linux-kernel - Re: [PATCH v2 2/5] bpf: Add function map->ops->map_traverse

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Date:	Wed, 22 Jul 2015 18:00:22 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Kaixu Xia <xiakaixu@...wei.com>, davem@...emloft.net,
	acme@...nel.org, mingo@...hat.com, a.p.zijlstra@...llo.nl,
	masami.hiramatsu.pt@...achi.com, jolsa@...nel.org
Cc:	wangnan0@...wei.com, linux-kernel@...r.kernel.org,
	pi3orama@....com, hekuang@...wei.com
Subject: Re: [PATCH v2 2/5] bpf: Add function map->ops->map_traverse_elem() to
 traverse map elems

On 7/22/15 1:09 AM, Kaixu Xia wrote:
> We want to traverse the map elements and make use
> of the map value one by one. So add new function
> map->ops->map_traverse_elem() to traverse map elements.
...
> @@ -26,6 +28,7 @@ struct bpf_map_ops {
>   	void *(*map_lookup_elem)(struct bpf_map *map, void *key);
>   	int (*map_update_elem)(struct bpf_map *map, void *key, void *value, u64 flags);
>   	int (*map_delete_elem)(struct bpf_map *map, void *key);
> +	int (*map_traverse_elem)(bpf_map_traverse_callback func, struct bpf_map *map);

I think 'traverse' is too error prone.
Better to add 'map_delete_all_elem' callback.
Then convert bpf_prog_array_map_clear() into such callback
and similar callback from perf_event_array.
Then call it with from bpf_map_release() and drop
if (map->map_type == PROG_ARRAY) there.
Just unconditionally map->ops->map_delete_all_elem().
Though looking at patch 3. I don't see why you need that.
prog_array has to do it due to potential circular dependency
between prog_fd of a program that uses a prog_array_fd and
stores prog_fd inside that prog_array.
In case of perf_event_array no such thing possible.
Just release them as part of map_free.

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