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

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Date:   Mon, 26 Mar 2018 09:47:21 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "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 9:35 AM, Steven Rostedt wrote:
> On Mon, 26 Mar 2018 09:25:07 -0700
> Alexei Starovoitov <ast@...com> wrote:
>
>> 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?
>
> Please just create a new function called tracepoint_find_by_name(), and
> use that. I don't see any benefit in using a for_each* function for
> such a simple routine. Not to mention, you then don't need to know the
> internals of a tracepoint in kernel/bpf/syscall.c.

It's a standard pattern in the kernel to stop for_each*() iterator
when callback function returns non-null.
Few examples:
idr_for_each
cfs_hash_for_each
and there are plenty more.

I don't mind to _rename_ for_each_kernel_tracepoint() into
tracepoint_find_by_name(), but keeping exported function
just to be used by out-of-tree modules would be wrong message for
the kernel community in general.
With my patch the for_each_kernel_tracepoint() will be used by bpf side
and out-of-tree can trivially hack their callbacks to keep working.
imo that's a better approach then renaming it.