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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Igor Druzhinin [mailto:igor.druzhinin@xxxxxxxxxx]
> Sent: 28 February 2019 14:11
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; 
> linux-kernel@xxxxxxxxxxxxxxx
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 
> davem@xxxxxxxxxxxxx; Igor
> Druzhinin <igor.druzhinin@xxxxxxxxxx>
> 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@xxxxxxxxxx>

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

> ---
> 
> 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


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