[PATCH v2 01/10] compiler/gcc: Emit build-time warning for GCC prior to version 4.8

[Will Deacon]

Prior to version 4.8, GCC may miscompile READ_ONCE() by erroneously
discarding the 'volatile' qualifier:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145

We've been working around this using some nasty hacks which make
READ_ONCE() both horribly complicated and also prevent us from enforcing
that it is only used on scalar types. Since GCC 4.8 is pretty old for
kernel builds now, emit a warning if we detect it during the build.

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 init/Kconfig            | 4 ++--
 scripts/Kconfig.include | 6 ++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index a34064a031a5..bdc2f1b1667b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -10,11 +10,11 @@ config DEFCONFIG_LIST
 	default "arch/$(ARCH)/defconfig"
 
 config CC_IS_GCC
-	def_bool $(success,$(CC) --version | head -n 1 | grep -q gcc)
+	def_bool $(cc-is-gcc)
 
 config GCC_VERSION
 	int
-	default $(shell,$(srctree)/scripts/gcc-version.sh $(CC)) if CC_IS_GCC
+	default $(gcc-version) if CC_IS_GCC
 	default 0
 
 config CC_IS_CLANG
diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
index d4adfbe42690..4e645a798b56 100644
--- a/scripts/Kconfig.include
+++ b/scripts/Kconfig.include
@@ -40,3 +40,9 @@ $(error-if,$(success, $(LD) -v | grep -q gold), gold linker '$(LD)' not supporte
 
 # gcc version including patch level
 gcc-version := $(shell,$(srctree)/scripts/gcc-version.sh $(CC))
+
+# Return y if the compiler is GCC, n otherwise
+cc-is-gcc := $(success,$(CC) --version | head -n 1 | grep -q gcc)
+
+# Warn if the compiler is GCC prior to 4.8
+$(warning-if,$(if-success,[ $(gcc-version) -lt 40800 ],$(cc-is-gcc),n),"Your compiler is old and may miscompile the kernel due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 - please upgrade it.")
-- 
2.25.0.341.g760bfbb309-goog