Re: [PATCH] virtio-net: disable delayed refill when setting up xdp

[Bui Quang Minh <minhquangbui99@gmail.com>]

On 4/3/25 14:24, Paolo Abeni wrote:
> On 4/2/25 7:42 AM, Bui Quang Minh wrote:
>> When setting up XDP for a running interface, we call napi_disable() on
>> the receive queue's napi. In delayed refill_work, it also calls
>> napi_disable() on the receive queue's napi. This can leads to deadlock
>> when napi_disable() is called on an already disabled napi. This commit
>> fixes this by disabling future and cancelling all inflight delayed
>> refill works before calling napi_disabled() in virtnet_xdp_set.
>>
>> Fixes: 4941d472bf95 ("virtio-net: do not reset during XDP set")
>> Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
>> ---
>>   drivers/net/virtio_net.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 7e4617216a4b..33406d59efe2 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -5956,6 +5956,15 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>>   	if (!prog && !old_prog)
>>   		return 0;
>>   
>> +	/*
>> +	 * Make sure refill_work does not run concurrently to
>> +	 * avoid napi_disable race which leads to deadlock.
>> +	 */
>> +	if (netif_running(dev)) {
>> +		disable_delayed_refill(vi);
>> +		cancel_delayed_work_sync(&vi->refill);
> AFAICS at this point refill_work() could still be running, why don't you
> need to call flush_delayed_work()?

AFAIK, the cancel_delayed_work_sync (this is a synchronous version) 
provides somewhat stronger guarantee than the flush_delayed_work. 
Internally, the cancel_delayed_work_sync will also call to __flush_work. 
The cancel_delayed_work_sync temporarily disables the work before 
calling __flush_work, so that even if refill_work tries to re-queue 
itself, that re-queue will fail. As the refill_work can actually 
re-queue itself, I think we must use cancel_delayed_work_sync here.

Thanks,
Quang Minh.