linux-kernel - Re: [PATCH] tracing: Export key trace event symbols

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Date:	Mon, 27 Apr 2015 14:12:10 -0500
From:	Ron Rechenmacher <ron@...l.gov>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	<linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] tracing: Export key trace event symbols

Steven Rostedt wrote on 04/24/15 16:24:
> On Mon, 20 Apr 2015 16:38:11 -0500
> Ron Rechenmacher <ron@...l.gov> wrote:
>
>> If symbols are not exported, modules can no longer register additional
>> (module specified) tracepoints like they use to be able to (i.e linux-3.15.x).
>> Somewhere on or about commit de7b2973903c6cc50b31ee5682a69b2219b9919d
>> (Author: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>> Date:   Tue Apr 8 17:26:21 2014 -0400
>> tracepoint: Use struct pointer instead of name hash for reg/unreg tracepoints)
>> modules which attempted to register additional tracing functions would
>> get "Unknown symbol" errors. For example: "... Unknown symbol
>> __tracepoint_sched_switch (err 0)"
>> Symbols can be exported using the kernel's EXPORT_TRACEPOINT_SYMBOL_GPL macro
>> to allow modules to once again register their own tracing functions (for at
>> least some key points in the kernel as provided by this patch).
>>
>> Signed-off-by: Ron Rechenmacher <ron@...l.gov>
>> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=96051
>
> Hi Ron,
>
> I was talking with Mathieu on IRC and asked him how LTTng gets its
> kernel tracepoints, and he told me he uses
> for_each_kernel_tracepoint(). That will iterate over all tracepoints
> that have been added in the kernel (and is exported GPL).
>
> You can still use that to get the handle onto any tracepoint you need.
> It's pretty straight forward (I just wrote a simple module to test it
> out), and just compare against the tp->name, to find what you want.

Excellent. Thanks Steve. I was able to get this to work 100% for me.
What should be done about the bug report (96051)?
Will you handle it or should I change it to "resolved as invalid"?

>
> I still would like to get more usage out of the internal code, but this
> is your work around you wanted. No need to export new symbols. Just a
> little more setup time on module load.

Do you think for_each_kernel_tracepoint() and, additionally, the
tracepoint_probe_register() and tracepoint_probe_unregister() functions
will be around for quite some time? I would imagine as long as
LTTng uses them.

Thanks,
Ron

>
> -- Steve
>
> Here's my mod....
>
>
> #include <linux/module.h>
> #include <linux/ftrace.h>
> #include <linux/tracepoint.h>
>
> static func(struct tracepoint *tp, void *ignore)
> {
> 	printk("tracepoint: %s\n", tp->name);
> }
>
> static int __init my_tp_init(void)
> {
> 	for_each_kernel_tracepoint(func, NULL);
> 	return 0;
> }
>
> static void __exit my_tp_exit(void)
> {
> }
>
> module_init(my_tp_init);
> module_exit(my_tp_exit);
>
> MODULE_AUTHOR("My name here");
> MODULE_DESCRIPTION("Me!");
> MODULE_LICENSE("GPL");
>

-- 
Ron Rechenmacher
Engineer
Fermi National Accelerator Laboratory
Batavia, IL  60510
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