Subject: Re: [PATCH bpf-next 04/17] xdp: add proper __rcu annotations to redirect map entries

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 11 Jun 2021 01:19:04 +0200

Martin KaFai Lau <[email-masked]> writes:

> On Wed, Jun 09, 2021 at 12:33:13PM +0200, Toke Høiland-Jørgensen wrote:
> [ ... ]
>
>> @@ -551,7 +551,8 @@ static void cpu_map_free(struct bpf_map *map)
>>  	for (i = 0; i < cmap->map.max_entries; i++) {
>>  		struct bpf_cpu_map_entry *rcpu;
>>  
>> -		rcpu = READ_ONCE(cmap->cpu_map[i]);
>> +		rcpu = rcu_dereference_check(cmap->cpu_map[i],
>> +					     rcu_read_lock_bh_held());
> Is rcu_read_lock_bh_held() true during map_free()?

Hmm, no, I guess not since that's called from a workqueue. Will fix!

>> @@ -149,7 +152,8 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
>>  			       u64 map_flags)
>>  {
>>  	struct xsk_map *m = container_of(map, struct xsk_map, map);
>> -	struct xdp_sock *xs, *old_xs, **map_entry;
>> +	struct xdp_sock __rcu **map_entry;
>> +	struct xdp_sock *xs, *old_xs;
>>  	u32 i = *(u32 *)key, fd = *(u32 *)value;
>>  	struct xsk_map_node *node;
>>  	struct socket *sock;
>> @@ -179,7 +183,7 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
>>  	}
>>  
>>  	spin_lock_bh(&m->lock);
>> -	old_xs = READ_ONCE(*map_entry);
>> +	old_xs = rcu_dereference_check(*map_entry, rcu_read_lock_bh_held());
> Is it actually protected by the m->lock at this point?

True, can just add that to the check.

>>  void xsk_map_try_sock_delete(struct xsk_map *map, struct xdp_sock *xs,
>> -			     struct xdp_sock **map_entry)
>> +			     struct xdp_sock __rcu **map_entry)
>>  {
>>  	spin_lock_bh(&map->lock);
>> -	if (READ_ONCE(*map_entry) == xs) {
>> -		WRITE_ONCE(*map_entry, NULL);
>> +	if (rcu_dereference(*map_entry) == xs) {
> nit. rcu_access_pointer()?

Yup.