Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload

[Li Zefan]

>>>>>> If the correspoding module is unloaded before ftrace_profile_disable()
>>>>>> is called, event->profile_disable() won't be called, which can
>>>>>> cause oops:
>>>>>>
>>>>>>   # insmod trace-events-sample.ko
>>>>>>   # perf record -f -a -e sample:foo_bar sleep 3 &
>>>>>>   # sleep 1
>>>>>>   # rmmod trace_events_sample
>>>>>>   # insmod trace-events-sample.ko
>>>>>>   OOPS!
>>>>>>
>>>>>> Signed-off-by: Li Zefan <lizf@xxxxxxxxxxxxxx>
>>>>>
>>>>> Hrmm, feel fragile, why don't we check if all a modules tracepoints are
>>>>> unused on unload?
>>>>>
>>>> I don't think it's fragile. We are profiling via a module's
>>>> tracepoint, so we should pin the module, via module_get().
>>>> If event->profile_enable() has been calld, we should make
>>>> sure it's profile_disable() will be called.
>>> What I call fragile is that everyone registering a tracepoint 
>>> callback will now apparently need to worry about modules, _that_ 
>>> is fragile.
>>>
>>> Either make module unload look at tracepoint users, or place the 
>>> try_get_module() in the registration hooks so that regular users 
>>> don't need to worry about it.
>> The bug found by Li needs to be fixed obviously.
>>
>> I tend to agree with you that this does not appear to be the best 
>> place to do it: so you suggest to implicitly increase the module 
>> refcount on callback registr instead? (and releasing it when 
>> unregistering)
>>
>> Same end result, slightly cleaner place to bump the refcount.
> 
> Yes, because the user of tracepoints should never need to care about
> modules.
> 

I'm afraid it is not feasible to bump module refcnt implicitly
in tracepoint_probe_register().

If a tracepoint is registered in module_init, and unregistered
in module_exit (see sample/tracepoints), the module is unloadable:

 insmod
 ->call mod->init()
   ->trace_reg_foo()
     ->module_get()

 rmmod
 ->check mod refcnt
 ->call mod->exit()
   ->trace_unreg_foo()
     ->module_put()
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