linux-kernel - Re: [RFC][PATCH] writeback: Unduplicate writeback reason

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Date:	Wed, 14 Dec 2011 14:28:44 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	linux-fsdevel@...r.kernel.org, Curt Wohlgemuth <curtw@...gle.com>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] writeback: Unduplicate writeback reason

On Tue, Dec 13, 2011 at 08:14:00PM -0500, Steven Rostedt wrote:
> Names of the writeback reasons are used in both the main kernel as well
> as for parsing the tracepoint format file. Instead of duplicating the
> names in two locations making it likely that they may become out of
> sync, use some macro magic to make sure all the names stay in sync. Any
> update only needs to happen in one spot for it to take place in all
> locations.
> 
> Note, this is an RFC patch, and it probably needs much better comments
> (well, it currently has no comments), and the C() macro probably should
> have a different name too.

I'm not sure this is a pattern we want to repeat all over the place -
print_symbolic() is quite widely used and adding macro redefinitions
all over the place doesn't fill me with joy.

AFAICT this code doesn't need a declared array to work so you can
just use a preprocessor construct like this (as used in XFS):

#define value_1 	1
#define value_2		2
.....

or

enum {
	value_1	= 1,
	value_2 = 2,
	.....
}

followed by:

#define VALUES	\
	{ value_1,	"Value 1" }, \
	{ value_2,	"Value 2" }, \
	.....

And it just uses print_symbolic(__entry->value, VALUES); to print
them out.

If this construct does everything requiredi, then I think it is a
much better pattern to use because it's easy to maintain, doesn't
require an array to be declared in a C file and doesn't require
macro tricks to do it's job....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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