Coding style when using atomic_dec_and_test

Coding style when using atomic_dec_and_test

[Camila Alvarez Inostroza]

I understand that atomic_dec_and_test returns 1 when the result of the
operation is 0, and 0 otherwise.
I've seen two ways of handling the resulting operation, below are a couple
of examples:
(1) if (!atomic_dec_and_test(&rd->refcount))
           return;
      call_rcu(&rd->rcu, free_rootdomain);

(2)  if (atomic64_dec_and_test(&map->refcnt)) {
           /* bpf_map_free_id() must be called first */
           ...
        }

Is it prefered to use one option over the other? Or is it just personal
preference?

Thanks in advance!
Camila Alvarez

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Re: Coding style when using atomic_dec_and_test

[Valdis Klētnieks]

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On Sat, 30 Mar 2024 21:52:29 -0300, Camila Alvarez Inostroza said:

> I've seen two ways of handling the resulting operation, below are a couple
> of examples:
> (1) if (!atomic_dec_and_test(&rd->refcount))
>            return;
>       call_rcu(&rd->rcu, free_rootdomain);
>
> (2)  if (atomic64_dec_and_test(&map->refcnt)) {
>            /* bpf_map_free_id() must be called first */
>            ...
>         }
>
> Is it prefered to use one option over the other? Or is it just personal
> preference?

Yes. :)

Basically, it will depend on what the code logic structure is like.  If the logical
way to do it is "if it fails, return now", then you do (1).  If the logical structure
is "if it succeeds, do this before continuing", you do (2).  In other cases,
you may want to do the "structured goto" the kernel uses to unwind allocations
and locks that happened before the failure.

The above is not an exhaustive list.

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