Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Sender: Tejun Heo <htejun@gmail.com>
Date:   Mon, 12 Dec 2022 12:13:47 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Josef Bacik <josef@toxicpanda.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
        Yi Zhang <yi.zhang@redhat.com>
Subject: Re: [PATCH-block v2 1/3] bdi, blk-cgroup: Fix potential UAF of blkcg
Message-ID: <Y5enmzQM7BIiEv9n@slm.duckdns.org>
References: <20221211222058.2946830-1-longman@redhat.com>
 <20221211222058.2946830-2-longman@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20221211222058.2946830-2-longman@redhat.com>
Precedence: bulk

On Sun, Dec 11, 2022 at 05:20:56PM -0500, Waiman Long wrote:
>  static void blkcg_destroy_blkgs(struct blkcg *blkcg)
>  {
> +	/*
> +	 * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
> +	 * references gone.
> +	 */
> +	if (WARN_ON_ONCE(!css_tryget(&blkcg->css)))
> +		return;

Wouldn't it make more sense to use percpu_ref_is_zero()? It's not like the
obtained extra reference does anything, right?

Thanks.

-- 
tejun