linux-kernel - Re: [PATCH] tracing/profile: Fix profile_disable vs module

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Date:	Thu, 27 Aug 2009 09:01:45 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload

Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@...dmis.org) wrote:
>> This patch solves the problem that Li originally reported. If something 
>> registers a trace point belonging to a module, then it ups the ref count 
>> of the module. This prevents a process from registering a probe to a 
>> tracepoint belonging to a module and then having the module disappear.
>>
>> Doing the example with perf in Li's original post, now errors on the 
>> rmmod, with "ERROR: Module trace_events_sample is in use".
>>
>> Mathieu, can I have your acked-by on this?
>>
> 
> Sorry, it looks buggy.
> 

And the patch itself is buggy.

> It does not deal with the fact that tracepoints with the same name and
> arguments can be present in more than one module, or in a combination of
> kernel core and modules.
> 
> The struct tracepoint_entry is specific to a a tracepoint name, used for
> registration, but is eventually tied to all tracepoint instrumentation
> instances for this tracepoint name.
> 
> Mathieu
> 

...

>> -tracepoint_update_probe_range(struct tracepoint *begin, struct tracepoint *end)
>> +tracepoint_update_probe_range(struct module *mod,
>> +			      struct tracepoint *begin, struct tracepoint *end)
>>  {
>>  	struct tracepoint *iter;
>>  	struct tracepoint_entry *mark_entry;
>> @@ -286,9 +291,15 @@ tracepoint_update_probe_range(struct tracepoint *begin, struct tracepoint *end)
>>  	for (iter = begin; iter < end; iter++) {
>>  		mark_entry = get_tracepoint(iter->name);
>>  		if (mark_entry) {
>> +			if (mod && !mark_entry->mod) {
>> +				if (!try_module_get(mod))
>> +					goto disable;

You can hit this code-path even when you unregister a probe,
so the right way is:
	module_get() when iter->state changed from 0 from 1
	module_put() when iter->state changed from 1 to 0

But still has some other problems. You don't fail the registration,
profile_enable() will return success even when the module is
being destructed.

>> +				mark_entry->mod = mod;
>> +			}
>>  			set_tracepoint(&mark_entry, iter,
>>  					!!mark_entry->refcount);
>>  		} else {
>> + disable:
>>  			disable_tracepoint(iter);
>>  		}
>>  	}
--
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