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

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Date:   Mon, 26 Jun 2017 15:12:52 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joelaf@...gle.com>
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, 26 Jun 2017 12:02:12 -0700
Joel Fernandes <joelaf@...gle.com> wrote:

> >> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> >> index e7973e10398c..240c6df95ea6 100644
> >> --- a/kernel/trace/trace_events.c
> >> +++ b/kernel/trace/trace_events.c
> >> @@ -343,6 +343,28 @@ void trace_event_enable_cmd_record(bool enable)
> >>       mutex_unlock(&event_mutex);
> >>  }
> >>
> >> +void trace_event_enable_tgid_record(bool enable)  
> >
> > This should return a value.
> >  
> >> +{
> >> +     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

Thinking about this more. Just allocate the array as soon as the option
is enabled, regardless if an trace event is set. That will make it a
lot simpler. If it fails to allocate, you can simply bail out with
-ENOMEM, and the setting of the option will return that. Then we can
even remove the WARN_ONCE() at allocation failure, as the user will
know what happened.

-- Steve