linux-kernel - Re: [RFC PATCH v2] Tracepoint: register/unregister struct tracepoint

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Message-ID: <1670411611.2880.1394729656679.JavaMail.zimbra@efficios.com>
Date:	Thu, 13 Mar 2014 16:54:16 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	"Frank Ch. Eigler" <fche@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, 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: [RFC PATCH v2] Tracepoint: register/unregister struct
 tracepoint

----- Original Message -----
> From: "Frank Ch. Eigler" <fche@...hat.com>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> Cc: "Steven Rostedt" <rostedt@...dmis.org>, 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>
> Sent: Thursday, March 13, 2014 12:35:31 PM
> Subject: Re: [RFC PATCH v2] Tracepoint: register/unregister struct tracepoint
> 
> Hi -
> 
> On Thu, Mar 13, 2014 at 12:10:48PM -0400, Mathieu Desnoyers wrote:
> 
> > [...]  Moreover, tracers are responsible for unregistering the probe
> > before the module containing its associated tracepoint is unloaded.
> 
> Could you spell out please how a tracer is supposed to know early
> enough that the module is going to be unloaded?

There are two ways this can be done.

One use-case is when the probe and the callsite are within the same module,
or if the module containing the probe has a symbol dependency on the callsite.
In this case the probe normally unregisters itself from a module exit function
before the module unloads.

The other use-case is if the tracer has a module coming/going notifier
tracking the module's tracepoint callsites. The going notifier should
be run before the tracepoint.c going notifier. A notifier with negative
priority should have this effect, since the tracepoint.c notifier has
priority 0.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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