[PATCH 0/2] kbuild: GCOV cleanups following the prefer-atomic fix

[PATCH 0/2] kbuild: GCOV cleanups following the prefer-atomic fix

[Konstantin Khorenko]

Two cleanups Peter Oberparleiter suggested while reviewing
commit 56cb9b7d96b2 ("gcov: use atomic counter updates to fix
concurrent access crashes"):

https://lore.kernel.org/e1b6e768-51af-468b-a729-0ed3fceef2f1@linux.ibm.com

Patch 1 makes the prefer-atomic try-run test reuse $(CFLAGS_GCOV)
instead of spelling the flags out again, so the test compiles with the
exact GCOV flags, -fno-tree-loop-im included.

Patch 2 moves the CFLAGS_GCOV block into scripts/Makefile.gcov,
included via include-$(CONFIG_GCOV_KERNEL) like the other
instrumentation makefiles. The try-run also stops running on builds
without GCOV.

Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
Konstantin Khorenko (2):
      kbuild: use $(CFLAGS_GCOV) in the prefer-atomic try-run test
      kbuild: move GCOV flags to scripts/Makefile.gcov

 MAINTAINERS           |  1 +
 Makefile              | 22 +---------------------
 scripts/Makefile.gcov | 21 +++++++++++++++++++++
 3 files changed, 23 insertions(+), 21 deletions(-)
---
base-commit: 551c722f40809618230001baccf219193e22fc5a
change-id: 20261001-b4-prep-gcov-82edd7efd6b9

Best regards,
--  
Konstantin Khorenko <khorenko@virtuozzo.com>

[PATCH 1/2] kbuild: use $(CFLAGS_GCOV) in the prefer-atomic try-run test

[Konstantin Khorenko]

Reference $(CFLAGS_GCOV) in the try-run test introduced by
commit 56cb9b7d96b2 ("gcov: use atomic counter updates to fix
concurrent access crashes"), otherwise it can miss potential future
changes in the list of flags used for GCOV profiling like it misses
-fno-tree-loop-im at the moment.

The outcome of the test is unchanged: -fno-tree-loop-im does not affect
the undefined symbols the test compares.

Suggested-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
 Makefile | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/Makefile b/Makefile
index 6b8b812f23b92..3b69b973f7598 100644
--- a/Makefile
+++ b/Makefile
@@ -1184,11 +1184,11 @@ CFLAGS_GCOV	+= -fno-tree-loop-im
 # undefined symbols (e.g. libatomic calls that the kernel cannot link).
 CFLAGS_GCOV	+= $(call try-run,\
 	echo 'long long x; void f(void){x++;}' | \
-	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -w -fprofile-arcs \
-	-ftest-coverage -x c - -c -o "$$TMP.base" && \
+	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -w $(CFLAGS_GCOV) \
+	-x c - -c -o "$$TMP.base" && \
 	echo 'long long x; void f(void){x++;}' | \
-	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -w -fprofile-arcs \
-	-ftest-coverage -fprofile-update=prefer-atomic \
+	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -w $(CFLAGS_GCOV) \
+	-fprofile-update=prefer-atomic \
 	-x c - -c -o "$$TMP" && \
 	$(NM) "$$TMP.base" | grep ' U ' > "$$TMP.ubase" || true ; \
 	$(NM) "$$TMP" | grep ' U ' > "$$TMP.utest" || true ; \

-- 
2.43.0

[PATCH 2/2] kbuild: move GCOV flags to scripts/Makefile.gcov

[Konstantin Khorenko]

 * split the CFLAGS_GCOV block out of the top-level Makefile into
   scripts/Makefile.gcov
 * include it via include-$(CONFIG_GCOV_KERNEL) like neighbour
   KASAN/KCOV/UBSAN makefiles do
 * keep the include before scripts/Makefile.gcc-plugins: the try-run
   must not get -fplugin= options, plugins do not exist on clean builds

CFLAGS_GCOV is no longer defined when CONFIG_GCOV_KERNEL is off:
 * its user in scripts/Makefile.lib is guarded by the CONFIG_GCOV_KERNEL
 * in i915/xe header an empty value is harmless

The try-run compiler invocations also stop running on builds without
GCOV.

Suggested-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
 MAINTAINERS           |  1 +
 Makefile              | 22 +---------------------
 scripts/Makefile.gcov | 21 +++++++++++++++++++++
 3 files changed, 23 insertions(+), 21 deletions(-)

diff --git a/MAINTAINERS b/MAINTAINERS
index 360977678f707..fabf33758351f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -10897,6 +10897,7 @@ M:	Peter Oberparleiter <oberpar@linux.ibm.com>
 S:	Maintained
 F:	Documentation/dev-tools/gcov.rst
 F:	kernel/gcov/
+F:	scripts/Makefile.gcov
 
 GDB KERNEL DEBUGGING HELPER SCRIPTS
 M:	Jan Kiszka <jan.kiszka@siemens.com>
diff --git a/Makefile b/Makefile
index 3b69b973f7598..f3ea94ceb1cec 100644
--- a/Makefile
+++ b/Makefile
@@ -1176,27 +1176,6 @@ endif
 # Ensure compilers do not transform certain loops into calls to wcslen()
 KBUILD_CFLAGS += -fno-builtin-wcslen
 
-CFLAGS_GCOV	:= -fprofile-arcs -ftest-coverage
-ifdef CONFIG_CC_IS_GCC
-CFLAGS_GCOV	+= -fno-tree-loop-im
-# Use atomic counter updates to avoid concurrent-access crashes in GCOV.
-# Only enable if -fprofile-update=prefer-atomic does not introduce new
-# undefined symbols (e.g. libatomic calls that the kernel cannot link).
-CFLAGS_GCOV	+= $(call try-run,\
-	echo 'long long x; void f(void){x++;}' | \
-	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -w $(CFLAGS_GCOV) \
-	-x c - -c -o "$$TMP.base" && \
-	echo 'long long x; void f(void){x++;}' | \
-	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -w $(CFLAGS_GCOV) \
-	-fprofile-update=prefer-atomic \
-	-x c - -c -o "$$TMP" && \
-	$(NM) "$$TMP.base" | grep ' U ' > "$$TMP.ubase" || true ; \
-	$(NM) "$$TMP" | grep ' U ' > "$$TMP.utest" || true ; \
-	cmp -s "$$TMP.ubase" "$$TMP.utest",\
-	-fprofile-update=prefer-atomic)
-endif
-export CFLAGS_GCOV
-
 # change __FILE__ to the relative path to the source directory
 ifdef building_out_of_srctree
 KBUILD_CPPFLAGS += -fmacro-prefix-map=$(srcroot)/=
@@ -1214,6 +1193,7 @@ include-$(CONFIG_KCSAN)		+= scripts/Makefile.kcsan
 include-$(CONFIG_KMSAN)		+= scripts/Makefile.kmsan
 include-$(CONFIG_UBSAN)		+= scripts/Makefile.ubsan
 include-$(CONFIG_KCOV)		+= scripts/Makefile.kcov
+include-$(CONFIG_GCOV_KERNEL)	+= scripts/Makefile.gcov
 include-$(CONFIG_RANDSTRUCT)	+= scripts/Makefile.randstruct
 include-$(CONFIG_KSTACK_ERASE)	+= scripts/Makefile.kstack_erase
 include-$(CONFIG_AUTOFDO_CLANG)	+= scripts/Makefile.autofdo
diff --git a/scripts/Makefile.gcov b/scripts/Makefile.gcov
new file mode 100644
index 0000000000000..16c5b876440c2
--- /dev/null
+++ b/scripts/Makefile.gcov
@@ -0,0 +1,21 @@
+# SPDX-License-Identifier: GPL-2.0-only
+CFLAGS_GCOV	:= -fprofile-arcs -ftest-coverage
+ifdef CONFIG_CC_IS_GCC
+CFLAGS_GCOV	+= -fno-tree-loop-im
+# Use atomic counter updates to avoid concurrent-access crashes in GCOV.
+# Only enable if -fprofile-update=prefer-atomic does not introduce new
+# undefined symbols (e.g. libatomic calls that the kernel cannot link).
+CFLAGS_GCOV	+= $(call try-run,\
+	echo 'long long x; void f(void){x++;}' | \
+	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -w $(CFLAGS_GCOV) \
+	-x c - -c -o "$$TMP.base" && \
+	echo 'long long x; void f(void){x++;}' | \
+	$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -w $(CFLAGS_GCOV) \
+	-fprofile-update=prefer-atomic \
+	-x c - -c -o "$$TMP" && \
+	$(NM) "$$TMP.base" | grep ' U ' > "$$TMP.ubase" || true ; \
+	$(NM) "$$TMP" | grep ' U ' > "$$TMP.utest" || true ; \
+	cmp -s "$$TMP.ubase" "$$TMP.utest",\
+	-fprofile-update=prefer-atomic)
+endif
+export CFLAGS_GCOV

-- 
2.43.0

Re: [PATCH 0/2] kbuild: GCOV cleanups following the prefer-atomic fix

[Peter Oberparleiter]

On 01.10.2026 18:57, Konstantin Khorenko wrote:
> Two cleanups Peter Oberparleiter suggested while reviewing
> commit 56cb9b7d96b2 ("gcov: use atomic counter updates to fix
> concurrent access crashes"):
> 
> https://lore.kernel.org/e1b6e768-51af-468b-a729-0ed3fceef2f1@linux.ibm.com
> 
> Patch 1 makes the prefer-atomic try-run test reuse $(CFLAGS_GCOV)
> instead of spelling the flags out again, so the test compiles with the
> exact GCOV flags, -fno-tree-loop-im included.
> 
> Patch 2 moves the CFLAGS_GCOV block into scripts/Makefile.gcov,
> included via include-$(CONFIG_GCOV_KERNEL) like the other
> instrumentation makefiles. The try-run also stops running on builds
> without GCOV.
> 
> Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>

Looks great, thanks for following up on this!

For both patches:

Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Tested-by: Peter Oberparleiter <oberpar@linux.ibm.com>

-- 
Peter Oberparleiter
Linux on IBM Z Development - IBM Germany R&D