There are people building the module with M= that's supposed to be used for external modules. This got broken in e9aa7c285d20 ("btrfs: enable W=1 checks for btrfs").
$ make M=fs/btrfs scripts/Makefile.lib:10: *** Recursive variable 'KBUILD_CFLAGS' references itself (eventually). Stop. make: *** [Makefile:1755: modules] Error 2 There's a difference compared to 'make fs/btrfs/btrfs.ko' which needs to rebuild a few more things and also the dependency modules need to be available. It could fail with eg. WARNING: Symbol version dump "Module.symvers" is missing. Modules may not have dependencies or modversions. In some environments it's more convenient to rebuild just the btrfs module by M= so let's make it work. The problem is with recursive variable evaluation in += so the conditional C options are stored in a temporary variable to avoid the recursion. Signed-off-by: David Sterba <dste...@suse.com> --- fs/btrfs/Makefile | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile index b634c42115ea..b4fb997eda16 100644 --- a/fs/btrfs/Makefile +++ b/fs/btrfs/Makefile @@ -7,10 +7,12 @@ subdir-ccflags-y += -Wmissing-format-attribute subdir-ccflags-y += -Wmissing-prototypes subdir-ccflags-y += -Wold-style-definition subdir-ccflags-y += -Wmissing-include-dirs -subdir-ccflags-y += $(call cc-option, -Wunused-but-set-variable) -subdir-ccflags-y += $(call cc-option, -Wunused-const-variable) -subdir-ccflags-y += $(call cc-option, -Wpacked-not-aligned) -subdir-ccflags-y += $(call cc-option, -Wstringop-truncation) +condflags := \ + $(call cc-option, -Wunused-but-set-variable) \ + $(call cc-option, -Wunused-const-variable) \ + $(call cc-option, -Wpacked-not-aligned) \ + $(call cc-option, -Wstringop-truncation) +subdir-ccflags-y += $(condflags) # The following turn off the warnings enabled by -Wextra subdir-ccflags-y += -Wno-missing-field-initializers subdir-ccflags-y += -Wno-sign-compare -- 2.29.2