netdev - Re: linux-next: build failure after merge of the net tree

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Date:   Tue, 14 Feb 2017 10:54:30 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        David Miller <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>, linux-next@...r.kernel.org,
        linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: linux-next: build failure after merge of the net tree

Em Tue, Feb 14, 2017 at 02:23:26PM +0100, Jiri Olsa escreveu:
> On Tue, Feb 14, 2017 at 09:50:20AM -0300, Arnaldo Carvalho de Melo wrote:
> 
> SNIP
> 
> > 
> > What I think Ingo meant with dependency at the build system level is to
> > somehow state that if file A gets changed, then tool B must be rebuilt.
> > 
> > Now that samples/bpf and tools/perf/ depend on tools/lib/bpf/ I _always_
> > build both, ditto for tools/objtool, that shares a different library
> > with tools/perf/, tools/lib/subcmd/:
> > 
> > ENTRYPOINT make -C /git/linux/tools/perf O=/tmp/build/perf && \
> >            rm -rf /tmp/build/perf/{.[^.]*,*} && \
> >            make NO_LIBELF=1 -C /git/linux/tools/perf O=/tmp/build/perf && \
> >            make -C /git/linux/tools/objtool O=/tmp/build/objtool && \
> >            make -C /git/linux O=/tmp/build/linux allmodconfig && \
> >            make -C /git/linux O=/tmp/build/linux headers_install && \
> >            make -C /git/linux O=/tmp/build/linux samples/bpf/
> > 
> > This is the default action for my
> > docker.io/acmel/linux-perf-tools-build-fedora:rawhide container.
> > 
> > It is published, so a:
> > 
> >    docker pull docker.io/acmel/linux-perf-tools-build-fedora:rawhide
> > 
> > And then run it before pushing things upstream would catch these kinds
> > of errors.
> > 
> > But that would possibly disrupt too much people's workflow, that is why
> > using the Kbuild originated tools/build/ we have to somehow express that
> > when a change is made in a file then a tool that uses that file needs to
> > be rebuilt.
> 
> we already have the check in the check-headers.sh script,
> an AFAICS there's no 'rebuild' option here.. just warn or fail
> because the headers update needs to be done manualy

... when needed. And that will only be detected if you try to build
tools using what is in tools/include/linux/bpf.h

Tools using tools/lib/bpf/ _must_ use what is in tools/include/.

So lemme see if my reasoning is right:

tools/lib/bpf/bpf.c has:

  #include <linux/bpf.h>

Now, samples/bpf/ will build tools/lib/bpf/bpf.o:

# Libbpf dependencies
LIBBPF := ../../tools/lib/bpf/bpf.o

HOSTCFLAGS += -I$(objtree)/usr/include
HOSTCFLAGS += -I$(srctree)/tools/lib/
HOSTCFLAGS += -I$(srctree)/tools/testing/selftests/bpf/
HOSTCFLAGS += -I$(srctree)/tools/lib/ -I$(srctree)/tools/include
HOSTCFLAGS += -I$(srctree)/tools/perf

HOSTCFLAGS_bpf_load.o += -I$(objtree)/usr/include -Wno-unused-variable

So it will never include tools/include/uapi/linux/bpf.h, which it
should.

Because the workflow people working on sample/bpf/ is to first install
the new headers using a variation of:

  make headers_install

So they will get the new bpf.h, not use tools/include/uapi/linux/bpf.h,
b00m.

They should use tools/include/uapi/linux/bpf.h, which is the one we know
builds well with tools/lib/bpf/bpf.c, since we tested it last time we
made the copy.
 
> > Makefile rules probably would be enough, but then it would have to be
> > done at the tools/build/ level and all tools using shared components
> > would have to use it to trigger the rebuild.
 
> we can move/invoke the check-headers.sh script in some upper dir

Most of the time I just ignore that warning, only when I find spare time
I go look if the changes in the kernel copy, i.e. upstream, should
trigger changes in the tools using its copy in tools/include/.

- Arnaldo