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

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Message-ID: <20130718152701.GB6588@redhat.com>
Date:	Thu, 18 Jul 2013 17:27:01 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.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

On 07/18, Masami Hiramatsu wrote:
>
> (2013/07/17 23:43), Oleg Nesterov wrote:
> > Once again, I am still not sure and I am asking for your review.
>
> OK,

Good ;)

> > 	- 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.

Speaking of event_enable_write() it needs the same mutex anyway.

> 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.

And this inc/dec needs event_mutex too, and the code is not trivial.

But yes, sure, I am not saying that it is always a win performance-wise.
In particular, with the patches I sent event/format holds event_mutex
between .start and .stop. But again, this is only to make the patch
simple. We can narrow the scope of this lock, we can switch to i_mutex
(needs the trivial change in invalidate_event_files) which should not
be contended.

And of course, sometimes it is better to do the "hard work" in .open()
and make .read/write as fast/simple as possible. But not in event/*
case, I think.

> > 	- 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?

But this check is not necessarily trivial too.

And to remind, personally I do not really like the fact that the
opened file blocks rmdir or unregister_probe_event().

To summarise. I believe that this approach is better (and simpler)
in general. But I understand that "better" is subjective, so I won't
argue. Not to mention, it can be simply wrong so I will heavily rely
on your/Steven's review anyway.

Oleg.

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