linux-kernel - [PATCH] kbuild: if_changed: document single use per target limitation

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Message-Id: <20180718091336.4540-1-dirk@gouders.net>
Date:   Wed, 18 Jul 2018 11:13:36 +0200
From:   Dirk Gouders <dirk@...ders.net>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Jonathan Corbet <corbet@....net>
Cc:     Dirk Gouders <dirk@...ders.net>, linux-kbuild@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] kbuild: if_changed: document single use per target limitation

Users of if_changed could easily feel invited to use it to divide a
recipe into parts like:

a: prereq FORCE
	$(call if_changed,do_a)
	$(call if_changed,do_b)

But this is problematic, because if_changed should not be used more
than once per target: in the above example, if_changed stores the
command-line of the given command in .a.cmd and when a is up-to-date
with respect to prereq, the file .a.cmd contains the command-line for
the last command executed, i.e. do_b.

When the recipe is then executed again, without any change of
prerequisites, the command-line check for do_a will fail, do_a will be
executed and stored in .a.cmd.  The next check, however, will still see
the old content (the file isn't re-read) and if_changed will skip
do_b, because the command-line test will not recognize a change.  On
the next execution of the recipe the roles will flip: do_a is OK but
do_b not and it will be executed.  And so on...

Signed-off-by: Dirk Gouders <dirk@...ders.net>
---
 Documentation/kbuild/makefiles.txt | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index 048fc39a6b91..01fae5a01ce7 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/Documentation/kbuild/makefiles.txt
@@ -1105,6 +1105,12 @@ When kbuild executes, the following steps are followed (roughly):
 		target: source(s) FORCE
 	#WRONG!#	$(call if_changed, ld/objcopy/gzip/...)

+        Note: if_changed should not be used more than once per target.
+              It stores the executed command in a corresponding .cmd
+        file and multiple calls would result in overwrites and
+        unwanted results when the target is up to date and only the
+        tests on changed commands trigger execution of commands.
+
     ld
 	Link target. Often, LDFLAGS_$@ is used to set specific options to ld.

-- 
2.16.1