selftests/bpf: Prevent runqslower from racing on building bpftool

runqslower's Makefile is building/installing bpftool into
$(OUTPUT)/sbin/bpftool, which coincides with $(DEFAULT_BPFTOOL). In practice
this means that often when building selftests from scratch (after `make
clean`), selftests are racing with runqslower to simultaneously build bpftool
and one of the two processes fail due to file being busy. Prevent this race by
explicitly order-depending on $(BPFTOOL_DEFAULT).

Fixes: a2c9652f751e ("selftests: Refactor build to remove tools/lib/bpf from include path")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200805004757.2960750-1-andriin@fb.com

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index e7a8cf8..48425f9 100644 (file)
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -142,7 +142,9 @@ VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux)                                \
                     /boot/vmlinux-$(shell uname -r)
 VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
 
-$(OUTPUT)/runqslower: $(BPFOBJ)
+DEFAULT_BPFTOOL := $(SCRATCH_DIR)/sbin/bpftool
+
+$(OUTPUT)/runqslower: $(BPFOBJ) | $(DEFAULT_BPFTOOL)
        $(Q)$(MAKE) $(submake_extras) -C $(TOOLSDIR)/bpf/runqslower     \
                    OUTPUT=$(SCRATCH_DIR)/ VMLINUX_BTF=$(VMLINUX_BTF)   \
                    BPFOBJ=$(BPFOBJ) BPF_INCLUDE=$(INCLUDE_DIR) &&      \
@@ -164,7 +166,6 @@ $(OUTPUT)/test_netcnt: cgroup_helpers.c
 $(OUTPUT)/test_sock_fields: cgroup_helpers.c
 $(OUTPUT)/test_sysctl: cgroup_helpers.c
 
-DEFAULT_BPFTOOL := $(SCRATCH_DIR)/sbin/bpftool
 BPFTOOL ?= $(DEFAULT_BPFTOOL)
 $(DEFAULT_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile)    \
                    $(BPFOBJ) | $(BUILD_DIR)/bpftool