netdev - Re: [PATCH v5 bpf-next 06/10] tracepoint: compute num

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Date:   Mon, 26 Mar 2018 09:25:07 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:     rostedt <rostedt@...dmis.org>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        netdev <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        linux-api <linux-api@...r.kernel.org>,
        "Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [PATCH v5 bpf-next 06/10] tracepoint: compute num_args at build
 time

On 3/26/18 9:17 AM, Mathieu Desnoyers wrote:
>>
>> re: 'added complexity'...
>> -       for (iter = begin; iter < end; iter++)
>> -               fct(*iter, priv);
>> +               return NULL;
>> +       for (iter = begin; iter < end; iter++) {
>> +               ret = fct(*iter, priv);
>> +               if (ret)
>> +                       return ret;
>> +       }
>> +       return NULL;
>>
>> where do you see 'added complexity' ?
>> Isn't the above diff self-explanatory that for_each_tracepoint_range()
>> can be used not only to iterate over all tracepoints
>> (just do 'return NULL') from callback _and_ to find one particular
>> tracepoint as patch 7 does ?
>
> I am not arguing about your proposed implementation. I am arguing about
> the lack of justification behind this change. Why is this change needed ?
> What is it allowing you to do that cannot be done using the private data
> pointer ?

commit log of patch 6 states:

"for_each_tracepoint_range() api has no users inside the kernel.
Make it more useful with ability to stop for_each() loop depending
via callback return value.
In such form it's used in subsequent patch."

and in patch 7:

+static void *__find_tp(struct tracepoint *tp, void *priv)
+{
+       char *name = priv;
+
+       if (!strcmp(tp->name, name))
+               return tp;
+       return NULL;
+}
...
+       struct tracepoint *tp;
...
+       tp = for_each_kernel_tracepoint(__find_tp, tp_name);
+       if (!tp)
+               return -ENOENT;

still not obvious?