Re: [PATCH 2/2] tracing: Export ftrace API for kernel modules

[Atsushi Tsuji <a-tsuji@bk.jp.nec.com>]

Steven Rostedt wrote:
> On Wed, 2009-09-16 at 15:09 +0900, Atsushi Tsuji wrote:
> 
>>> I don't want to disable preemption when I don't have to. The function
>>> tracer that is called can. But actually, it's ever more that that. If
>>> you only register a single function, it will call that function
>>> directly. Then there will always be a race window between when the
>>> function gets called and disabling preemption, even if the called
>>> function disables preemption as the first thing it does.
>> Thank you for detailed explanation.
>>
>> I may be wrong, but I think function_trace_probe_call using 
>> register_ftrace_function_probe is almost enough for modules,
> 
> Heh, I forgot about function_probe. Yeah, looking at that, it does seem
> that it would be safe for modules.
> 
>> since it disables preemption while a probe is calling and it
>> called every time even if only one probe function is registered.
>> So is it enough to make a new registering function using
>> it and upping module ref count for module safe?  
> 
> Yes, upping the module ref count for every function probe would be
> required. Unfortunately, this would require adding another variable to
> struct ftrace_func_probe, which I hate to do.
> 
> 
>> Or should I make another handler for modules not using
>> function_trace_probe_call?
> 
> Maybe another handler might be better. But it may be similar to
> function_probe.

I see. I'll try to make a new handler for modules.

Thanks,
Atsushi

> 
>>>>> It will still need to up the mod ref count when a probe is added, but it
>>>>> can also remove it.
>>>>>
>>>>>
>>>>> The problem with the current method, is that a probe can be executing at
>>>>> anytime. Here's an example if we did it your way.
>>>>>
>>>>> 1. module installed
>>>>> 2. module adds probe
>>>>> 3. function X in kernel calls probe but gets preempted.
>>>>> 4. module removes probe
>>>>> 5. module unistalled
>>>>> 6. function X in kernel continues to run probe but probe no longer
>>>>> exists --- Oops!
>>>> Agreed, if mcount doesn't disable preemption, this will happen.
>>> And it does not.
>> I think the preemption is disabled in not register_ftrace_function
>> but register_ftrace_function_probe, is that wrong?
> 
> No that's correct. It's been a while since I worked on the probe code,
> so I forgot about it :-)
> 
> -- Steve
> 
> 
> 
>