Re: [PATCH bpf-next v2 1/3] bpf: xdp, update devmap comments to reflect napi/rcu usage

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Sat, 25 Jan 2020 19:58:51 -0800
John Fastabend <john.fastabend@gmail.com> wrote:

> Now that we rely on synchronize_rcu and call_rcu waiting to
> exit perempt-disable regions (NAPI) lets update the comments
> to reflect this.
> 
> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
> Acked-by: Björn Töpel <bjorn.topel@intel.com>
> Acked-by: Song Liu <songliubraving@fb.com>
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> ---
>  kernel/bpf/devmap.c | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index da9c832..f0bf525 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -193,10 +193,12 @@ static void dev_map_free(struct bpf_map *map)
>  
>  	/* At this point bpf_prog->aux->refcnt == 0 and this map->refcnt == 0,
>  	 * so the programs (can be more than one that used this map) were
> -	 * disconnected from events. Wait for outstanding critical sections in
> -	 * these programs to complete. The rcu critical section only guarantees
> -	 * no further reads against netdev_map. It does __not__ ensure pending
> -	 * flush operations (if any) are complete.
> +	 * disconnected from events. The following synchronize_rcu() guarantees
> +	 * both rcu read critical sections complete and waits for
> +	 * preempt-disable regions (NAPI being the relavent context here) so we
                                                   ^^^^^^^^
Spelling: relevant

I would hate to block the patch this close to the release deadline, so
maybe DaveM can just adjust this before applying?

> +	 * are certain there will be no further reads against the netdev_map and
> +	 * all flush operations are complete. Flush operations can only be done
> +	 * from NAPI context for this reason.
>  	 */
>  
>  	spin_lock(&dev_map_lock);
> @@ -498,12 +500,11 @@ static int dev_map_delete_elem(struct bpf_map *map, void *key)
>  		return -EINVAL;
>  
>  	/* Use call_rcu() here to ensure any rcu critical sections have
> -	 * completed, but this does not guarantee a flush has happened
> -	 * yet. Because driver side rcu_read_lock/unlock only protects the
> -	 * running XDP program. However, for pending flush operations the
> -	 * dev and ctx are stored in another per cpu map. And additionally,
> -	 * the driver tear down ensures all soft irqs are complete before
> -	 * removing the net device in the case of dev_put equals zero.
> +	 * completed as well as any flush operations because call_rcu
> +	 * will wait for preempt-disable region to complete, NAPI in this
> +	 * context.  And additionally, the driver tear down ensures all
> +	 * soft irqs are complete before removing the net device in the
> +	 * case of dev_put equals zero.
>  	 */
>  	old_dev = xchg(&dtab->netdev_map[k], NULL);
>  	if (old_dev)



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer