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

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Date:   Mon, 26 Mar 2018 08:42:49 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rostedt <rostedt@...dmis.org>
CC:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        <netdev@...r.kernel.org>, <kernel-team@...com>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [PATCH v5 bpf-next 06/10] tracepoint: compute num_args at build
 time

On 3/26/18 8:14 AM, Mathieu Desnoyers wrote:
> ----- On Mar 26, 2018, at 11:02 AM, rostedt rostedt@...dmis.org wrote:
>
>> On Fri, 23 Mar 2018 19:30:34 -0700
>> Alexei Starovoitov <ast@...com> wrote:
>>
>>> From: Alexei Starovoitov <ast@...nel.org>
>>>
>>> add fancy macro to compute number of arguments passed into tracepoint
>>> at compile time and store it as part of 'struct tracepoint'.
>>> The number is necessary to check safety of bpf program access that
>>> is coming in subsequent patch.
>>>
>>> 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.
>>
>> I believe this is used by LTTng.
>
> Indeed, and by SystemTAP as well.
>
> What justifies the need to stop mid-iteration ? A less intrusive alternative
> would be to use the "priv" data pointer to keep state telling further calls
> to return immediately. Does performance of iteration over tracepoints really
> matter here so much that stopping iteration immediately is worth it ?

I'm sure both you and Steven are not serious when you object
to _in-tree_ change to for_each_kernel_tracepoint() that
affects _out-of_tree_ modules?

Just change your module to 'return NULL' instead of plain 'return'.