linux-kernel - Re: 2.6.33 GP fault only when built with tracing

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Date:	Fri, 19 Mar 2010 11:22:07 -0700
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: 2.6.33 GP fault only when built with tracing

On 03/18/10 17:59, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@...dmis.org) wrote:
>> On Thu, 2010-03-18 at 16:26 -0700, Randy Dunlap wrote:
>>> I can build/boot 2.6.33 with CONFIG_TRACE/TRACING disabled successfully,
>>> but when I enable lots of tracing config options and then boot with
>>> ftrace=nop on the kernel command line, I see a GP fault when the parport &
>>> parport_pc modules are loading/initializing.
>>
>> Do you see it without adding the "ftrace=nop"? The only thing that
>> should do is expand the ring buffer on boot up.
>>
>>>
>>> It happens in drivers/parport/share.c::parport_register_device(), when that
>>> function calls try_module_get().
>>>
>>> If I comment out the trace_module_get() calls in include/linux/module.h,
>>> the kernel boots with no problems.
>>
>>
>> Interesting. Well, trace_module_get() is a TRACE_EVENT tracepoint. But
>> should be disabled here. It may be something to do with DEFINE_TRACE. 
>>
>> (added Mathieu to Cc since he wrote that code)
> 
> can you try replacing the "local_read(__module_ref_addr(module, cpu))" argument
> with "0" ?

Yes, that boots with no problems.

> Arguments with side-effects are not skipped by the jump over disabled
> instrumentation. This is why we should do that part within the probe declaration
> in the TRACE_EVENT macros.
> 
> But if we find out that the problem really is this argument, then it should be
> fixed, because something would be wrong with it (just moving it to TRACE_EVENT
> is not a proper solution).
> 
> Thanks,
> 
> Mathieu


-- 
~Randy
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