netdev - Re: [PATCH bpf-next 1/2] xsk: remove AF_XDP socket from map when the socket is released

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Date:   Mon, 6 May 2019 12:04:57 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...il.com>,
        ast@...nel.org, netdev@...r.kernel.org
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        magnus.karlsson@...el.com, magnus.karlsson@...il.com,
        bruce.richarson@...el.com, bpf@...r.kernel.org,
        Bruce Richardson <bruce.richardson@...el.com>
Subject: Re: [PATCH bpf-next 1/2] xsk: remove AF_XDP socket from map when the
 socket is released

On 05/04/2019 06:06 PM, Björn Töpel wrote:
> From: Björn Töpel <bjorn.topel@...el.com>
> 
> When an AF_XDP socket is released/closed the XSKMAP still holds a
> reference to the socket in a "released" state. The socket will still
> use the netdev queue resource, and block newly created sockets from
> attaching to that queue, but no user application can access the
> fill/complete/rx/tx rings. This results in that all applications need
> to explicitly clear the map entry from the old "zombie state"
> socket. This should be done automatically.
> 
> After this patch, when a socket is released, it will remove itself
> from all the XSKMAPs it resides in, allowing the socket application to
> remove the code that cleans the XSKMAP entry.
> 
> This behavior is also closer to that of SOCKMAP, making the two socket
> maps more consistent.
> 
> Reported-by: Bruce Richardson <bruce.richardson@...el.com>
> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
[...]


> +static void __xsk_map_delete_elem(struct xsk_map *map,
> +				  struct xdp_sock **map_entry)
> +{
> +	struct xdp_sock *old_xs;
> +
> +	spin_lock_bh(&map->lock);
> +	old_xs = xchg(map_entry, NULL);
> +	if (old_xs)
> +		xsk_map_del_node(old_xs, map_entry);
> +	spin_unlock_bh(&map->lock);
> +
> +}
> +
>  static void xsk_map_free(struct bpf_map *map)
>  {
>  	struct xsk_map *m = container_of(map, struct xsk_map, map);
> @@ -78,15 +142,16 @@ static void xsk_map_free(struct bpf_map *map)
>  	bpf_clear_redirect_map(map);
>  	synchronize_net();
>  
> +	spin_lock_bh(&m->lock);
>  	for (i = 0; i < map->max_entries; i++) {
> +		struct xdp_sock **entry = &m->xsk_map[i];
>  		struct xdp_sock *xs;
>  
> -		xs = m->xsk_map[i];
> -		if (!xs)
> -			continue;
> -
> -		sock_put((struct sock *)xs);
> +		xs = xchg(entry, NULL);
> +		if (xs)
> +			__xsk_map_delete_elem(m, entry);
>  	}
> +	spin_unlock_bh(&m->lock);
>  

Was this tested? Doesn't the above straight run into a deadlock?

>From xsk_map_free() you iterate over the map with m->lock held. Once you
xchg'ed the entry and call into __xsk_map_delete_elem(), you attempt to
call map->lock on the same map once again. What am I missing?

Thanks,
Daniel