Re: [PATCH bpf-next] bpf: make list_for_each_entry portable

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Fri, May 10, 2024 at 1:27 AM Jose E. Marchesi
> <jose.marchesi@oracle.com> wrote:
>>
>>
>> > On Thu, May 9, 2024 at 1:47 AM Jose E. Marchesi
>> > <jose.marchesi@oracle.com> wrote:
>> >> +/* A `break' executed in the head of a `for' loop statement is bound
>> >> +   to the current loop in clang, but it is bound to the enclosing loop
>> >> +   in GCC.  Note both compilers optimize the outer loop out with -O1
>> >> +   and higher.  This macro shall be used to annotate any loop that
>> >> +   uses cond_break within its header.  */
>> >> +#ifdef __clang__
>> >> +#define __compat_break
>> >> +#else
>> >> +#define __compat_break for (int __control = 1; __control; --__control)
>> >> +#endif
>> > ..
>> >> +       __compat_break
>> >>         for (i = zero; i < cnt; cond_break, i++) {
>> >>                 struct elem __arena *n = bpf_alloc(sizeof(*n));
>> >
>> > This is too ugly. It ruins the readability of the code.
>> > Let's introduce can_loop macro similar to cond_break
>> > that returns 0 or 1 instead of break/continue and use it as:
>> >
>> >         for (i = zero; i < cnt && can_loop; i++) {
>> >
>> > pw-bot: cr
>>
>> I went with the ugliness because I was trying to avoid rewriting the
>> loops in the tests, assuming the tests were actually testing using
>> cond_break in these particular locations would result in a particular
>> number of iterations.
>>
>> The loops
>>
>>   for (i = zero; i < cnt; cond_break, i++) BODY
>>
>> and
>>
>>   for (i = zero; i < cnt && can_loop; i++) BODY
>>
>> are not equivalent if can_loop implements the same logic than
>> cond_break.
>
> It's off by one and it's fine.
> The loops don't and shouldn't expect the precise number allowed
> by may_goto.

Ok, understood.

I assume you also want to use can_loop also in the definition of
list_for_each_entry?

> btw there are tests that use cond_break inside {}.
> They don't need to change.

Yes I noticed these.  Won't touch them.

>
>>
>> The may_goto instructions are somehow patched at run-time, and in a
>> predictable way since the tests are checking for explicit iteration
>> counts, right?
>
> They're patched by the verifier, but they're unpredictable.
> Right now it's a simpler counter, but sooner or later
> it will be time based.