linux-kernel - Re: Reason for unused flags argument in trace_raw_output

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Date:   Thu, 7 Oct 2021 10:31:48 -0400
From:   William Cohen <wcohen@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mingo@...hat.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Reason for unused flags argument in trace_raw_output_* function?

On 10/7/21 09:20, Steven Rostedt wrote:
> On Wed, 6 Oct 2021 23:49:22 -0400
> William Cohen <wcohen@...hat.com> wrote:
> 
>> Hi,
>>
>> I have some code that analyzes x86_64 binaries
>> (https://developers.redhat.com/articles/2021/08/09/debugging-function-parameters-
>> dyninst#liveness_analysis) and list which function parameters are unused.
>>  I noticed that trace_raw_output_* functions have an unused flags
>> argument.  I assume this is is to make the argument list match up with
>> some other function, but I couldn't see which function(s) the
>> trace_raw_output_* functions are trying to match up with.  Which
>> functions are the trace_raw_output_* function's arguments trying to match?
>>
> 
> The match of other printing functions is in kernel/trace/trace_output.c
> 
> And is called from kernel/trace/trace.c where one place it does pass in
> flags:
> 
> 
> static enum print_line_t print_trace_fmt(struct trace_iterator *iter)
> {
> 	[..]
> 	if (event)
> 		return event->funcs->trace(iter, sym_flags, event);

Hi Steve,

Thanks for pointing to where the trace_raw_output_* functions are called from.

> 
> Now if they are still used, is something we should investigate.

For the trace_raw_output_* functions the flags arg all seems to be unused.  Are there other trace functions should examining for unused args?  With the extensive macro use I could see where some vestigial unused argument gets left behind.

> 
> This code has changed a lot over the years when that "flags" parameter was
> introduced. It could now just be a relic from the old implementations.
> 
> Thanks for pointing this out. If I get time I'll look deeper into it, but
> feel free to look at it yourself.
> 
> -- Steve
> 

Thanks again,

-Will