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Subject: Re: [PATCH v3] virtio_net: avoid using netif_tx_disable() for
 serializing tx routine
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <276ac5bb-90f7-5fb6-a826-0bb05ecfa069@redhat.com>
 <20181017104419.13003-1-ake@igel.co.jp>
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From:   ake <ake@igel.co.jp>
Message-ID: <abd5887e-e332-190c-0a4c-d729320cae76@igel.co.jp>
Date:   Thu, 18 Oct 2018 12:25:14 +0900
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On 2018/10/18 0:09, Michael S. Tsirkin wrote:
> On Wed, Oct 17, 2018 at 07:44:12PM +0900, Ake Koomsin wrote:
>> Commit 713a98d90c5e ("virtio-net: serialize tx routine during reset")
>> introduces netif_tx_disable() after netif_device_detach() in order to
>> avoid use-after-free of tx queues. However, there are two issues.
>>
>> 1) Its operation is redundant with netif_device_detach() in case the
>>    interface is running.
>> 2) In case of the interface is not running before suspending and
>>    resuming, the tx does not get resumed by netif_device_attach().
>>    This results in losing network connectivity.
>>
>> It is better to use netif_tx_lock_bh()/netif_tx_unlock_bh() instead for
>> serializing tx routine during reset. This also preserves the symmetry
>> of netif_device_detach() and netif_device_attach().
>>
>> Fixes commit 713a98d90c5e ("virtio-net: serialize tx routine during reset")
>> Signed-off-by: Ake Koomsin <ake@igel.co.jp>
> 
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> 
> Thanks a lot for debugging!
> Seems like stable material to me, right?

Yes. With this patch, we can avoid network connectivity lost
because of tx not get re-enabled under some situation. Plus, it avoids
redundant operation between netif_device_detach() and
netif_tx_disable().

I tested the patch on Linux net-next and QEMU master by suspending/
resuming the virtual machine repeatedly. The network looks no problem
and has no connectivity lost so far. I tested with both user-mode
networking and tap interface.

Best Regards

>> ---
>>  drivers/net/virtio_net.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 3f5aa59c37b7..3e2c041d76ac 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -2267,8 +2267,9 @@ static void virtnet_freeze_down(struct virtio_device *vdev)
>>  	/* Make sure no work handler is accessing the device */
>>  	flush_work(&vi->config_work);
>>  
>> +	netif_tx_lock_bh(vi->dev);
>>  	netif_device_detach(vi->dev);
>> -	netif_tx_disable(vi->dev);
>> +	netif_tx_unlock_bh(vi->dev);
>>  	cancel_delayed_work_sync(&vi->refill);
>>  
>>  	if (netif_running(vi->dev)) {
>> @@ -2304,7 +2305,9 @@ static int virtnet_restore_up(struct virtio_device *vdev)
>>  		}
>>  	}
>>  
>> +	netif_tx_lock_bh(vi->dev);
>>  	netif_device_attach(vi->dev);
>> +	netif_tx_unlock_bh(vi->dev);
>>  	return err;
>>  }
>>  
>> -- 
>> 2.19.1