linux-kernel - Re: [RFC PATCH 0/4] tracing: fix open/delete fixes

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Message-ID: <51E7A4CF.2050404@hitachi.com>
Date:	Thu, 18 Jul 2013 17:18:23 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/4] tracing: fix open/delete fixes

(2013/07/17 23:43), Oleg Nesterov wrote:
> On 07/17, Masami Hiramatsu wrote:
>>
>> At a glance, you're trying to change which operation will be
>> failed. Currently, user can not remove an event while someone
>> opens files which related to the event. And this approach
>> changes that the someone can remove the event even if the
>> files are opened (and read/write operation will be failed).
>> Am I understand correctly?
> 
> Yes.
> 
> Once again, I am still not sure and I am asking for your review.

OK,

> But to me this looks much better. To simplify the discussion, lets
> consider ftrace_enable_fops in particular.
> 
> 	- Why should .open() block rmdir or unregister_uprobe_event?

Because it is opened and under preparing for use. :)
But, yeah, if we expect there is only one user using
ftrace, accessing the removing event file is meaningless.
It should be failed.

> 	- Why do we need .open() at all? Whatever it can do to
> 	  validate file/call/etc, .read/write can do the same.

Currently, just for preparing and reserving.

> 	- If we kill .open/release, we do not need the nontrivial
> 	  refcounting. Everything becomes simple, no need to keep
> 	  the state "in between".

That also means to refrain checking existence under locking mutex
in all operations. And we have to check it, which I actually concern.
refcounting is not so small and itself is complex, but it just
needs to inc/dec on open/close.


> 	  We need event_mutex anyway (and note that other f_op's can
> 	  also rely on other locks taken by trace_remove_event_call),
> 	  the validation degrades to the trivial != NULL check.
> 
> 	- This also simplifies trace_remove_event_call() paths, we
> 	  know that once it takes event_mutex nobody can play with
> 	  ftrace_event_file/ftrace_event_call we are going to free.

Hmm, it seems that we can remove only refcount check, or more?

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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