linux-kernel - Re: [for-next][PATCH 08/20] tracing: Warn if a tracepoint is not set via debugfs

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Date:	Mon, 10 Mar 2014 22:58:20 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	fche@...hat.com (Frank Ch. Eigler)
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Berg <johannes.berg@...el.com>
Subject: Re: [for-next][PATCH 08/20] tracing: Warn if a tracepoint is not
 set via debugfs

On Mon, 10 Mar 2014 22:41:31 -0400
fche@...hat.com (Frank Ch. Eigler) wrote:

> 
> Hi -
> 
> 
> >> From: Steven Rostedt <rostedt@...dmis.org>
> >> 
> >> Tracepoints were made to allow enabling a tracepoint in a module before that
> >> module was loaded. When a tracepoint is enabled and it does not exist, the
> >> name is stored and will be enabled when the tracepoint is created.
> >> 
> >> The problem with this approach is that when a tracepoint is enabled when
> >> it expects to be there, it gives no warning that it does not exist.
> 
> So it is a deferred-activation kind of call, with no way of knowing
> when or if the tracepoints will start coming in.  Why is that
> supported at all, considering that clients could monitor modules
> coming & going via the module_notifier chain, and update registration
> at that time?

That's my argument.

> 
> 
> >> +	entry = get_tracepoint(name);
> >> +	/* Make sure the entry was enabled */
> >> +	if (!entry || !entry->enabled)
> >> +		ret = -ENODEV;
> 
> For what it's worth, I agree with Mathieu that this sort of successful
> failure result code is a problem for tracking what needs cleanup and
> what doesn't.  (In systemtap's case, if this change gets merged, we'll
> have to treat -ENODEV as if it were 0.)

Does systemtap enable tracepoints before they are created? That is, do
you allow enabling of a tracepoint in a module that is not loaded yet?

If not, than you want this as an error.

-- Steve
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