parent;
> +	root = cgrp->root;
> +	sb = root->sb;
> +
> +	if (atomic_read(&cgrp->count)
> +	    || !list_empty(&cgrp->children)
> +	    || cgroup_has_css_refs(cgrp)) {
>  		mutex_unlock(&cgroup_mutex);
>  		return -EBUSY;
>  	}
> 

I think the last statement deserves a comment that after re-acquiring the lock,
we need to check if count, children or references changed. Otherwise looks good,
though I've not yet tested it.

Acked-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>

-- 
	Balbir
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