Re: [PATCH][rc1] cgroup: catch bad css refcnt at css_put

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

KAMEZAWA Hiroyuki wrote:
> This is a patch for checking css->refcnt's sanity at css_put().
> 
> BTW, I noticed that...css->refcnt may overflow if used with memcg...
> Now, refcnt is incremented per a page. Paul, do you have any idea ?

atomic64_t ?

But for 4K pagesize, it won't overflow until when the referenced
memory is > 8T?

> (Ah, yes. "don't use css->refcnt per page" is maybe reasonable but
>  it will be big change..)
> 
> ==
> __css_put() doesn't check a bug as refcnt goes to minus.
> I think it should be caught. This patch adds a check for it.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Acked-by: Li Zefan <lizf@cn.fujitsu.com>

> ---
>  kernel/cgroup.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6.32-rc1/kernel/cgroup.c
> ===================================================================
> --- linux-2.6.32-rc1.orig/kernel/cgroup.c
> +++ linux-2.6.32-rc1/kernel/cgroup.c
> @@ -3708,8 +3708,10 @@ static void check_for_release(struct cgr
>  void __css_put(struct cgroup_subsys_state *css)
>  {
>  	struct cgroup *cgrp = css->cgroup;
> +	int val;
>  	rcu_read_lock();
> -	if (atomic_dec_return(&css->refcnt) == 1) {
> +	val = atomic_dec_return(&css->refcnt);
> +	if (val == 1) {
>  		if (notify_on_release(cgrp)) {
>  			set_bit(CGRP_RELEASABLE, &cgrp->flags);
>  			check_for_release(cgrp);
> @@ -3717,6 +3719,7 @@ void __css_put(struct cgroup_subsys_stat
>  		cgroup_wakeup_rmdir_waiter(cgrp);
>  	}
>  	rcu_read_unlock();
> +	WARN_ON(val < 1);

When we run into this, it'll probably fill up the syslog quickly,
so I think WARN_ON_ONCE() is a bit better.

>  }
>