* using bpf_map_update_elem and bpf_map_get_next_key at the same time when looping through the hash map
@ 2021-07-06 3:11 G
2021-07-06 18:46 ` Yonghong Song
0 siblings, 1 reply; 2+ messages in thread
From: G @ 2021-07-06 3:11 UTC (permalink / raw)
To: bpf
Hi BPF Experts
I'm having an issue with using "bpf_map_update_elem" and "bpf_map_get_next_key" at the same time when looping through the bpf HashMap.
My program turns to an infinite loop and the pseudocode is as following:
------------------------------------------------------------------------------
bpf.MapCreate // type=BPF_MAP_TYPE_HASH size=128
for { bpf.MapUpdate } // add(update) 128 elements at once
then loop through the map to update each element
bpf.MapGetNextKey(fd, nil, &scankey) // find first key
for {
bpf.MapUpate(fd, &scankey, &val, BPF_EXIST)
bpf.MapGetNextKey(fd, &scankey, &scankey)
}
------------------------------------------------------------------------------
I have tried to read the relevant kernel code, and seems like it is moving the element to the top of the has bucket when calling the “bpf_map_update_elem” even the element already exists in the hash map. See the following source code:
------------------------------------------------------------------------------
// kernel/bpf/hashtab.c
htab_map_update_elem {
...
/* add new element to the head of the list, so that
* concurrent search will find it before old elem
*/
hlist_nulls_add_head_rcu(&l_new->hash_node, head);
...
}
------------------------------------------------------------------------------
Therefore, when I was trying to traversing the two elements in the same hash a bucket, it ran into an infinite loop by repeatedly getting the key of these two elements. Not sure my understanding for "bpf_map_update_elem"and "bpf_map_get_next_key" is correct or not. My question is: is that behave as the design? or is it a bug for the bpf hashmap? Please let me know, thanks.
Best regards
W.Gao
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: using bpf_map_update_elem and bpf_map_get_next_key at the same time when looping through the hash map
2021-07-06 3:11 using bpf_map_update_elem and bpf_map_get_next_key at the same time when looping through the hash map G
@ 2021-07-06 18:46 ` Yonghong Song
0 siblings, 0 replies; 2+ messages in thread
From: Yonghong Song @ 2021-07-06 18:46 UTC (permalink / raw)
To: G, bpf
On 7/5/21 8:11 PM, G wrote:
> Hi BPF Experts
>
> I'm having an issue with using "bpf_map_update_elem" and "bpf_map_get_next_key" at the same time when looping through the bpf HashMap.
> My program turns to an infinite loop and the pseudocode is as following:
> ------------------------------------------------------------------------------
> bpf.MapCreate // type=BPF_MAP_TYPE_HASH size=128
> for { bpf.MapUpdate } // add(update) 128 elements at once
>
> then loop through the map to update each element
> bpf.MapGetNextKey(fd, nil, &scankey) // find first key
> for {
> bpf.MapUpate(fd, &scankey, &val, BPF_EXIST)
> bpf.MapGetNextKey(fd, &scankey, &scankey)
> }
> ------------------------------------------------------------------------------
>
> I have tried to read the relevant kernel code, and seems like it is moving the element to the top of the has bucket when calling the “bpf_map_update_elem” even the element already exists in the hash map. See the following source code:
> ------------------------------------------------------------------------------
> // kernel/bpf/hashtab.c
> htab_map_update_elem {
> ...
> /* add new element to the head of the list, so that
> * concurrent search will find it before old elem
> */
> hlist_nulls_add_head_rcu(&l_new->hash_node, head);
> ...
> }
> ------------------------------------------------------------------------------
>
> Therefore, when I was trying to traversing the two elements in the same hash a bucket, it ran into an infinite loop by repeatedly getting the key of these two elements. Not sure my understanding for "bpf_map_update_elem"and "bpf_map_get_next_key" is correct or not. My question is: is that behave as the design? or is it a bug for the bpf hashmap? Please let me know, thanks.
bpf_map_get_next_key() is added after bpf_map_update_elem(). So the
above behavior is in the kernel already for sometimes.
bpf_map_get_next_key() is not super reliable for hash table as if some
deletion happens, the get_next_key may start from the beginning.
The recommendation is to use bpf_map_*_batch() interface.
If your kernel does not implement bpf_map_*_batch() interface, I think
it would be best you call bpf_map_get_next_key() for ALL elements before
doing any update/delete.
>
> Best regards
> W.Gao
>
^ permalink raw reply [flat|nested] 2+ messages in thread
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