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Re: [Qemu-block] [Qemu-devel] [PATCH for-2.9] mirror: Fix backwards mirr

From: Daniel P. Berrange
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH for-2.9] mirror: Fix backwards mirror_yield parameters
Date: Wed, 15 Mar 2017 10:59:24 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

On Wed, Mar 15, 2017 at 10:26:44AM +0000, Daniel P. Berrange wrote:
> On Wed, Mar 15, 2017 at 06:18:35PM +0800, Stefan Hajnoczi wrote:
> > On Fri, Mar 10, 2017 at 02:49:22PM -0600, Eric Blake wrote:
> > > And here's where I'm stuck: the makefiles are broken.  Touching
> > > scripts/tracetool/format/h.py does NOT cause tracetool to be re-run by a
> > > mere 'make'; I've had to resort to 'make -B block/trace.h-timestamp' to
> > > get things to rebuild.  And this is in spite of the fact that h.py
> > > _should_ be getting listed in $(tracetool-y) by trace/Makefile.objs, and
> > > $(tracetool-y) is listed as a dependency of %/trace.h-timestamp in the
> > > top-level Makefile.  I would appreciate anyone with advice or an idea on
> > > how to patch Makefile to get the dependency working without me having to
> > > manually kick it.
> > 
> > Also CCing Daniel Berrange.  He recently touched the tracing Makefiles
> > and may have ideas.
> I've been looking at this and I'm damned if I understand what's broken.
> All the required dependancies look to be expressed in the Makefile
>   %/trace.h: %/trace.h-timestamp
>   %/trace.h-timestamp: $(SRC_PATH)/%/trace-events $(tracetool-y)
> and $(tracetool-y) expands to the list of source files
>   tracetool-y = $(SRC_PATH)/scripts/tracetool.py
>   tracetool-y += $(shell find $(SRC_PATH)/scripts/tracetool -name "*.py")
> If I do 'touch hw/net/trace-events', then hw/net/trace.h gets rebuilt,
> but if do 'touch scripts/tracetool.py' it doesn't get built. So somehow
> make seems to be dropping the $(tracetool-y) deps despite being listed
> against the %/trace-h-timestamp file, and despite earlier deps on
> trace-events being honoured

Oh this is a fun one.  While the $(tracetool-y) variable *is* defined at
the time the build rules execute, it is *not* defined at the time make
evaluates dependancies, so it expanded to be empty ! I copied you on
the easy fix

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