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

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Message-ID: <20180326141104.6d63660a@gandalf.local.home>
Date:   Mon, 26 Mar 2018 14:11:04 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexei Starovoitov <ast@...com>
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 Mon, 26 Mar 2018 10:55:51 -0700
Alexei Starovoitov <ast@...com> wrote:

> An email ago you were ok to s/return/return NULL/ in your out-of-tree
> module, but now flip flop to add new function approach just to
> reduce the work you need to do in lttng?
> We're not talking about changing __kmalloc signature here.
> My patch extends for_each_kernel_tracepoint() api similar to other
> for_each_*() iterators and improves possible uses of it.

Alexei, do you have another use case for using
for_each_kernel_tracepoint() other than the find_tp? If so, then I'm
sure Mathieu can handle the change.

But I think it's cleaner to add a tracepoint_find_by_name() function.
If you come up with another use case for using the for_each* function
then we'll consider changing it then.


> One thing is to be nice to out-of-tree and do not break them
> for no reason, but arguing that kernel shouldn't add a minor extension
> to for_each_kernel_tracepoint() api is really taking the whole thing
> to next level.

That's not the point. I disagree with the reason for the change, and
believe that it would be cleaner to add a find_by_name() function.
Which would make your patch set even cleaner. 

Instead of having in the bpf code:

static void *__find_tp(struct tracepoint *tp, void *priv)
{
	char *name = priv;

	if (!strcmp(tp->name, name))
		return tp;
	return NULL;
}

[..]

	tp = for_each_kernel_tracepoint(__find_tp, tp_name);
	if (!tp)
		return -ENOENT;


You would simply have:

	tp = tracepoint_find_by_name(tp_name);
	if (!tp)
		return -ENOENT;

That would make the code more obvious to what it is doing. And this
does not impede your patch set at all.

-- Steve