[PATCH v3] selftests/cgroup: Make the object directory an order-only prerequisite

[Fuad Tabba <fuad.tabba@linux.dev>]

The cgroup_util.o rule in libcgroup.mk lists the object directory as a
normal prerequisite. The KVM selftests include libcgroup.mk, so for
them that directory is their own $(OUTPUT)/lib. After any incremental
KVM build that rebuilds an object there, the directory is newer than
cgroup_util.o, and the next build recompiles cgroup_util.o and relinks
every test.

Make the directory an order-only prerequisite, as libvfio.mk does.

Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---

Notes:
    This was patch 2/2 of v2 [1]. Sean took patch 1 through kvm-x86 and
    asked for this one to go through the cgroup tree [2].
    
    Changes since v2:
    - Collected Michal's Acked-by.
    - Rebased on cgroup/for-7.5.
    - Dropped the Fixes: tag, as the patch only saves a rebuild.
    
    [1] https://lore.kernel.org/all/20260922071945.693714-1-fuad.tabba@linux.dev/
    [2] https://lore.kernel.org/all/avJqZzKX1ZVAMGr-@seanjc-mac.roam.internal/

 tools/testing/selftests/cgroup/lib/libcgroup.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/cgroup/lib/libcgroup.mk b/tools/testing/selftests/cgroup/lib/libcgroup.mk
index 7a73007204c39..bcf6fe2516223 100644
--- a/tools/testing/selftests/cgroup/lib/libcgroup.mk
+++ b/tools/testing/selftests/cgroup/lib/libcgroup.mk
@@ -13,7 +13,7 @@ EXTRA_HDRS := $(selfdir)/clone3/clone3_selftests.h
 $(LIBCGROUP_O_DIRS):
 	mkdir -p $@
 
-$(LIBCGROUP_O): $(OUTPUT)/%.o : $(CGROUP_DIR)/%.c $(EXTRA_HDRS) $(LIBCGROUP_O_DIRS)
+$(LIBCGROUP_O): $(OUTPUT)/%.o : $(CGROUP_DIR)/%.c $(EXTRA_HDRS) | $(LIBCGROUP_O_DIRS)
 	$(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c $< -o $@
 
 EXTRA_CLEAN += $(LIBCGROUP_O)

base-commit: 80bf33fd523ee1df2d265d34970df670a44a2ccd
-- 
2.47.3