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

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Message-ID: <4A94D3A8.1090902@cn.fujitsu.com>
Date:	Wed, 26 Aug 2009 14:18:16 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload

CC: Mathieu

Peter Zijlstra wrote:
> On Tue, 2009-08-25 at 12:47 +0200, Peter Zijlstra wrote:
>> On Tue, 2009-08-25 at 12:39 +0200, Ingo Molnar wrote:
>>
>>>> Do you really wish to burden every tracepoint user with the extra 
>>>> logic needed to deal with modules?
>>> Not necessarily - i'm just outlining why i think that the 'dont 
>>> allow subsystems to utilize tracepoint callbacks' is a restriction 
>>> we should not live with voluntarily.
>> Well, unless someone has a bright idea that's what it comes down to. 
> 
> OK, I still think modules probing their own tracepoints its stupid [*],
> but what you could do is iterate the tracepoint's callback list and see
> if it has a callback outside of the module code section and then fail
> the unload.
> 

I don't think this can work, what if a new callback is registered after
this check? Seems there's no syncronization to guarantee the result of
the check can remain valid during module unload.

And the other proposal that bumping module refcnt in register_tracepoint_xxx(),
I found it hard to do so, because register() won't know where the
tracepoint is. For example:

   register_tracepoint_foo() is in module bar, while
   DEFINE_TRACE(foo) and trace_foo() is in module foo.

Note, you're allowed to load bar without foo.

The simplest fix is the patch I sent, which you don't like. Maybe
Mathieu, the auther of tracepoint, or some one else has some idea?

> [*] in the really utterly fundamentally wrong stupid class.

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