linux-kernel - [PATCH] kbuild: do not append NOSTDINC_FLAGS to avoid rebuild in package targets

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Date:	Tue,  7 Jun 2016 14:18:01 +0900
From:	Masahiro Yamada <yamada.masahiro@...ionext.com>
To:	linux-kbuild@...r.kernel.org
Cc:	Rik van Riel <riel@...hat.com>,
	Paulo Zanoni <paulo.r.zanoni@...el.com>,
	Michal Marek <mmarek@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	Michal Marek <mmarek@...e.com>, linux-kernel@...r.kernel.org
Subject: [PATCH] kbuild: do not append NOSTDINC_FLAGS to avoid rebuild in package targets

For package building, the scripts/package/Makefile invokes
"$(MAKE) KBUILD_SRC=", so the top Makefile is invoked recursively.

Notice NOSTDINC_FLAGS is exported and assigned with "+=", not ":=".
It means, NOSTDINC_FLAGS is accumulated in the call loop of package
build: top Makefile -> scripts/package/Makefile -> top Makefile.

Before, it was not a big deal because GCC just ignores the repeated
compile options, and $(call if_changed,...) compared the old/new
commands as a set of arguments.

However, the situation was changed by commit 9c8fa9bc08f6 ("kbuild:
fix if_change and friends to consider argument order").  Now Kbuild
compares old/new commands more precisely.

Since then, "make" followed by "make targz-pkg" always rebuilds the
whole kernel.  The NOSTDINC_FLAGS is added just once for "make",
whilst twice for "make targz-pkg" as mentioned above.  So, Kbuild
considers that the build command has changed despite that we want a
tarball straight away if everything is already built.

The easiest way to fix this problem is to change "+=" to ":="
for NOSTDINC_FLAGS assignment.

This effectively reverts commit e8f5bdb02ce0 ("[PATCH] Makefile
include path ordering").  Its log says that the arch Makefile may
override the include path order, but I see no arch Makefile touching
NOSTDINC_FLAGS.  So, this change should have no impact unless
something outside of the kernel needs to override it.

Reported-by: Paulo Zanoni <paulo.r.zanoni@...el.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
---

This patch is fixing the problem reported by:
http://www.gossamer-threads.com/lists/linux/kernel/2454440

Paulo,
Thanks for your report!

Rik,
This patch is reverting your commit applied more than a decide ago.
As far as I see the kernel tree, I am not sure if it is still needed.
I am not familiar with Xen at all, so your review is very appreciated.

 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 8d1301a..58c06be 100644
--- a/Makefile
+++ b/Makefile
@@ -767,7 +767,7 @@ KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
 endif

 # arch Makefile may override CC so keep this after arch Makefile is included
-NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
+NOSTDINC_FLAGS := -nostdinc -isystem $(shell $(CC) -print-file-name=include)
 CHECKFLAGS     += $(NOSTDINC_FLAGS)

 # warn about C99 declaration after statement
-- 
1.9.1