RE: [PATCH] xen-netback: don't populate the hash cache on XenBus disconnect

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Igor Druzhinin [mailto:igor.druzhinin@citrix.com]
> Sent: 28 February 2019 14:11
> To: xen-devel@lists.xenproject.org; netdev@vger.kernel.org; linux-kernel@vger.kernel.org
> Cc: Wei Liu <wei.liu2@citrix.com>; Paul Durrant <Paul.Durrant@citrix.com>; davem@davemloft.net; Igor
> Druzhinin <igor.druzhinin@citrix.com>
> Subject: [PATCH] xen-netback: don't populate the hash cache on XenBus disconnect
> 
> Occasionally, during the disconnection procedure on XenBus which
> includes hash cache deinitialization there might be some packets
> still in-flight on other processors. Handling of these packets includes
> hashing and hash cache population that finally results in hash cache
> data structure corruption.
> 
> In order to avoid this we prevent hashing of those packets if there
> are no queues initialized. In that case RCU protection of queues guards
> the hash cache as well.
> 
> Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>

Reviewed-by: Paul Durrant <paul.durrant@citrix.com>

> ---
> 
> Found this while applying the previous patch to our patchqueue. Seems it
> never went to the mailing list and, to my knowledge, the problem is still
> present. From my recollection, it only happened on stress frontend on/off
> test with Windows guests (since only those detach the frontend completely).
> So better late than never.
> 
> ---
>  drivers/net/xen-netback/hash.c      | 2 ++
>  drivers/net/xen-netback/interface.c | 7 +++++++
>  2 files changed, 9 insertions(+)
> 
> diff --git a/drivers/net/xen-netback/hash.c b/drivers/net/xen-netback/hash.c
> index 0ccb021..10d580c 100644
> --- a/drivers/net/xen-netback/hash.c
> +++ b/drivers/net/xen-netback/hash.c
> @@ -454,6 +454,8 @@ void xenvif_init_hash(struct xenvif *vif)
>  	if (xenvif_hash_cache_size == 0)
>  		return;
> 
> +	BUG_ON(vif->hash.cache.count);
> +
>  	spin_lock_init(&vif->hash.cache.lock);
>  	INIT_LIST_HEAD(&vif->hash.cache.list);
>  }
> diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-netback/interface.c
> index 182d677..6da1251 100644
> --- a/drivers/net/xen-netback/interface.c
> +++ b/drivers/net/xen-netback/interface.c
> @@ -153,6 +153,13 @@ static u16 xenvif_select_queue(struct net_device *dev, struct sk_buff *skb,
>  {
>  	struct xenvif *vif = netdev_priv(dev);
>  	unsigned int size = vif->hash.size;
> +	unsigned int num_queues;
> +
> +	/* If queues are not set up internally - always return 0
> +	 * as the packet going to be dropped anyway */
> +	num_queues = READ_ONCE(vif->num_queues);
> +	if (num_queues < 1)
> +		return 0;
> 
>  	if (vif->hash.alg == XEN_NETIF_CTRL_HASH_ALGORITHM_NONE)
>  		return fallback(dev, skb, NULL) % dev->real_num_tx_queues;
> --
> 2.7.4