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;
Subject: Re: [PATCH -next 0/4] blk-cgroup: synchronize del_gendisk() with
 configuring cgroup policy
To:     Tejun Heo <tj@kernel.org>, Yu Kuai <yukuai1@huaweicloud.com>
Cc:     hch@infradead.org, josef@toxicpanda.com, axboe@kernel.dk,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, yi.zhang@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20221217030908.1261787-1-yukuai1@huaweicloud.com>
 <Y6DP3aOSad8+D1yY@slm.duckdns.org>
 <e01daffe-a3e3-8bf2-40ee-192a9e70d911@huaweicloud.com>
 <Y6HcWiJbaWjN3jlt@slm.duckdns.org>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <c2fecf04-6e9e-25a1-f14c-47862837cc54@huaweicloud.com>
Date:   Wed, 21 Dec 2022 09:10:33 +0800
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <Y6HcWiJbaWjN3jlt@slm.duckdns.org>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

Hi,

在 2022/12/21 0:01, Tejun Heo 写道:
> Hello,
> 
> On Tue, Dec 20, 2022 at 05:19:12PM +0800, Yu Kuai wrote:
>> Yes, that sounds good. BTW, queue_lock is also used to protect
>> pd_alloc_fn/pd_init_fn，and we found that blkcg_activate_policy() is
>> problematic:
>>
>> blkcg_activate_policy
>>   spin_lock_irq(&q->queue_lock);
>>   list_for_each_entry_reverse(blkg, &q->blkg_list
>>    pd_alloc_fn(GFP_NOWAIT | __GFP_NOWARN,...) -> failed
>>
>>    spin_unlock_irq(&q->queue_lock);
>>    // release queue_lock here is problematic, this will cause
>> pd_offline_fn called without pd_init_fn.
>>    pd_alloc_fn(__GFP_NOWARN,...)
> 
> So, if a blkg is destroyed while a policy is being activated, right?
Yes, remove cgroup can race with this, for bfq null pointer deference
will be triggered in bfq_pd_offline().

> 
>> If we are using a mutex to protect rq_qos ops, it seems the right thing
>> to do do also using the mutex to protect blkcg_policy ops, and this
>> problem can be fixed because mutex can be held to alloc memroy with
>> GFP_KERNEL. What do you think?
> 
> One worry is that switching to mutex can be more headache due to destroy
> path synchronization. Another approach would be using a per-blkg flag to
> track whether a blkg has been initialized.
I think perhaps you mean per blkg_policy_data flag? per blkg flag should
not work in this case.

Thanks,
Kuai
> 
> Thanks.
>