linux-kernel - Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE

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Date:   Fri, 19 Jan 2018 14:24:11 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mhocko@...nel.org, akpm@...ux-foundation.org,
        pbonzini@...hat.com, liliang.opensource@...il.com,
        yang.zhang.wz@...il.com, quan.xu0@...il.com, nilal@...hat.com,
        riel@...hat.com
Subject: Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

On 01/18/2018 12:44 AM, Michael S. Tsirkin wrote:
> On Wed, Jan 17, 2018 at 01:10:11PM +0800, Wei Wang wrote:
>>   
>> +static void virtballoon_changed(struct virtio_device *vdev)
>> +{
>> +	struct virtio_balloon *vb = vdev->priv;
>> +	unsigned long flags;
>> +	__u32 cmd_id;
>> +	s64 diff = towards_target(vb);
>> +
>> +	if (diff) {
>> +		spin_lock_irqsave(&vb->stop_update_lock, flags);
>> +		if (!vb->stop_update)
> Why do you ignore stop_update for freeze?
> This means new wq entries can be added during remove
> causing use after free issues.

I think stop_update isn't needed, because the lock has already been 
handled internally by the APIs. Similar examples like 
mem_cgroup_css_free() in "mm/memcontrol.c", there is no such locks used 
for cancel_work_sync(&memcg->high_work).

Best,
Wei