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

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Message-ID: <20140311142650.GB13756@redhat.com>
Date:	Tue, 11 Mar 2014 10:26:50 -0400
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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

Hi, Steven -

> > 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.

Was there an answer?


> > >> +	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?

We have no formal opinion on whether or not this makes sense.  If the
kernel permits it, fine.

> If not, than you want this as an error.

But it's not exactly an error!  It's a success of sorts, and means
that later on we have to unregister the callback, just as if it were
successful.


- FChE
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