netdev - Re: [PATCH net V2] tuntap: synchronize through tfiles array instead of tun->numqueues

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Date:   Wed, 08 May 2019 10:36:13 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     jasowang@...hat.com
Cc:     netdev@...r.kernel.org, mst@...hat.com, yuehaibing@...wei.com,
        xiyou.wangcong@...il.com, weiyongjun1@...wei.com,
        eric.dumazet@...il.com
Subject: Re: [PATCH net V2] tuntap: synchronize through tfiles array
 instead of tun->numqueues

From: Jason Wang <jasowang@...hat.com>
Date: Tue,  7 May 2019 00:03:36 -0400

> @@ -1313,6 +1315,10 @@ static int tun_xdp_xmit(struct net_device *dev, int n,
>  
>  	tfile = rcu_dereference(tun->tfiles[smp_processor_id() %
>  					    numqueues]);
> +	if (!tfile) {
> +		rcu_read_unlock();
> +		return -ENXIO; /* Caller will free/return all frames */
> +	}
>  
>  	spin_lock(&tfile->tx_ring.producer_lock);
>  	for (i = 0; i < n; i++) {

The only way we can see a NULL here is if a detach happened in parallel,
and if that happens we should retry the tfile[] indexing after resampling
numqueues rather than dropping the packet.