https://git.kernel.dk/cgit/blktrace/patch/?id=f4f8ef7cdea138cfaa2f3ca0ee31fa23d3bcf1cc

From f4f8ef7cdea138cfaa2f3ca0ee31fa23d3bcf1cc Mon Sep 17 00:00:00 2001
From: Gwendal Grignou <gwendal@chromium.org>
Date: Thu, 16 Jan 2020 12:33:26 -0800
Subject: fix parallel build of btt and blkiomon

rbtree.c is used by both binaries. It is possible that when make -C btt
is invoked rbtree.o does not exist yet, but is already schedule by the
compilation of blkiomon. That could result in recompiling rbtree.o again
for btt/btt.
In that case, at install time, make will recompile blkiomon which can
fail in gentoo, because CC variable is not overriden by ebuild script at
install time. (see https://bugs.gentoo.org/705594)

Add a dependency on SUBDIRS to wait for all binary in . to be compiled.
It will guarante rbtree.o exists.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 5917814..eb3c6a1 100644
--- a/Makefile
+++ b/Makefile
@@ -14,7 +14,7 @@ all: $(ALL) $(SUBDIRS)
 
 # We always descend into subdirs because they contain their own dependency
 # information which we don't track in this top level Makefile.
-$(SUBDIRS):
+$(SUBDIRS): $(PROGS)
 	$(MAKE) -C $@
 .PHONY: $(SUBDIRS)
 
-- 
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