[coreboot] romtool and makefile breakage

[Carl-Daniel Hailfinger]

On 04.04.2009 20:40, svn at coreboot.org wrote:
> Author: stepan
> Date: 2009-04-04 20:40:46 +0200 (Sat, 04 Apr 2009)
> New Revision: 4068
>
> Modified:
>    trunk/coreboot-v2/src/arch/ppc/Config.lb
> Log:
> small workaround for romtool incompatibility with ppc ports
>
> Signed-off-by: Stefan Reinauer <stepan at coresystems.de>
> Acked-by: Stefan Reinauer <stepan at coresystems.de>
>   

No offense intended, but almost every commit intended to fix the build
since the introduction of romtool broke the build for some boards.

I'm very tempted to simply suggest reverting romtool until someone
figures out how to get a reliable and stable abuild.

I mean, look at the commits which tried to fix romtool breakage:
4049, 4054, 4061, 4062, 4067, 4068
and the build is still broken!

It seems the cause of the breakage is not understood.

Can we please require a full _parallel_ abuild on a
_multicore/multiprocessor_ machine for each "bugfix" before it is committed?

By the way, each of the make rules in the code snippet below is a pure
race condition:

> Modified: trunk/coreboot-v2/src/arch/ppc/Config.lb
> ===================================================================
> --- trunk/coreboot-v2/src/arch/ppc/Config.lb	2009-04-04 18:24:21 UTC (rev 4067)
> +++ trunk/coreboot-v2/src/arch/ppc/Config.lb	2009-04-04 18:40:46 UTC (rev 4068)
> @@ -1,10 +1,15 @@
>  ldscript init/ldscript.lb
>  
> -makerule coreboot.rom 
> +makerule coreboot.strip 
>  	depends	"coreboot" 
>  	action	"cp $< $@"
>  end
>  
> +makerule coreboot.rom 
> +	depends	"coreboot.strip" 
> +	action "cp $< $@"
> +end
> +
>  dir init
>  dir lib
>  dir boot
>   

"cp" and "cat" should be banned in makefiles. They are not atomic and if
they touch any files specified in any dependency rule, they WILL
randomly BREAK parallel compilation (or just result in silently
corrupted images) and work without problems if compilation is not
parallelized. Since almost everybody doesn't run abuild in parallel mode
on a multicore machine (except our build-bot), the breakage happens only
on the build-bot.

I just checked our v2 makefiles and every single usage of cp and every
single usage of cat looks broken.

Solution:
If you really have to use cp and cat, make sure the destination file is
not mentioned in any dependency rule. Use temporary files as destination
files and rename (mv) them to the real destination file. Make sure the
temp file and the destination file are in the same directory

Example:
Broken rule:

makerule a
	depends	"b"
	action "cp $< $@"
end

Fixed rule:
makerule a
	depends	"b"
	action "cp $< $someuniquetempname"
	action "mv $someuniquetempname $@"
end


someuniquetempname must be created (in the same directory as the
destination file of the rule) with mkstemp (or mktemp, if mkstemp is
unavailable) in that rule (not globally, or you get another race
condition), but my make-fu is too weak for this.

And while we're on the topic of makefile race conditions, please note
that gcc is smart enough to create output files atomically, but almost
every other program (romcc, romtool, cat, cp) is not. Non-smart programs
MUST (in the RFC sense) always output to files not specified in any
dependency rule. Creating smart programs is hard.


Sorry for the lengthy mail, it's meant to be helpful.


Regards,
Carl-Daniel

-- 
http://www.hailfinger.org/