A recent change to the optimization pipeline in LLVM reveals some
fragility around the inlining of LoongArch's __percpu functions, which
manifests as a BUILD_BUG() failure:
In file included from kernel/sched/build_policy.c:17:
In file included from include/linux/sched/cputime.h:5:
In file included from include/linux/sched/signal.h:5:
In file included from include/linux/rculist.h:11:
In file included from include/linux/rcupdate.h:26:
In file included from include/linux/irqflags.h:18:
arch/loongarch/include/asm/percpu.h:97:3: error: call to '__compiletime_assert_51' declared with 'error' attribute: BUILD_BUG failed
97 | BUILD_BUG();
| ^
include/linux/build_bug.h:59:21: note: expanded from macro 'BUILD_BUG'
59 | #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
| ^
include/linux/build_bug.h:39:37: note: expanded from macro 'BUILD_BUG_ON_MSG'
39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
| ^
include/linux/compiler_types.h:425:2: note: expanded from macro 'compiletime_assert'
425 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
| ^
include/linux/compiler_types.h:413:2: note: expanded from macro '_compiletime_assert'
413 | __compiletime_assert(condition, msg, prefix, suffix)
| ^
include/linux/compiler_types.h:406:4: note: expanded from macro '__compiletime_assert'
406 | prefix ## suffix(); \
| ^
<scratch space>:86:1: note: expanded from here
86 | __compiletime_assert_51
| ^
1 error generated.
If these functions are not inlined (which the compiler is free to do
even with functions marked with the standard 'inline' keyword), the
BUILD_BUG() in the default case cannot be eliminated since the compiler
cannot prove it is never used, resulting in a build failure due to the
error attribute.
Mark these functions as __always_inline to guarantee inlining so that
the BUILD_BUG() only triggers when the default case genuinely cannot be
eliminated due to an unexpected size.
Cc: <stable@xxxxxxxxxxxxxxx>
Closes: https://github.com/ClangBuiltLinux/linux/issues/1955
Fixes: 46859ac8af52 ("LoongArch: Add multi-processor (SMP) support")
Link: https://github.com/llvm/llvm-project/commit/1a2e77cf9e11dbf56b5720c607313a566eebb16e
Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
---
Changes in v2:
- Change 'inline' to __always_inline for all functions that contain
BUILD_BUG() (Huacai)
- Notate that 'inline' does not guarantee inlining in the commit message
to further clarify the change to __always_inline.
- Link to v1: https://lore.kernel.org/r/20231101-loongarch-always-inline-percpu-ops-v1-1-b8f2e9a71729@xxxxxxxxxx
---
arch/loongarch/include/asm/percpu.h | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/arch/loongarch/include/asm/percpu.h b/arch/loongarch/include/asm/percpu.h
index b9f567e66016..313852fba845 100644
--- a/arch/loongarch/include/asm/percpu.h
+++ b/arch/loongarch/include/asm/percpu.h
@@ -32,7 +32,7 @@ static inline void set_my_cpu_offset(unsigned long off)
#define __my_cpu_offset __my_cpu_offset
#define PERCPU_OP(op, asm_op, c_op) \
-static inline unsigned long __percpu_##op(void *ptr, \
+static __always_inline unsigned long __percpu_##op(void *ptr, \
unsigned long val, int size) \
{ \
unsigned long ret; \
@@ -63,7 +63,7 @@ PERCPU_OP(and, and, &)
PERCPU_OP(or, or, |)
#undef PERCPU_OP
-static inline unsigned long __percpu_read(void *ptr, int size)
+static __always_inline unsigned long __percpu_read(void *ptr, int size)
{
unsigned long ret;
@@ -100,7 +100,8 @@ static inline unsigned long __percpu_read(void *ptr, int size)
return ret;
}
-static inline void __percpu_write(void *ptr, unsigned long val, int size)
+static __always_inline void __percpu_write(void *ptr, unsigned long val,
+ int size)
{
switch (size) {
case 1:
@@ -132,8 +133,8 @@ static inline void __percpu_write(void *ptr, unsigned long val, int size)
}
}
-static inline unsigned long __percpu_xchg(void *ptr, unsigned long val,
- int size)
+static __always_inline unsigned long __percpu_xchg(void *ptr, unsigned long val,
+ int size)
{
switch (size) {
case 1:
---
base-commit: 278be83601dd1725d4732241f066d528e160a39d
change-id: 20231101-loongarch-always-inline-percpu-ops-cf77c161871f
Best regards,
--
Nathan Chancellor <nathan@xxxxxxxxxx>
