linux-kernel - Re: [PATCH v4 1/3] tracing: Add support for recording tgid of tasks

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Date:   Mon, 26 Jun 2017 12:10:04 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Michael Sartain <mikesart@...il.com>, kernel-team@...roid.com,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v4 1/3] tracing: Add support for recording tgid of tasks

On Mon, Jun 26, 2017 at 12:02 PM, Joel Fernandes <joelaf@...gle.com> wrote:
> On Mon, Jun 26, 2017 at 9:52 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>> On Sun, 25 Jun 2017 22:38:42 -0700
[..]
>>
>>> +{
>>> +     struct trace_event_file *file;
>>> +     struct trace_array *tr;
>>> +
>>> +     mutex_lock(&event_mutex);
>>> +     do_for_each_event_file(tr, file) {
>>> +             if (!(file->flags & EVENT_FILE_FL_ENABLED))
>>> +                     continue;
>>> +
>>> +             if (enable) {
>>> +                     tracing_start_tgid_record();
>>
>> If we fail to start, the bit should not be set, and we should return
>> failure. Note, it can only fail on the first try, as once it is
>> allocated, you don't need to worry about it failing. Thus, if it fails,
>> break out of the loop now and return failure.
>>
>
> That seems Ok with me to do, I think a valid point.
>
> I think that I should do it in the second call to
> tracing_start_tgid_record too then (__ftrace_event_enable_disable) to
> error out if the allocation fails.
>
> While going this code I again, I noticed another potential issue in
> __ftrace_event_enable_disable

Sorry I hit send too soon!

I was saying, While going through this code again I noticed another
potential issue in __ftrace_event_enable_disable in existing code:

Here we do:
ret = call->class->reg(call, TRACE_REG_REGISTER, file);

But in the error handling path we only do a
tracing_stop_cmdline_record() and not clear the
EVENT_FILE_FL_RECORDED_CMD_BIT flag. Is that not an issue? I am
guessing because enabling of the event itself will fail, that's Ok.
But just to keep it consistent, I am thinking if we should just clear
the bit here.

Thanks,
Joel