linux-kernel - Re: [PATCH 1/7] tracing: Introduce trace_create_cpu_file() and tracing_get

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Date:	Mon, 22 Jul 2013 19:14:35 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Alexander Z Lam <azl@...gle.com>,
	David Sharp <dhsharp@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Vaibhav Nagarnaik <vnagarnaik@...gle.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] tracing: Introduce trace_create_cpu_file() and
	tracing_get_cpu()

On 07/22, Steven Rostedt wrote:
>
> Now here's why I'm emailing you.  What Oleg is doing here is instead of
> creating this extra trace_cpu structure, he's using the inode->i_cdev to
> store the CPU information (he's wrapped this with helper functions so we
> can use any inode structure). He sets inode->i_cdev to CPU+1 or to
> RING_BUFFER_ALL_CPUS (when all CPU info is needed).

This doesn't really matter, but RING_BUFFER_ALL_CPUS is encoded as NULL
so we do not need to change init_tracer_debugfs(). inode_init_always()
clears ->i_cdev.

Al, I will appreciate it if can ack/nack this hack.

I chose i_cdev because it shares the same union with i_pipe/bdev, this
(I hope) obviously means that vfs can never use this pointer unless it
checks S_ISCHR().

Oleg.

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