Subject: Re: [PATCH net-next v6 4/5] virtio-net: add spin lock for ctrl cmd access

From: Heng Qi <[email-masked]>
Date: Thu, 7 Dec 2023 12:47:09 +0800

在 2023/12/7 下午12:19, Jason Wang 写道:
> On Wed, Dec 6, 2023 at 9:03 PM Heng Qi <[email-masked]> wrote:
>>
>>
>> 在 2023/12/6 下午8:27, Paolo Abeni 写道:
>>> On Tue, 2023-12-05 at 19:05 +0800, Heng Qi wrote:
>>>> 在 2023/12/5 下午4:35, Jason Wang 写道:
>>>>> On Tue, Dec 5, 2023 at 4:02 PM Heng Qi <[email-masked]> wrote:
>>>>>> Currently access to ctrl cmd is globally protected via rtnl_lock and works
>>>>>> fine. But if dim work's access to ctrl cmd also holds rtnl_lock, deadlock
>>>>>> may occur due to cancel_work_sync for dim work.
>>>>> Can you explain why?
>>>> For example, during the bus unbind operation, the following call stack
>>>> occurs:
>>>> virtnet_remove -> unregister_netdev -> rtnl_lock[1] -> virtnet_close ->
>>>> cancel_work_sync -> virtnet_rx_dim_work -> rtnl_lock[2] (deadlock occurs).
>>>>
>>>>>> Therefore, treating
>>>>>> ctrl cmd as a separate protection object of the lock is the solution and
>>>>>> the basis for the next patch.
>>>>> Let's don't do that. Reasons are:
>>>>>
>>>>> 1) virtnet_send_command() may wait for cvq commands for an indefinite time
>>>> Yes, I took that into consideration. But ndo_set_rx_mode's need for an
>>>> atomic
>>>> environment rules out the mutex lock.
>>>>
>>>>> 2) hold locks may complicate the future hardening works around cvq
>>>> Agree, but I don't seem to have thought of a better way besides passing
>>>> the lock.
>>>> Do you have any other better ideas or suggestions?
>>> What about:
>>>
>>> - using the rtnl lock only
>>> - virtionet_close() invokes cancel_work(), without flushing the work
>>> - virtnet_remove() calls flush_work() after unregister_netdev(),
>>> outside the rtnl lock
>>>
>>> Should prevent both the deadlock and the UaF.
>>
>> Hi, Paolo and Jason!
>>
>> Thank you very much for your effective suggestions, but I found another
>> solution[1],
>> based on the ideas of rtnl_trylock and refill_work, which works very well:
>>
>> [1]
>> +static void virtnet_rx_dim_work(struct work_struct *work)
>> +{
>> +    struct dim *dim = container_of(work, struct dim, work);
>> +    struct receive_queue *rq = container_of(dim,
>> +            struct receive_queue, dim);
>> +    struct virtnet_info *vi = rq->vq->vdev->priv;
>> +    struct net_device *dev = vi->dev;
>> +    struct dim_cq_moder update_moder;
>> +    int i, qnum, err;
>> +
>> +    if (!rtnl_trylock())
>> +        return;
> Don't we need to reschedule here?
>
> like
>
> if (rq->dim_enabled)
>         sechedule_work()
>
> ?

I think no, we don't need this.

The work of each queue will be called by "net_dim()->schedule_work()"
when napi traffic changes (before schedule_work(), the dim->profile_ix
of the corresponding rxq has been updated).
So we only need to traverse and update the profiles of all rxqs in the
work which is obtaining the rtnl_lock.

Thanks!

>
> Thanks
>
>> +
>> +    for (i = 0; i < vi->curr_queue_pairs; i++) {
>> +        rq = &vi->rq[i];
>> +        dim = &rq->dim;
>> +        qnum = rq - vi->rq;
>> +
>> +        if (!rq->dim_enabled)
>> +            continue;
>> +
>> +        update_moder = net_dim_get_rx_moderation(dim->mode,
>> dim->profile_ix);
>> +        if (update_moder.usec != rq->intr_coal.max_usecs ||
>> +            update_moder.pkts != rq->intr_coal.max_packets) {
>> +            err = virtnet_send_rx_ctrl_coal_vq_cmd(vi, qnum,
>> +                                   update_moder.usec,
>> +                                   update_moder.pkts);
>> +            if (err)
>> +                pr_debug("%s: Failed to send dim parameters on rxq%d\n",
>> +                     dev->name, qnum);
>> +            dim->state = DIM_START_MEASURE;
>> +        }
>> +    }
>> +
>> +    rtnl_unlock();
>> +}
>>
>>
>> In addition, other optimizations[2] have been tried, but it may be due
>> to the sparsely
>> scheduled work that the retry condition is always satisfied, affecting
>> performance,
>> so [1] is the final solution:
>>
>> [2]
>>
>> +static void virtnet_rx_dim_work(struct work_struct *work)
>> +{
>> +    struct dim *dim = container_of(work, struct dim, work);
>> +    struct receive_queue *rq = container_of(dim,
>> +            struct receive_queue, dim);
>> +    struct virtnet_info *vi = rq->vq->vdev->priv;
>> +    struct net_device *dev = vi->dev;
>> +    struct dim_cq_moder update_moder;
>> +    int i, qnum, err, count;
>> +
>> +    if (!rtnl_trylock())
>> +        return;
>> +retry:
>> +    count = vi->curr_queue_pairs;
>> +    for (i = 0; i < vi->curr_queue_pairs; i++) {
>> +        rq = &vi->rq[i];
>> +        dim = &rq->dim;
>> +        qnum = rq - vi->rq;
>> +        update_moder = net_dim_get_rx_moderation(dim->mode,
>> dim->profile_ix);
>> +        if (update_moder.usec != rq->intr_coal.max_usecs ||
>> +            update_moder.pkts != rq->intr_coal.max_packets) {
>> +            --count;
>> +            err = virtnet_send_rx_ctrl_coal_vq_cmd(vi, qnum,
>> +                                   update_moder.usec,
>> +                                   update_moder.pkts);
>> +            if (err)
>> +                pr_debug("%s: Failed to send dim parameters on rxq%d\n",
>> +                     dev->name, qnum);
>> +            dim->state = DIM_START_MEASURE;
>> +        }
>> +    }
>> +
>> +    if (need_resched()) {
>> +        rtnl_unlock();
>> +        schedule();
>> +    }
>> +
>> +    if (count)
>> +        goto retry;
>> +
>> +    rtnl_unlock();
>> +}
>>
>> Thanks a lot!
>>
>>> Side note: for this specific case any functional test with a
>>> CONFIG_LOCKDEP enabled build should suffice to catch the deadlock
>>> scenario above.
>>>
>>> Cheers,
>>>
>>> Paolo