linux-kernel - Re: [PATCH] tracing: Export tracing_start() and tracing

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Date:   Mon, 7 Jun 2021 09:14:32 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Ingo Molnar <mingo@...hat.com>, kernel@...s.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: Export tracing_start() and tracing_stop()

On Mon, 7 Jun 2021 09:25:19 +0100
Christoph Hellwig <hch@...radead.org> wrote:

> On Wed, Jun 02, 2021 at 10:01:18AM +0200, Vincent Whitchurch wrote:
> > tracing_stop() is very useful during hands-on debugging for getting the
> > trace to stop exactly when the problem is detected.  Export this to
> > modules.
> > 
> > Personally, I haven't yet found the need to use tracing_start() from
> > code since I usually start tracing via tracefs, but export that too for
> > symmetry since it may have its uses together with tracing_stop().  
> 
> NAK, no exports for unused symbols.

Normally I would NAK this NAK because this is not normal functionality
that modules could use. It's for debugging, similar to trace_printk(),
and there should be no used symbols in any modules. This is something
for debugging purposes only and should never be in shipped kernels.

That said though, tracing_stop() is probably not what is wanted
(unless its for a suspend to ram thing). According to the above
description, the author really wants to use "tracing_off()" and not
"tracing_stop()" as tracing_off() is faster and can be turned back on
in user-space with the "tracing_on" file in tracefs, where as,
tracing_stop() can not be. tracing_stop() needs a tracing_start() to
get it going again.

And tracing_off() is already EXPORT_SYMBOL_GPL() (as it is commonly
used for debugging of modules). Again, it shouldn't have any in-kernel
users in modules, because, like I stated above, it's similar to
trace_printk() which should be removed before pushing to Linus.

I'll NAK this patch for a different reason. Use tracing_off() instead.

-- Steve