linux-kernel - Re: Better way to force a rebuild

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Message-ID: <20081210200242.GA15692@uranus.ravnborg.org>
Date:	Wed, 10 Dec 2008 21:02:42 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kbuild@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	zippel@...ux-m68k.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Better way to force a rebuild

On Wed, Dec 10, 2008 at 02:48:35PM -0500, Steven Rostedt wrote:
> 
> Hi,
> 
> In include/linux/kernel.h I currently have the following lines at the 
> bottom of the file:
> 
> /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
> #ifdef CONFIG_FTRACE_MCOUNT_RECORD
> # define REBUILD_DUE_TO_FTRACE_MCOUNT_RECORD
> #endif
> 
> 
> This is only there to force a rebuild of all files when 
> CONFIG_FTRACE_MCOUNT_RECORD is modified. Since the modification of that 
> config causes the build to act different, we need to rebuild all C 
> objects. I added the #ifdef in kernel.h as a hack to force the rebuild by 
> the config dependencies used in kbuild.
> 
> My question is, is there a better way to force a full rebuild on a 
> modification of a config?

Why is that hack required in the first place?
We rebuild if:
- target file is missing (not the case here)
- any prerequisite files are newer than target (not the case here)
- any CONFIG_* options used by any prerequisite has changed (not the case here)
  [It is due to your hack]
- gcc changed (not the case here)
- options are added/deleted to gcc (this should be the case for relevant files)


Looking a bit deeper into this.
I see that we have in Makefile.build:

define rule_cc_o_c
        $(call echo-cmd,checksrc) $(cmd_checksrc)                         \
        $(call echo-cmd,cc_o_c) $(cmd_cc_o_c);                            \
        $(cmd_modversions)                                                \
        $(cmd_record_mcount)   

So you want everyting to be rebuild if the command above changes.
That should be doable - and it is indeed a bug that we do not do so.
The integration of record_mcount needs to be improved.


	Sam
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