linux-kernel - Re: Re: [RFC PATCH V2] tracing: Check f_dentry before accessing event_file/call in inode->i

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Date:	Fri, 19 Jul 2013 14:21:04 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	jovi.zhangwei@...wei.com, Jiri Olsa <jolsa@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: Re: [RFC PATCH V2] tracing: Check f_dentry before accessing
 event_file/call in inode->i_private

(2013/07/18 23:51), Oleg Nesterov wrote:
> On 07/18, Masami Hiramatsu wrote:
>>
>> (2013/07/17 23:51), Oleg Nesterov wrote:
>>> Well, perhaps you are right... But this TRACE_EVENT_FL_REF_MASK code
>>> is new too, it is not that we only need a small fixlets to finish it.
>>
>> Would you mean that TRACE_EVENT_FL_REF_MASK may also have some problems?
> 
> It was you who initially pointed that it does have problems ;)
> 
> And, _afaics_ your patch which tries to fix this problem is not
> exactly correct.

Hm,

> It removes trace_array_get/put from tracing_open_generic_file() and
> tracing_release_generic_file(). This assumes that "call->flags++" is
> enough, but it is not.

No, it replaces trace_array_get/put with ftrace_event_file_get/put
which calls trace_array_get/put inside.
(Just one point, previous ftrace_event_file_get has a racy point
when it does tr->ref++, it should be fixed.)

> Yes, the next patch adds the "flags & TRACE_EVENT_FL_REF_MASK" check
> into trace_remove_event_call() path. But this is still racy wrt
> instance_delete() unless I missed something.
> 
> IOW, I believe that either .open() should do trace_array_get(), or
> __trace_remove_event_dirs() needs another for-each-file loop which
> checks file->call->flags & TRACE_EVENT_FL_REF_MASK.

Agreed :)

>>> So I think that it makes sense to discuss the alternatives before we
>>> decide what exactly we should do.
>>
>> Your approach is also interesting for me, indeed. However, it is so
>> different from current one. I think you should clarify what bug you
>> would like to solve and how.
> 
> The same bugs which Steven's 1/4 tries to solve ;)

OK, let me confirm that, would you mean we still need 2/4 - 4/4?

Thank you,
-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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