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

[Li Zefan <lizf@cn.fujitsu.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. 
> 

I think it's safe to call cgroup_call_pre_destroy() without cgroup_lock.
If cgroup_call_pre_destroy() gets called, it means the cgroup fs has sub-dirs,
so any remount/umount will fail, which means root->subsys_list won't be
changed during rmdir(), so using for_each_subsys() in cgroup_call_pre_destroy()
is safe.

> 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.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> ---
>  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;
>  	}
> 

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