linux-kernel - Re: [PATCH tracing/kprobes 5/7] tracing/kprobes: Add argument name support

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Date:	Fri, 11 Sep 2009 12:11:29 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	rostedt@...dmis.org
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Jim Keniston <jkenisto@...ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Tom Zanussi <tzanussi@...il.com>
Subject: Re: [PATCH tracing/kprobes 5/7] tracing/kprobes: Add argument name
 support

Steven Rostedt wrote:
> On Thu, 2009-09-10 at 19:53 -0400, Masami Hiramatsu wrote:
> 
>> @@ -870,9 +892,13 @@ print_kprobe_event(struct trace_iterator *iter, int flags)
>>   {
>>   	struct kprobe_trace_entry *field;
>>   	struct trace_seq *s =&iter->seq;
>> +	struct trace_event *event;
>> +	struct trace_probe *tp;
>>   	int i;
>>
>>   	field = (struct kprobe_trace_entry *)iter->ent;
>> +	event = ftrace_find_event(field->ent.type);
>> +	tp = container_of(event, struct trace_probe, event);
> 
> Can this function be called the data is in the ring buffer, but the
> probe has been unregistered? If so, the result of ftrace_find_event be
> NULL?

Hmm, it will depend on ftrace implementation. Before releasing
trace_probe, kprobe tracer tries to unregister event call.
If it's correctly locking mutex or some rw_lock for both of
unregistering and printing, it will be safe.

Unfortunately, it seems not :-(.

In trace_events.c,
1054 static void __trace_remove_event_call(struct ftrace_event_call *call)
1055 {
1056         ftrace_event_enable_disable(call, 0);
1057         if (call->event)
1058                 __unregister_ftrace_event(call->event);

What we need to do is calling unregister_ftrace_event() instead of
__unregister_ftrace_event.


Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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