netdev - RE: [PATCH bpf-next v2 1/8] xdp: simplify devmap cleanup

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Date:   Tue, 07 Jan 2020 09:32:33 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Björn Töpel <bjorn.topel@...il.com>,
        netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        bpf@...r.kernel.org, davem@...emloft.net,
        jakub.kicinski@...ronome.com, hawk@...nel.org,
        john.fastabend@...il.com, magnus.karlsson@...el.com,
        jonathan.lemon@...il.com,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: RE: [PATCH bpf-next v2 1/8] xdp: simplify devmap cleanup

Björn Töpel wrote:
> From: Björn Töpel <bjorn.topel@...el.com>
> 
> After the RCU flavor consolidation [1], call_rcu() and
> synchronize_rcu() waits for preempt-disable regions (NAPI) in addition
> to the read-side critical sections. As a result of this, the cleanup
> code in devmap can be simplified

OK great makes sense. One comment below.

> 
> * There is no longer a need to flush in __dev_map_entry_free, since we
>   know that this has been done when the call_rcu() callback is
>   triggered.
> 
> * When freeing the map, there is no need to explicitly wait for a
>   flush. It's guaranteed to be done after the synchronize_rcu() call
>   in dev_map_free(). The rcu_barrier() is still needed, so that the
>   map is not freed prior the elements.
> 
> [1] https://lwn.net/Articles/777036/
> 
> Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>
> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
> ---
>  kernel/bpf/devmap.c | 43 +++++--------------------------------------
>  1 file changed, 5 insertions(+), 38 deletions(-)
> 
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index 3d3d61b5985b..b7595de6a91a 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -201,7 +201,7 @@ static struct bpf_map *dev_map_alloc(union bpf_attr *attr)
>  static void dev_map_free(struct bpf_map *map)
>  {
>  	struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
> -	int i, cpu;
> +	int i;
>  
>  	/* 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
> @@ -221,18 +221,6 @@ static void dev_map_free(struct bpf_map *map)
>  	/* Make sure prior __dev_map_entry_free() have completed. */
>  	rcu_barrier();
>  

The comment at the start of this function also needs to be fixed it says,

  /* 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.
   */

also comment in dev_map_delete_elem() needs update.

> -	/* To ensure all pending flush operations have completed wait for flush
> -	 * list to empty on _all_ cpus.
> -	 * Because the above synchronize_rcu() ensures the map is disconnected
> -	 * from the program we can assume no new items will be added.
> -	 */
> -	for_each_online_cpu(cpu) {
> -		struct list_head *flush_list = per_cpu_ptr(dtab->flush_list, cpu);
> -
> -		while (!list_empty(flush_list))
> -			cond_resched();
> -	}
> -
>  	if (dtab->map.map_type == BPF_MAP_TYPE_DEVMAP_HASH) {
>  		for (i = 0; i < dtab->n_buckets; i++) {
>  			struct bpf_dtab_netdev *dev;
> @@ -345,8 +333,7 @@ static int dev_map_hash_get_next_key(struct bpf_map *map, void *key,
>  	return -ENOENT;
>  }

Otherwise LGTM thanks.