From: "Jose E. Marchesi" <jose.marchesi@oracle.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, David Faust <david.faust@oracle.com>,
Cupertino Miranda <cupertino.miranda@oracle.com>
Subject: Re: [PATCH bpf-next] bpf: make list_for_each_entry portable
Date: Fri, 10 May 2024 19:16:42 +0200 [thread overview]
Message-ID: <87plttsi2t.fsf@oracle.com> (raw)
In-Reply-To: <CAADnVQ+-w1cD83mE0u0QhCP4cCvSSB1-GNoqErYRRhtxcwkTmg@mail.gmail.com> (Alexei Starovoitov's message of "Fri, 10 May 2024 10:03:51 -0700")
> 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.
prev parent reply other threads:[~2024-05-10 17:25 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-09 8:46 [PATCH bpf-next] bpf: make list_for_each_entry portable Jose E. Marchesi
2024-05-09 21:48 ` Alexei Starovoitov
2024-05-10 8:26 ` Jose E. Marchesi
2024-05-10 17:03 ` Alexei Starovoitov
2024-05-10 17:16 ` Jose E. Marchesi [this message]
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