CMake: Failed to compile with -j20.

[logic-wei]

How to reproduce:

1. mkdir build; cd build
2. cmake ../examples/c/
3. make -j20 # It's easily pass
4. make clean
5. make -j20 # It's easily fail.

The key log:

[ 25%] No configure step for 'bpftool' [20/720]
[ 29%] Performing build step for 'libbpf'
[ 29%] Performing build step for 'bpftool'
[ 29%] Performing build step for 'blazesym'
[ 29%] Performing build step for 'bpftool'
[ 30%] Performing build step for 'libbpf'
make[3]: warning: jobserver unavailable: using -j1. Add '+' to parent make rule.
make[3]: warning: jobserver unavailable: using -j1. Add '+' to parent make rule.
make[3]: warning: jobserver unavailable: using -j1. Add '+' to parent make rule.
make[3]: warning: jobserver unavailable: using -j1. Add '+' to parent make rule.
INSTALL bpf.h libbpf.h btf.h libbpf_common.h libbpf_legacy.h bpf_helpers.h bpf_helper_defs.h bpf_tracing.h bpf_endian.h bpf_core_read.h skel_internal.h libbpf_version.h usdt.bpf.h
INSTALL bpf.h libbpf.h btf.h libbpf_common.h libbpf_legacy.h bpf_helpers.h bpf_helper_defs.h bpf_tracing.h bpf_endian.h bpf_core_read.h skel_internal.h libbpf_version.h usdt.bpf.h
install: cannot create regular file '/home/logic/workspace/opensource/libbpf-bootstrap/build/libbpf/bpf/bpf_endian.h': File exists
make[3]: *** [Makefile:164: install_headers] Error 1
INSTALL /home/logic/workspace/opensource/libbpf-bootstrap/build/libbpf/libbpf/libbpf.pc
make[2]: *** [CMakeFiles/libbpf.dir/build.make:86: libbpf/src/libbpf-stamp/libbpf-build] Error 2
make[1]: *** [CMakeFiles/Makefile2:120: CMakeFiles/libbpf.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
INSTALL /home/logic/workspace/opensource/libbpf-bootstrap/build/libbpf/libbpf/libbpf.a
INSTALL ../include/uapi/linux/bpf.h ../include/uapi/linux/bpf_common.h ../include/uapi/linux/btf.h
[ 30%] Built target libbpf-build
Finished release profile [optimized + debuginfo] target(s) in 0.05s
[ 31%] No install step for 'blazesym'
[ 31%] Completed 'blazesym'
[ 31%] Built target blazesym
... libbfd: [ on ]
... libbfd: [ on ]
... clang-bpf-co-re: [ on ]
... clang-bpf-co-re: [ on ]
... llvm: [ on ]
... llvm: [ on ]
... libcap: [ on ]
... libcap: [ on ]
[ 32%] No install step for 'bpftool'
[ 32%] Built target bpftool-build
[ 34%] Completed 'bpftool'
[ 34%] Built target bpftool
make: *** [Makefile:91: all] Error 2

[logic-wei]

I think the problem is that the libbpf is compiled as dependency in parrallel. It's a race condition. More than one libbpf compiling process are creating the same file using command "install". So it raise the error.

The two same logs can prove this:

INSTALL bpf.h libbpf.h btf.h libbpf_common.h libbpf_legacy.h bpf_helpers.h bpf_helper_defs.h bpf_tracing.h bpf_endian.h bpf_core_read.h skel_internal.h libbpf_version.h usdt.bpf.h
INSTALL bpf.h libbpf.h btf.h libbpf_common.h libbpf_legacy.h bpf_helpers.h bpf_helper_defs.h bpf_tracing.h bpf_endian.h bpf_core_read.h skel_internal.h libbpf_version.h usdt.bpf.h

[logic-wei]

Before the error log, we can see these log below:

[ 4%] Creating directories for 'blazesym'
[ 5%] Creating directories for 'libbpf'
[ 5%] Creating directories for 'bpftool'
[ 4%] Creating directories for 'bpftool'
[ 5%] Creating directories for 'libbpf'
[ 6%] No download step for 'libbpf'
[ 9%] No download step for 'blazesym'
[ 9%] No download step for 'bpftool'
[ 9%] No download step for 'bpftool'
[ 9%] No download step for 'libbpf'
[ 14%] No update step for 'libbpf'
[ 14%] No update step for 'blazesym'
[ 14%] No update step for 'bpftool'
[ 14%] No update step for 'libbpf'
[ 14%] No update step for 'bpftool'
[ 20%] No patch step for 'libbpf'
[ 20%] No patch step for 'blazesym'
[ 20%] No patch step for 'bpftool'
[ 20%] No patch step for 'bpftool'
[ 20%] No patch step for 'libbpf'
[ 24%] No configure step for 'bpftool'
[ 24%] No configure step for 'blazesym'
[ 24%] No configure step for 'libbpf'
[ 24%] No configure step for 'bpftool'
[ 25%] No configure step for 'libbpf'
[ 28%] Performing build step for 'libbpf'
[ 28%] Performing build step for 'bpftool'
[ 30%] Performing build step for 'libbpf'
[ 30%] Performing build step for 'bpftool'
[ 30%] Performing build step for 'blazesym'

There're two build tasks of the libbpf and bpftool building at the same time! So that's why it fail.