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

From: Li Zefan
Date: Tue Aug 25 2009 - 02:35:16 EST

>>>>>>>> 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()
> Not tracepoint_probe_{un,}register(), in {un,}register_trace_$call().

Is there any difference?

static inline int register_trace_##name(void (*probe)(proto)) \
{ \
int ret; \
void (*func)(void) = reg; \
ret = tracepoint_probe_register(#name, (void *)probe); \
if (func && !ret) \
func(); \
return ret; \

> Basically avoid module unload when a tracepoint from that module has
> registered callbacks.

TRACE_EVENT() won't prevent this. Instead at module unload, a module
notifier callback will be called to unregistread those tracepoint callbacks.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at