linux-kernel - Re: [PATCH] [BUGFIX]cgroup: fix potential deadlock in pre

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 12 Nov 2008 14:58:06 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"menage@...gle.com" <menage@...gle.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] [BUGFIX]cgroup: fix potential deadlock in pre_destroy.

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@...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;
>  	}
> 
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/