can add a new bpf helper function bpf_map_compare_and_update_elem?

can add a new bpf helper function bpf_map_compare_and_update_elem?

[Yahui Chen]

Suppose we have a map, MAP_A, and the user program does the following:

1. bpf_map_lookup_elem(MAP_A, key, value)
2. change the value
3. bpf_map_update_elem(MAP_A, key, value, FLAG)

At the same time, the kernel's BPF program may also be modifying the value.

Then we have concurrency problems.

Therefore, can we add a helper function like compare and swap?

Let's call it bpf_map_compare_and_update_elem.

So, the map operations will be modified as follows:

for true {
    1. bpf_map_lookup_elem(MAP_A, key, old_value)
    2. change value
    3. ret = bpf_map_compare_and_update_elem(MAP_A, key, old_value,
new_value, FLAG)
    4. if ret == 0 { break }
}


thank you

Re: can add a new bpf helper function bpf_map_compare_and_update_elem?

[Cong Wang]

On Thu, Nov 25, 2021 at 04:34:48PM +0800, Yahui Chen wrote:
> Suppose we have a map, MAP_A, and the user program does the following:
> 
> 1. bpf_map_lookup_elem(MAP_A, key, value)
> 2. change the value
> 3. bpf_map_update_elem(MAP_A, key, value, FLAG)
> 
> At the same time, the kernel's BPF program may also be modifying the value.
> 
> Then we have concurrency problems.
> 

This is why we have bpf spinlock. ;)


> Therefore, can we add a helper function like compare and swap?
> 

I don't think you can atomically compare and swap values larger than
CPU word size.


Thanks.

Re: can add a new bpf helper function bpf_map_compare_and_update_elem?

[Yahui Chen]

Cong Wang <xiyou.wangcong@gmail.com> 于2021年11月27日周六 上午4:25写道：
>
> On Thu, Nov 25, 2021 at 04:34:48PM +0800, Yahui Chen wrote:
> > Suppose we have a map, MAP_A, and the user program does the following:
> >
> > 1. bpf_map_lookup_elem(MAP_A, key, value)
> > 2. change the value
> > 3. bpf_map_update_elem(MAP_A, key, value, FLAG)
> >
> > At the same time, the kernel's BPF program may also be modifying the value.
> >
> > Then we have concurrency problems.
> >
>
> This is why we have bpf spinlock. ;)
>

I get a bpf_spinlock example from
https://patchwork.ozlabs.org/project/netdev/patch/20190124041403.2100609-2-ast@kernel.org/:
```
Example:
struct hash_elem {
    int cnt;
    struct bpf_spin_lock lock;
};
struct hash_elem * val = bpf_map_lookup_elem(&hash_map, &key);
if (val) {
    bpf_spin_lock(&val->lock);
    val->cnt++;
    bpf_spin_unlock(&val->lock);
}
```

But, I think bpf_spinlock cannot be used in user mode.

>
> > Therefore, can we add a helper function like compare and swap?
> >
>
> I don't think you can atomically compare and swap values larger than
> CPU word size.
>

Yep, you are right.
So I imagined adding something code like `memcmp` to the kernel before
update bpf map element.
If memcmp success then update the element, otherwise return fail.

New helper function looks like:
bpf_map_compare_and_update_elem(MAP_A, key, old_value,
new_value, FLAG)

>
> Thanks.