Re: [PATCH] libxl: fix incremental parallel build

From: Wei Liu <wei.liu2@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Ian Jackson <Ian.Jackson@eu.citrix.com>
Subject: Re: [PATCH] libxl: fix incremental parallel build
Date: Fri, 1 Sep 2017 16:28:24 +0100	[thread overview]
Message-ID: <20170901152824.vg525wpi6sc2i32t@citrix.com> (raw)
In-Reply-To: <59A6800202000078001755C4@prv-mh.provo.novell.com>

On Wed, Aug 30, 2017 at 01:06:10AM -0600, Jan Beulich wrote:
> Short-circuit absolute paths of generated headers to their pathless
> equivalents, to avoid two racing invocations of the same rule producing
> them.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> This wants to be backported to 4.9 and 4.8.
> 
> --- a/tools/libxl/Makefile
> +++ b/tools/libxl/Makefile
> @@ -264,6 +264,13 @@ $(LIBXL_OBJS) $(LIBXL_TEST_OBJS) $(LIBXL
>  	$(TEST_PROG_OBJS) $(SAVE_HELPER_OBJS): libxl.h
>  $(LIBXL_OBJS) $(LIBXL_TEST_OBJS): libxl_internal.h
>  
> +# libacpi sources (living in another subdirectory) will have their inclusions
> +# of headers from this directory recorded as absolute paths in the .*.o.d
> +# files.  In order to not invoke the subsequent rule twice (and perhaps in a
> +# racing way when doing a parallel build), short-circuit the absolute paths to
> +# the local ones first.
> +$(CURDIR)/_libxl_type%.h: _libxl_type%.h ;

I don't quite get this. I normally build with -j8 and never saw a race.

Do you mean parallel build in which two makes enter libxl? Is that
possible?

Why does libacpi matter? All dependencies files (*.o.d) should be local
to libxl anyway.

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